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Contents

ForewordPrefaceAcknowledgementsAbouttheAuthorDedication

1WhyGetanOscillscope?2ALittleHistory3EveryScopeHasTheseElements4ProbesandAccessories5ScopeSectionsinDetail6InputModes7Let’sPutaScopetoWork8IfYouAreGoingtoBuyOne—Specifications

Appendix1SoftwareOscilloscopes—CapableandFree!Appendix2QSTProductReview:TektronixTBS1042andRigolDS1052EOscilloscopesAppendix3QSTProductReview:OsciumiMOS-204PortableOscilloscope

Foreword

Apopular activity amongamateurs isbuilding,modifying, restoringor repairingequipment.Duringtheircareers,amateursassembleahomeworkshopappropriatefortheirinterests,usuallystartingwithafewbasictools,agoodsolderingironandperhapsamultimeter.Fromthere,youmightaddapower/SWRmeteroranantennaanalyzer.

When it comes to working inside a piece of equipment, one of the most useful tools is theoscilloscope. “Scopes” have been around for decades, helping countless amateurs “see” the signalsinsidetheirequipment.IsmySSBtransmitterproperlyadjusted?WhatdoesmyCWwaveformlooklike?Isthererippleonmypowersupply?Onceanexpensivetoolforonlythemosttechnicallysavvyamateur,today we have access to a variety of analog, digital or hybrid scopes at prices suitable for a homeworkshop.

Inthisbook,PaulDanzer,N1II,conveysawealthofinformationabouttheseusefultools.Startingwithan overview and short history lesson, Paul goes on to discuss oscilloscope functional blocks, probes,controlsandinputmodesandthendescribespracticalapplications.Heconcludeswithachaptertohelpyouunderstandscopespecificationsandfeaturessothatyoucanfindonethatwillbestsuityourneeds.

Wehopeyou’llfindthisbookausefuladditiontoyourlibrary.

DavidSumner,K1ZZChiefExecutiveOfficerNewington,ConnecticutFebruary2015

Preface

Asevery teacherknows,occasionallyyouare rewardedbya fewstudentswhowant toknowabitmoreonatopicthatyourlectureorthetextbookcovered.Thisisespecialytruewhenyoumentiontoaclass,asIdid,thattheoscilloscopeisaveryvaluabletoolinelectronics—hamradioorotherwise—simplybecauseitletsyou“see”whatisgoingon!

Yearsagothesameoscilloscopeblockdiagramandthesameexplanationwerefoundinmostbooks.Today, with the introduction of personal computers, digital technology and the ability to produceoscilloscopeswithmorecapabilityatalowercost,theentirefieldhaschanged.

Inparticular,therestillarequiteafewtotallyanalogoscilloscopesavailable,buttherearealsomanynewconfigurations.Somearetotallyself-containeddigitalinstruments,someareadigital/analoghybrid,somerequireapersonalcomputertoactastheprocessoranddisplay,andothersusesmartphonesphonesandtabletsastheirhost.

Most books and online descriptions reflect either the old analog configuration or advanced theorybeyond what many students and radio amateurs can profitably use. This book was written to discussoscilloscopesinamiddleground—pastthesimpleanalogscope,butlessthanagraduateleveltreatiseindataprocessingandsignalcomputations.

Today’s technologies havemade very capable oscilloscopes available to radio amateurs formanyuses in the ham shack. There are two reasons for this availability. First, the new digital scopes havedisplaced the older, very expensive scopes in businesses and industrial labs.As a result, scopeswithcapabilitiesmostofuscouldonlydreamofyearsagoareoftenavailableusedatapriceofonetenthorlessoftheiroriginalprice.Second,thenewdigitalscopesare,duetotheuseofdigitalprocessing,notdependentonprecisionanalogcircuitsandthereforelessexpensivethantheirpredecessors.

Theresultisthattheabilityto“seewhatisgoingon”inourequipmentismuchmoreavailableandmuchmorecommoninthehamshack.

73,PaulDanzer,N1II(pastcallsigns:KN2DGR,K2DGR,W1DQJ)

Acknowledgements

Manythankstothefollowingindividuals,companiesandorganizationswhowereverygenerouswiththeirtimeandhelp:

JimBrannigan,WB2TPSDaveCisco,W4AXLSamDick,NV1PSethGolitzer,W1SHGDonHudson,KA1TZRChuckPenson,WA7ZZERonPollack,K2RPRichRoznoy,K1OFJoeVeras,K9OCOTimWalker,W1GIGMarkWilson,K1RO

AlexWongatDigilentInc.BryanLeeatOSCIUM,Dechnia,LLCChuckatwww.myvintagetv.comTekwiki,thecommunityofTektronixoscilloscopeenthusiasts,www.w140.com/tekwiki

AbouttheAuthor

Paul Danzer, N1II, started his Amateur Radio career as a teenager, which led to Bachelor’s andMaster’sDegreesinElectricalEngineering.Hisengineeringcareerspannedmorethan30years,andhewasawarded11patentswhilespecializing indigitalcircuits,digital systems,and radarsystems.Asaresulthehadagreatdealofhands-onexperiencewiththesubjectofthisbook,oscilloscopes.

Afterretiringfromengineering,hespentthreeyearsasaTechnicalEditoratARRLHeadquartersinNewington, Connecticut. There, he authored one book, co-authored a second and edited the ARRLHandbookandtheARRLOperatingManualaswellasseveralotherpublications.

PaulthenembarkedonanewcareerasaProfessorofComputerScienceatalocalcommunitycollege,teaching electronics, personal computer hardware, data communications andotherPC related subjects.After11yearsasa full timeprofessorhe isnowanAdjunctProfessorandspends the restofhis timewritingonAmateurRadiosubjects.

He has written more than 250 magazine articles for Amateur Radio publications and computerpublications.HisARRLappointmentsincludeTA(TechnicalAdvisor)andTS(TechnicalSpecialist).In2004,hewasawardedtheBillOrr,W6SAITechnicalWritingAwardbytheARRLBoardofDirectors.

Dedication

Tomywife Flo,who has patiently tolerated strange noises, strangewires, and all sorts of strangethingsattachedtotheroofofourhome.

Chapter1

WhyGetanOscilloscope?

Didn’tyoueversay,“IwishIcouldseewhatwasgoingon?”Thatisthequestionhamshavebeenaskingsincetheearliestdaysofhamradio.Bynature,notonlydo

hams like to experiment, but they alsowant toget themost out of their equipment.Canyou imagine acook,busypreparingameal,whocannotsmellortastethefood?Thatisexactlythesituationmanyhamsfindthemselvesinwhentheyaretesting,repairingorjustusingtheirgear.

Everypieceoftestequipmenthasitsplace,andanon-the-airtestcanbeveryrevealing.Buttoreallyunderstandwhatisgoingon—justlikeacookinthekitchenhastotastedisheswithhisorhertongue—youhavetoliterallyseewithyoureyestoreallyunderstandacircuitorequipment.

Someyearsagothisability toseewasbeyondthebudgetofmosthams,exceptforafewwhobuilttheir own oscilloscopes. Professional-grade scopes cost $1500 to $3500 and required periodiccalibrationwith test equipment thatwasmoreexpensive than the scopes themselves. Industrial surplusequipmentorlessergradescopes, includingkits,werenotcheap.Youwouldhavetowriteacheckfor$300to$600andyouwereneversurejusthowaccurateyourmeasurementswere.

Inadditiontorequiringperiodiccalibration,thisgenerationofscopesusedvacuumtubesthatthatranhot, increased theamountofand frequencyofcalibration,and increasinglycaused theoscilloscopes todeteriorateinperformance.

Since those days there have been two major changes: Solid state devices (transistors, diodes,integrated circuits) have replaced vacuum tube circuitry and digital techniques have replaced driftinganalog circuits. Perhaps the result of these changes canbest be seenby comparing the pictures in thischapter.

WhatDoOscilloscopesOldandNewLookLike?The older generation high-quality analog scope, such as the Tektronix unit shown in Figure 1.1,

represents a type of scope that was a laboratory standard through many model number variations.Generallythesescopesweighedmorethan60poundsandweremountedonacartbecausetheywerenotcarriedeasily.Powerinputwasabout700W,soasmalllabspacecouldgetquitehot.YoucanseethetubelineupofthisbeastinFigure1.2.

Technicianqualityscopessuchasmightbefoundinawell-equipped(andwell-funded)homelaboronthebenchofaTVrepairshopinthepasttypicallylookedliketheoneinFigure1.3—anEICO460.ForthemostpartthesescopesdidnotdoprecisionmeasurementastheTektronixdid,butstillansweredthequestion“Whatisgoingon?”InthecaseoftheTVrepairshoporthehamworkshop,itansweredthequestion“Isthesignalthere?”

Bycomparison,manyoftoday’sscopeslookliketheoneFigure1.4. Itmeasuresroughly2×8×7inches,weighs1.5pounds,andplugsintoacomputerUSBportforpower.Exceptfortheinputcircuit,therestofthescopeisdigital.Thereisnoscreenordisplay;thescopefeedsadesktoporlaptopcomputer.Calibrationisusuallybuiltin,andthedigitalcircuitsareoftengivenadditionaltaskssuchasactingasasignalgeneratorora spectrumanalyzer.Themostamazingpart is thecost—anywhere from less than$100toperhaps$350forfairlyaccuratemeasurements.

There is one othermajor change from the older analog oscilloscopes to the newer digital scopes.Often the scope is used to findout “WhatHappened?”Thismeans looking for something that happensbriefly, not continuously. With older analog scopes, their native mode of operation was continuousmeasurement.Youwould have towatch the display carefully to catch transient or “occasionally here,usuallynot” things.Tobeable tocaptureandhold transientmeasurements,youwouldneed toownanevenmoreexpensivestoragescope.Storagescopescouldcaptureanddisplayatransientwaveformonaspecialphosphorscreenthathadlong-termholdingcapability.

Thenativemodeof thenewdigitalscope isstorage. Inputwaveformsaresent toadigitalmemory.Unlesscommandedtoeraseorreplacetheolddatawithnewdata,theyautomaticallystoretheirinputstothelimitofthesizeoftheirmemory.

WhatCanYouDoWithYourScope?Lateron,inChapter7,wewilltakealookatmanyofthecommonusesofanoscilloscopeintheham

shack.Butfornow,let’sjustgetanideaofwhatthesedevicescandoforyou.IfyouopenedacopyoftheARRLHandbookof30or40yearsago, thefirstapplicationforscopes

would be to look at your transmitted AM signal. This was a very good way to tell if you had yourtransmitter adjusted correctly or if you were over modulating, thus spattering all over the band inadditiontosoundingterrible.InChapter7wewilltakealookatthisapplication,butfornowlookattheblockdiagraminFigure1.5.Hereisaproblemyourscopecouldhelpyouwith,andatleasttellyouwhatisgoingon.

Suddenlyyourfriendson therepeatercomplain thatyour2meter transceiverathomehasa terriblehum.Because you are conscious of public service and emergency communications, you run this radiofroma12Vstoragebatterywithatricklecharger.Youcanthinkofthreepossibilities.

First, is the hum coming from the trickle charger/battery combination?Out comes your scope, youconnect it at point A, from the +12 V line to ground, and this possibility is quickly confirmed oreliminated.Isthe12Vlineapuredcsignaloristhereanaccomponenttothewaveform?

Next, is the problem in the audio chain? Perhaps it is a badmicrophone cord or something in themicrophoneamplifier.ConnectthescopetopointBandnowyoucantellifthehumiscomingfromthissetofcircuits.

Finally,perhapsthesynthesizer/phase-lockedloopishavingaproblem.Yourfriendstellyouthatthehumsoundslike60Hz,solookatvariousplacesaroundthesynthesizer.Youdon’thavetoknowwhatwaveform you are looking at, nor do you need to see each individual signal in detail. Look for anenvelope thathasrepetitivechangescorresponding to theapproximateperiodofa60Hzwaveform—perhapsatpointC.

Is thisapproachguaranteed tohelpyousolve theproblem?Ofcoursenot,butyoucanactuallyseewhatisgoingonatthesekeypointsinthecircuit,soyoustandabetterchanceoffindingacure.Maybeyouwillbeluckyanditisjustabadgroundonyourmicrophonecord.

Figure1.6showsanotherverycommonuseforoscilloscopes.Withafewmorecomponentsyoucantestdiodes,capacitors,resistors,andtransistors.InChapter7wewilldiscussthisuseandothersinmoredetail. InFigure1.6, the componentbeing tested is a standarddiode—perhaps apower rectifier.Bypickingthecorrectcomponentsandoscilloscopesettings,aV-I(voltagevscurrent)curveappearsonthescopedisplay.Ifthediodeisokay,theV-Icurvewilllookliketheleft-handdrawing,ifshorted,thecenterdrawing,andifopentheright-handdrawing.

Anoscilloscopeinyourshackismorethanjustahandytestinstrument;itletsyoubothsolveproblems

and test new ideas. Chapter 2 will explain a bit of where oscilloscopes came from and how theinexpensive but very capable unitswe have todaywere developed for the older—occasionally verymucholder—technology.

Chapter2

ALittleHistory

We’vecomealongway.Inordertounderstandwhyoscilloscopeshavethedesignsandcapabilitiestheyhavetoday,itishelpfultoseewheretheycamefromandhowtheydeveloped.Inthenextchapteryouwillseethateveryoscilloscope—whetheritisbuiltentirelyinhardware,partlyinhardwareandpartlyin softwareor even totally in software—has the same four elementsor sections.One reason for thiscommonalityishistory.

Thechangesinoscilloscopesfromthelate1800stothepresentresultfromatwo-edgedsword.Asthetechnologychanged,therequirementsforoscilloscopeschangedandthecomponentsthatcouldbeusedinthedesignofoscilloscopeschanged—bothfactorschanginginparallel.Thischaptersummarizeshowtheoscilloscopeswehaveavailable tous inourhamshacksandworkbenchesdevelopedandchangedovertheyears.Afullhistorywouldoccupymorethanthisentirebook,butabriefglanceservestoexplainwhereweareandhowwegothere.Inparticular,thischapterexplainshowhamshavegonefromrarelyowning and using oscilloscopes to being able to afford and use today’s commonly available low-costscopesiftheysodesire.

EarlyInstrumentsTheneedto“see”avoltage,currentorotherphysicalitemdatesbacktotheearliestelectricaldesign.

It would come as no surprise that the limit to “seeing” was how fast themeasuring instrument couldrespond,howfastyoucouldseetheresponse,andperhapsmostimportant—howfastyoucouldwriteitdown.

Veryquickmechanical“scribers”—whattodaywewouldcallplotters—wereinvented.AsFigure2.1shows,atypicalearlymodelconsistedofamodifiedmetersuchasastandardD’Arsonvalvoltmeterwithanextendedpointer.Ontheendofthepointerwaseitherapenwitharollofpaperorametalpinorscribethatleftanimpressiononatreatedpaper.Themeterismountedoverthepaper.ThemotionofthepenprovidestheX-axis(amplitudeofthevoltagebeingmeasured),andthepapermotionprovidestheY-axis (time scale). In this crude implementation, the amplitude scale is not linear, and various clevermechanicalwayswereinventedtomakeitlinear.

Sincethemeterpointerdoesnotmoveveryquickly,othermeasurementtechniqueswerefound.Someusedmirrorsandlightonphotographicsensitivepapertoallowtheinstrumenttobemoreresponsiveandtoplothigherfrequencysignals.

TheCathodeRayTube(CRT)ChangedEverythingItisalwaysaproblemtostatewithabsolutecertaintywhowasthefirstpersontodosomethingorthe

firstpersontoinventsomething.Asanexample,theSmithsonianinWashingtonDChasanentireexhibitpayingtributetotheWrightBrothersformakingthefirstflight.ButinConnecticutthereisarecordofanearlierflight,describedinanewspaperofthetime.TheFrenchhavetheirowncandidateforfirstflight,asdotheGermansandothers.

Knowing that, let’s startwithKarlBraun,who is creditedwithmaking a cold-cathode ray tube in1897, and is recorded as using it to explore thewaveform of an alternating current voltage. Thus, ineffect, he made and used an oscilloscope. However, this was before the first recorded vacuum tubeamplifierdemonstratedbySirJohnAmbroseFlemingin1912.Undoubtedlythereareothercandidatestoclaimthattheyshouldhavethe“firsttodo”titleofthesedevelopments,butthesethingsdoformthebasisoftoday’soscilloscopes.

Fastforwardingnowtopre-WorldWarII,Figure2.2showsastateof-the-artoscilloscopeforhamsin1937.Thisdevice,theNationalRadioCompanymodelCRM,wouldberecognizedasafunctionalpieceoftestequipmentthatcouldbeusedbymosthamstoday—althoughoflimitedbandwidthandaccuracy.Theprice,asadvertisedinQSTinthelate1930s,was—readyforthisone?—$11.10plusanadditional$5.81forthecathoderaytube.

By1947hamshadgraduatedtotheNationalCRUoscilloscope,witha2-inchtube(Figure2.3).Fromthepictureitseemstohaveasweeptriggercontrol.Atthesametime,WorldWarIIsurplusgearwasverycommon,somanyhamswerebuildingtheirownversionoftheNationalscope.ThentheHeathCompany,which started out in life selling airplane kits, came out with its first electronic kit — the O-1oscilloscope.Key to thiskitwas,asyoumightguess, the largestockofwar-surpluscathoderay tubesavailableonthemarket.Figure2.4isanearlyHeathkitscopekitsellingfor$39.50.

Aboutthesametimeanewhomedevice—television—wascapturingimaginations.Usingtubesandhighvoltages,theseTVsetsrequiredfrequentrepair,somanyTVrepairshopssprangup.Figure2.5isanexampleofthetypicaloscilloscopefoundinmanyTVshops,aDumont274.RCA,whichatthetimewasaTVmanufacturer,acommunicationscompanyandaneducational institute, jumpedinwith itsstudent-orientedscopeinFigure2.6.Nowhamshadthreesourcesofoscilloscopesfortheshackandworkbench—kitssuchastheHeathandEico,moderatelypricedcommercialunitssuchasDumontandRCA,and,ofcourse,homebuilt.

MyHeathkitOscilloscopeCloneByTimWalker,W1GIG

Intheearly1960s,whenIhadayoungfamilyandwasstrugglingtofinishmynewhouseinUtica,NewYork,Ineededascopetoservicemyhi-figear.IhadpreviouslybuiltonefromtheARRLHandbookthatusedasmalldiameter913tubeforthedisplay,buta1-inchscopehasits limitations.Searching through thesurplusstores in theareaproduceda3AP1 tube(a3-inchCRT)andascopepowertransformer.OfcourseIhadthecurrentHeathcatalogandinitfound the plans for a very nice 3-inch scope.Remember,Heath used to include the circuitdiagram formanyproducts in their catalogs.Usingmostlyparts frommy junkbox I put thescopetogetherandusedithappily.

Acoupleofyearslater,afterbeingtransferredtoNewYorkCity,IfoundthatmynewofficewasinthemiddleofRadioRow,atthetimethesurpluselectronicscenteroftheuniverse.Youcanbet that Ispentmanya lunchhourcheckingoutall thestores.Oneday Icameupona3ACP1smilingatmefromthewindowofoneofmyfavoritestores.Theflatfaceofthetubepromisedmeamuchbetterscope.Forabout$5,Igotthetube,themumetalshieldandthespecial14-pinsocket.Nowhowtouseit?

I found that theHeathdesignhadbeenupdated touse the3BP1andminiature tubes. ItwasnowcalledtheIO-21.Theonlyproblemwasthatthe3ACP1hadanacceleratinganodethatrequires4000Vabovethecathode,soIrebuiltthehighvoltagepowersupplyformynew3ACP1.

Ihavebeenusingthisscopeforabout50yearsnow.Asyoucanseefromthephotographsitlookstired,thepartsareoldandlooklargeascomparedtotoday’sparts—butitstillworksandworkswell!

AdvancesinIndustryIn the 1960s, 70s and80s, commercial electronics labs outgrew theDumonts in favor of precision

laboratoryscopes.Tektronixwasperhapsthemostcommonnameseenincommercialdevelopmentandmilitarycontractorlabs,followedbyHewlett-Packard.Theyhadseveralthingsincommon:

Theywereexpensive—severalthousanddollarstomorethan$10,000.Theywereheavy—oftentheyweremountedonacartTheyrequiredagreatdealofmaintenance,includingtubereplacementandcalibration.Theyoftenrequiredairconditionedroomsbecausetheyradiatedalotofheatanddidnotfarewell

inhighhumidity.Typicalofthisgenerationistheapproximately70-poundTektronix535showninFigure2.7.At the

lowerleftisanexchangeablefrontendthatallowedthescopetobeusedforvariouspurposes,butthisroom-warmerdissipatedaroundahalf-kilowatt!Assucceedinggenerationsofscopeswerereplacedwithnewerandmorecapableunits,thesevacuumtubebasedunitsoftenweresoldforafewhundreddollars—whichmeantthattheyendedtheirlivesinahamworkshop.

Ofcourse,asyoumightexpect,theappearanceofthetransistorandintegratedcircuitchangedthingscompletely. By comparison the solid-state Tektronix 2215A inFigure 2.8 weighs around 15 pounds,dissipates40Wandistypicalofthislatergenerationofscopes.Ashappenedwiththeprecedingtube-basedscopes,newerandnewerunitsreplacedtheolderonesintheindustriallabs.Onceagain,forafewhundreddollars(andoftenless)thenewersolid-stateunitsreplacedtheolderonesinhamworkshops.

CompaniessuchasBKPrecision(Figure2.9)jumpedintosupplyrepairshopsandmorewell-fundedhams.Andof courseHeath remained a hamworkshop favoritewithmodels such as the 4554 (Figure2.10)Suddenlyhamsnolongerneedtoshopforobsoletetube-basedscopes!

Today’sChoicesToday—doyouwantanewscopefor$150-$350?Takea lookatFigure2.11.This instrument is

dualchannel,includesself-calibration,issolidstate,includeseverymodethattheolderscopeshadandmore.Butwhereisthedisplayscreen?

Theansweris,itisattachedtoyourdesktopPCorlaptopandusesthecomputer’smonitor.Onlytheanalogfrontend,A/Dconvertersandsomedigitalcontrolcircuitsareinthesmallboxshown(8incheshigh,2incheswideand7inchesdeep).Theinputisconvertedtoadigitalsignalandsenttothecomputer,generallythroughaUSBport.Thereisnopowersupplyorconnection—itispoweredbytheUSBport.Processingfordisplayisdonebysoftwareinthecomputer.

If you feel that you are missing the familiar oscilloscope front panel — well, just look at thesynthesizedpanelofaPC-basedscopeinFigure2.12.Notice thebarat the top.Notonly is thisdual-

channelscope,butmostunitslikethisalsofunctionasaspectrumanalyzerandstoragescopeortransientrecorder.

Ifyoustillliketheideaofaself-containedoscilloscopewithbuilt-indisplayscreen,low-costdigitalstorage oscilloscopes such as the one shown in Figure 2.13 are available from a number ofmanufacturers.Costingaslittleas$300andweighingabout5pounds,theyhaveacolorLCDdisplay.

Notenoughforyou?Howabout theOsciumscope inFigure2.14.Shirt-pocket size, itplugs intoatabletsuchasaniPad,andithasallthecapabilityandfeaturesofitspredecessorlargerunits.

Aswewillseeinthenextchapter,scopesthedescribedhere,startingwiththepost-WorldWarIIunitsuntil today, all have as aminimum the same four basic sections—whether built in hardware or parthardware-partsoftware.

Chapter3

EveryScopeHasTheseElements

Every oscilloscope contains the four functional parts shown in Figure 3.1. Notice the wordfunctional.Earlyinthehistoryofoscilloscopesthereweregenerallyfouractualcircuitsections,butnottoday.Whenmicroprocessors,fastanalog-to-digital(A/D)converters,personalcomputersandflatpaneldisplaysmadetheirappearance,oscilloscopesreflectedthesenewtechnologies.

Today’s scopes are part hardware, part software, sometimes part personal computer (or laptop,netbook,tablet,andsoon).Sometimestheyareevensoftwarealone.

Youmightalsonotice thata fifth functionalarea,apowersupply, isnot included in the figure.Thereason issimple—manymodernscopesdonothavean internalpowersupply.Theyusepower takenfromaUSBportorotherconnectiontoacomputinghost.

Thischapterbrieflydescribeseachofthefourfunctionalareas.Later,inChapter6,youwillfindmoredetailsonthesesectionsandtheircapabilities.

VerticalCircuitsHandletheInputSignalsOther than the power supply, the vertical circuit or vertical channel is the only part of an

oscilloscopethatmustbedesigned,atleastinpart,asananalogcircuit.Theobjectoftheverticalcircuitistoputthewaveformonthescreenwithminimumdistortion.Itmustalsobeabletolimitthevoltageoftheinputsignal—perhapsattenuateit,inconjunctionwithatestprobe,oramplifyit—sothatitfallswithinthelimitsofthecircuitsthatfollow.

Figure3.2 isasimplifiedblockdiagramofaninputstage.ThefirstswitchallowsyoutoselectDCandseetheabsolutelevelofasignal.ThesecondpositionisAC—ingeneralacapacitorblocksthedcvoltageandonlytheaccomponentisseen.Quiteoftenthereisathirdselection—GROUND.Thisisusedtopositionthetrace,withoutasignal,anywhereyouwishonthescreen.

Most scopes have an overlay on the screen, called a graticule. Analog scopes typically have thegraticuleasaphysicalplasticoverlay,whilemoderndigitalscopesoftenuseanelectronicpatternonthescreen.As seen inFigure3.3, it is customary tohave thegraticule calibrated incentimeters, andasaresulttheinputvoltagescaleisusuallystatedinvoltspercentimeter(V/cm).Thehorizontaltimescaleisusuallydescribedinseconds,milliseconds,microsecondsorothertimeunitspercentimeter—suchas3ms/cmor3millisecondspercentimeter.

AnexpandedblockdiagramisshowninFigure3.4,where two inputchannels (dual channels)areshown.Each inputchannel isprecededby theselectionswitch justdescribedandfollowedbycircuitsthatallowselectionof:

asingleinputchannelbothinputchannelssimultaneouslyandalternately,orbothinputchannelssimultaneouslyandcombined(addedtogetheroronesubtractedfromtheother).Anadditionalchoiceischopped,wherebothchannelsareshownsimultaneouslybutprocessedsoyou

seebothbuttheyarealternatelysampled,oneaftertheother.Inananalogscopetheoutputofthesestagesgoestoadditionalamplifier.Inmanymodernscopes,the

firststageoutputgoestoA/Dconverters(Figure3.5)andthechannelselectionisdoneinprocessingpasttheA/Dconverters.

TheHorizontalSectionSweepstheTraceAcrosstheScreenSince an oscilloscope usually shows a quantity — say a voltage — as it varies with time, the

horizontalaxisrepresentstime.Thestartoftime,orwhatisgenerallycalledt=0,isontheleftsideofthescreen.Howfastthetracemovesacrossthescreenisthesweeprate.Thiscanvaryfromsecondspercentimeter(s/cm)tonanosecondspercentimeter(ns/cm).Lowersweepratesareusedforvoltages thatvaryslowly.

Asanexample,supposeyouwantedtotakeacloselookata60Hzsinewave.Thetimeforonesuch

wave, or theperiod, iscalculated as 1/frequency or 1/60 of a second— or approximately 16.66ms(milliseconds).Most oscilloscopes have a graticule calibrated as 10 centimeters (cm)wide.A sweeprateof2ms/cmwouldhavethetracecrosstheentire10cmwidescreenin20ms,soabitmorethanonecycle(16.66ms)ofthe60Hzwaveformwouldbeseen.

Ifwedoubledthesweeprateto4ms/cmthetimetocrossthescreenwouldbe40ms,and40/16.66=approximately2.4.Inotherwords,2.4completesinewaveswouldbevisible.

Withacompletelyanalogscope,avoltagewouldbeapplied to twohorizontalplates in thedisplaycathoderaytube(CRT),withawaveformasshowninFigure3.6.Inthefigurethevoltage(A)initiallyputsadotattheleftsideofthescreen,andasthevoltageincreasesthetracemovestotherightsideofthescreen.

Today,ofcourse,mostscopesdonotuseCRTsandtheoutputgoestoadigitally-generateddisplay—usuallyaflatscreenLCDofsomesort.Inthiscase,theanalogvoltageofFigure3.6wouldbereplacedbyadigitalcount.Acountofzerowouldplacethedotattheleftedge,andasthecountincreasesthetracewould move to the right. Sweep speed would be controlled digitally. You can look at the horizontalpositionasthecount inadigitalcounter—thefaster theinputclocktothecounter, thefaster thetracegoesacross.Inotherwords,thefastertheinputclock,smallerthetimepercm.

Someoscilloscopeshaveanadditionalmode,where thesweepcircuit isdisabledandonechannel,saychannelA,isconnectedasusualtotheverticalaxisandthesecondchannel,channelB,isconnectedtothehorizontalaxis.Thispermitsanon-screenplotofonesignalagainstanother.Wherethefrequenciesoftwosignalsarerelated,sayoneisasinewaveat1000Hzandtheotherasinewaveat3000Hz,the

patternwilltellyouthefrequencyrelationshipbycountingthelobesonthescreen.ThisdisplayiscalledaLissajouspatternandtheoneshowninFigure3.7hasa3:1relationship.

At this point you can see that a oscilloscope today could be totally analog,with precision analogcircuitsandaCRTdisplay.Or,ascopecanbetotallydigital,convertingtheanaloginputimmediatelytodigitalandprocessingitandthesweepindigitalcircuitry.Manyscopesareinfactacombinationofthetwo.

TheSyncCircuitTriggerstheSweepAllscopes,digitaloranalog,musthaveawaytomakethedisplayedwaveformseemtostandstillon

thescreen.Thisisthefunctionofthesynchronizationorsynccircuits.Ifweletthesweepcircuitrunfree— there is no relationship to the input waveform— the waveform inFigure 3.8 is the result. Eachsuccessive inputwaveformstartsatadifferentpoint,andafteranumberof inputwaveforms theentirescreenfills.However,ifwetriggerthesweepsothateachsweepstartsastheinputwaveformequalsasetvoltage—as inFigure3.9—thenasinglewaveformisseen.Actually it ismanywaveforms,butperfectlysuperimposedoneachother.Figure3.9showstwotriggerpoints.FortriggerpointA,+1Visselectedwithapositiveslope.PointBshowswherethetriggerpointwouldbeif+1Visselectedwithanegativeslope.

Individualscopeshavesetsoftriggerselectionfeatures.Generallythereisavoltagelevelselectionthatdetermineswhereontheinputwaveformthesweepshouldstart.Oftenthereisaslopeselection—whether the selected voltage trigger point should be on a rising waveform or a falling waveform.Additional choices include picking either channel A or channel B for sync trigger, ac coupling or dccouplingtothesynccircuitandoftenalineor60Hzsyncinputonolderscopes.

Shouldyoudecidetobuyanolderscope,don’tbesurprisedifthesyncselectionalsohaspositionslabeledHORIZONTAL and VERTICAL. These were positions used to service analog TV sets before theswitchovertodigitalTV.

While the sweepusually is triggered—starts— immediately as the selected trigger point occurs,more complex scopes (oftenmore expensive scopes) often have a delay feature. The trigger selectionpicksthestartpoint,buttheactualsweepdoesnotstartuntilalaterselectedtime.Thisfeatureisknownasthesweepdelay.InChapter6wewilldiscussoneofthesedelayfeaturesthathassomeinterestinghamradioapplications.

DisplayTypesCurrentlytherearethreepopularoscilloscopedisplays:acathoderaytube(CRT),aflatpanelLCDor

TFTdisplay suchasusedwithmostpersonalcomputers, and…none.The first twoarephysical, andintegratedintotheoscilloscopecabinetorenclosure.ThelastrepresentsmodernscopesthatconsistofaprocessingfrontendpluggedintoaPC,tabletorevenasmartphone.

ClassicCathodeRayTubeThecathoderaytube,orCRT,wastheearliestoscilloscopedisplaycomponent.Forascreensizeof

fiveorsoinchesacross,thescopedesignerhadtouseapackagethatwasfairlydeep(12inchesormore)andallowedagreatdealofheattoberadiated.Mostimportantofall,theCRTrequiredapowersupplyofperhapsseveralthousandvolts.

ThebackendofaCRTresemblesastandardvacuumtube(Figure3.10)witha filament toheat thecathode.Electronsradiatingfromthecathodepassthroughagridandarethenpulledforwardtowardthescreenbyacombinationofapositivevoltageonahollowanodeandahighpositivevoltageappliedtoconductivelayersonthesidestowardthefrontofthetube.

The spot where the electron steam hits the phosphor-coated front screen illuminated a dot. Thepositionofthedotiscontrolledbyfourdeflectionplates.Figure3.11 illustrates theplates,showinganilluminatedspottowardtheupperleftcorner.TheelectronstreamispulledupbyhavingthevoltageonplateApositivewithrespect toplateB.In thesamewaythespot isonthe leftbecausethevoltageonplateCispositivewithrespecttoplateD.IftherewerenovoltagedifferencebetweenAandB,andnodifferencebetweenCandD,thespotwouldbeinthemiddleofthescreen.

Brightnessiscontrolledbythegrid,justasthegridinanordinaryvacuumtubecontrolstheflowofelectronstowardtheplate.TheverticalchannelisconnectedtoplatesAandB,andthesweepcircuitsconnecttoplatesCandD.

Self-ContinedFlatPanelDisplayUnliketheCRT,newerscopesincorporatingflatpaneldisplaysdonotrequireadeepenclosure,do

notneedhighvoltage anddonotgenerate agreatdealofheat.Agood scopedisplay, like agoodTVscreen,doesrequireadevicewithreasonableresolution,averyflatfaceandalonglife.

TypicalofsuchadisplayistheoneusedintheRigolDS1052e,picturedinFigure3.12.Ithasa5.6-inchdiagonalcolorLCD(liquid-crystaldisplay)screenusingTFT(thinfilmtransistor)technology.Verysimilardisplays arenowbeingofferedon the frontpanelofAmateurRadio transceivers fromseveralmanufacturers.

Thescopesizeisdrivenbythefrontpanel,containingthedisplayandthemanualcontrols(knobsandswitches). The depth, approximately 5 inches, is all that is needed to contain the circuitry of a verycapablescope.

NoDisplay,NoPowerSupplyImagine taking the scope in Figure 3.12, eliminating the front panel and just packaging the internal

circuitboard—perhapsinapackage1-1/2incheswidebyafewincheshighand5inchesorsodeep.ThefrontpanelholdsjacksfortwoprobesandtherearpanelaconnectorforaUSBcabletoapersonalcomputer.TheUSBportsuppliespowertothescopeandthesignallinesintheUSBcablefeedthetwochannelinputs,digitized,tothepersonalcomputer.Figure1.4inChapter1showsonesuchscope.

Figure3.13showsanevensmaller—althoughlesscapable—scope,theOsciumiMSO-204.ItplugsdirectlyintoaniPad,iPhoneorotherAppledevice.Similarmodels,roughly2.5×3.25×0.75inches,areavailablewithUSBconnections.

SomeScopesCanStoreYourWaveformToday’s technologyhasbroughtus some interestingcapabilities, somedirectlyand some indirectly.

Withanolder scope, ifyouhadawaveform thatyouwanted to lookat indetail, and itoccurredonlyonce,youwouldhavetolookveryquickly.Whentheimageseenthroughtheilluminatedphosphoronthescreenfaded,sodidyourcapabilitytolook.

Some older analog scopes had storage capability. The CRT had a special phosphor and voltagecircuitrythatpermittedalongpersistenceonthescreen.Generally,storagescopescommandedamuchhigherpricethanconventionalscopes.

Withtoday’sscopesthatdigitizetheincomingwaveforms,storagecapabilityisthenormalmode.Eachincrementofthewaveformisstoredasadigitalnumber.Duringeachnewsweepacrossthescreenthesenumbersarereplacedbythenewincomingwaveformvoltages—anewsetofnumbers.Wantstorageofawaveform?Justinhibitthenewsetofnumbers.Getonesweep,holdthenumbersandlookaslongasyouwant!

Sinceeachpointontheincomingwaveformisheldasadigitalnumber,youcangetindirectbenefits—math functions are almost free, since they are just digital processing. As an example, look at thetriangularwaveshape inFigure3.14.Want toknow thepeak-to-peakvoltage?Find thehighestnumber(pointAinthedrawing)andthelowestnumber(pointBinthedrawing), takethetwodigitalnumbers,subtract,andyouhavethepeak-to-peakvalue.

Wanttheperiod?Pickapointonthewaveform(herepointC,whichisazerocrossingwithapositiveslope)andacorrespondingpointD.Measurethetimebetweenthetwoandyouhavetheperiod.Calculateaverage?CalculateRMS?Thescopehasthedataindigitalform;thecalculationsarefairlysimplesinceyouareusingadigitalprocessor.Ofcourseyoudon’tpickthepointsanddothemath;thesoftwaredoesitforyou.

Wherewaveformstorageusedtobeanexpensiveoption,itisnowthenormwithanumberofindirectfeatures—calculationsandnumericalmeasurements—readilyavailable.

Chapter4

ProbesandAccessories

Supposeitisabeautifuldayoutside.Youlookthroughthewindow—thesunisshining,thegrassisgreen,theairisclearandyoucanseesomesmallbirdspeckingaroundonthegrass.Nowchangethingsabit.Leavetheoutsideasitwas,butmakethewindowdirty.Ithasbeensplashedbymud,coatedbygreasyfumesand stickydust, and it casts anunevengrayover the scene.Nowyou lookout and suddenly thebeautifuldaylooksdrearyandunappealing.Whatyouseeisnotwhatisthere.

Thesamesituationoccurswithanoscilloscope.Theprobeorprobesconnectthesignalyouwanttoseewith theoscilloscope inputcircuits.Theprobecanhaveamajoreffectonwhatyousee,yetmanypeopletakeitforgranted.

DoYouReallyNeedaProbe?Inanidealworld,youcouldfindaprobethatyoucouldignore.Suchaprobewouldbeeasytouse—

justconnect it toyourcircuit. Itwouldnotdistort thesignal inanyway, itwouldlookto thecircuitasthough nothingwere connected (does not load down the circuit), and random noise pulses from othercircuitelementswouldberejectediftherewereany!

Ofcoursethisisnotanidealworld,andaperfectprobedoesnotexist.Toseewhatdoesexist,firstwehavetorememberthatwhenyouarelookingatasignalatonefrequencyithascomponents—wewillseeshortlywhat ismeantbycomponents—atmanyfrequencies.Remember: ifyouare lookingata7MHzsquareorsawtoothwaveform,thisisthesame7MHzastheRFcomingoutofyourtransmitterwhenyouareoperatingon40meters.Youwouldnotexpecttoruna40metersignaldownasingle12or18inch longpieceofwirewithouthavingsomesortofproblem.Ataminimumyouwoulduseapieceofcoaxialcableorspecialshieldedwire—andthisiswheretheproblembegins.

Thereareafewverylimitedcaseswhereashort—say6inchlong—pieceofwirecouldbeused,suchas the four logicsignal inputson theOsciumiPadoscilloscopedescribed inChapter3.This isavery special casewhere the signalsare limited to relatively low frequencies (that scopehasa5MHzbandwidth).Actualsignalshapeislessimportantandlogicsignalpositionistheimportantitem.

WhatDoesaProbeLookLikefromtheSignal’sPointofView?Let’ssupposeyouhaveascopewitha100MHzbandwidthandtheprobeconsistsofapieceofcoax,

withthecenterofthecoaxconnectedtothesignalyouwishtosee.Figure4.1isamodelofapieceofcoax.Noticethatithasinductance,resistance,andcapacitance—thatis,itlookslikeanetworkmadeupoftheseelements.Ifyouarelookingata6meter(50MHz)signal,theinputsinewavetotheprobewouldshowupattheoutput—thatis,atthescopeinputjack.Therewouldbesomesmallloses,butasinewaveinwouldbeasinewaveout.Unfortunately,ifyouwerelookingatasquarewave,sawtooth,triangularoranyotherrepetitive,non-sinusoidalsignal,theoutputatthescopewouldnotlookexactlyastheinputto

theprobe.Inordertounderstandwherethisdifferencecomesfromwehavetolookatalittlemath—atechnique

knownasFourieranalysis.Insummary,thismathprinciplesaysthatforanyrepetitivewaveform(yesitholdsforallwaveforms)thewaveshapeisactuallycomposedofasetofoverlappingsinewaves.Figure4.2showshowthishappens,usingasymmetricalsquarewaveforillustration.

InFigure4.2Aasinewaveofthesamefrequencyasthesquarewaveisdrawnoverthesquarewave.InFigure4.2Basinewaveofthreetimesthefrequencyofthesquarewave,withaspecificamplitudeoflessthantheoriginalsinewave,isaddedtothefirstsinewave.Youcannowseehowthecombination,orsum,starts to look like thesquarewave. InFigure4.2CandFigure4.2Dsinewavesof five timesandseven times theoriginal frequency, eachwitha specific calculatedamplitude, are added.The result isalmostacompletesquarewave.

Toreallysynthesizeagoodsquarewave,moresinewaves—eachwithadifferentamplitudeatoddmultiplesofthebasefrequency—wouldbeaddeduntiltheresultwasaperfectsquarewave.Nowlet’slookata7MHzsquarewave.Itwouldconsistofsinewavesat7,21,35,49(andsoon)MHz—sinewavesatoddmultiplesof7MHz,eachwithitsownamplitude.

Looking back at Figure 4.1, the connecting coax has inductance, capacitance, and even someresistance.Eachof these inputwaveswouldhaveadifferentattenuationandphaseshift.So theoutputresultingfromthesimultaneousinputsinewavesat7,21,35MHz,andsoonwouldnolongeradduptothenicesquarewavewestartedwithattheinput.Forthisreasonanoscilloscopeprobegetsabitmoreinvolved.Figure4.3showsastandard,inexpensivescopeprobe.Thesolutiontotheproblemofunequalprobe response tovarying frequency is calledcompensation. In the photo notice the screwdriver slot,locatedjustbelowthegroundwireconnection.Thisisthecompensationadjustment.

ProbeCompensationEachprobemanufacturerincludesacompensationnetworktoequalizetheproberesponsetovarious

frequencies.Averyminimalnetwork is shown inFigure4.4.R1 andC1—whereC1 is variable—provide thecompensation for thecombination thatconsistsof thecoaxor shieldedwireand the scopeinput circuit. The scope input acts as the termination of this combination, with resistor values in themegohmregionandcapacitanceofperhaps5to25pF.

Asecondresistorandperhapsothercomponentswouldbeaddedtotheprobeforthe×10attenuationselection. This is usually controlled by a slide switch such as the one just below the compensationadjustmentinFigure4.3.Thistypeofprobeisgenerallyconsideredavoltageprobesinceit isusedtoexamine voltagewaveforms. The compensation network shown is veryminimal; some voltage probeshavemuchmorecomplicatedcompensationschemes.

Theresultofconnectingaprobetoagood-qualitysquarewaveisshowninFigure4.5.Well-adjustedcompensation is shown inFigure4.5A,andmisadjustedcompensation inFigure4.5Band4.5C.Thesefigureswereobtainedbysimplyrotatingthecompensationcontrolandcapturingtheresultingscopetrace.

ProbeTypesandCapabilitiesProbescomeintwogeneralvarieties—activeandpassive.Mostpeoplearefamiliarwithpassive

probes,suchasthosediscussedintheprecedingparagraphs.Theyhavetheadvantageofgenerallybeinginexpensiveandtheconnector,mostcommonlyaBNC,fitsmanyifnotmostoscilloscopes.

PassiveProbesPassiveprobesarewidelyused.For themostpart theyaregenerally interchangeableandrelatively

inexpensive.Theydocomeinseveraldifferenttypes.Allprobeshaveratingsforvoltagemaximumandfrequencyresponse.Often thefrequencyresponse(usuallycalledbandwidth) ismarkeddirectlyon theprobe.Thismayvaryfrom10MHztotypically100MHz.

Thegroundwireispartofthemeasuringcircuit,soatfrequencieswheretheinductanceofa6-inchpiece of wire could be significant — say 5, 10, or 20 MHz and up— it is possible that that thisinductancewillcauseringingorsmalldampedoscillationsonthewaveform.

Therearetwogeneraltypesofprobetips.AhookendisshowninFigure4.6.Byslidingbackthering(just behind the groundwire connection), the hook is exposed. Releasing the ring allows the hook toretract,trappingawireortestpointbetweenthehookandtheplasticbody.Otherdesignsuseopposingwirehooksinascissorsconfiguration,buttheyaregenerallymorefragile.

Theothertypeofprobeendisusuallyinsidethehooksleeve.Thisplasticsleeveslidesforwardandofforunscrews,exposingapointedprobeend(Figure4.7).Generallytheonlycontrolsonthesepassiveprobesarethecompensationadjustmentandthe×1or×10attenuationselection.

ActiveProbesActiveprobes areusedwherevery low loadingon thecircuit isnecessary.Quiteoften theyhavea

fieldeffecttransistor(FET)connectedtotheprobetip,whichmeansthetiphasaveryhighresistanceand

lowcapacitance.Theyare,ofcourse,moreexpensiveandusuallymateonlywithspecificoscilloscopesfor which they were designed. Active probes typically have an additional advantage of includingautomatic calibration. Since an amplifier is located in the tip, quite often this sort of probe has anextendedbandwidthofseveralhundredmegahertz.

HighVoltageProbesHighvoltageprobesareusedwhenthenormalvoltagelimitsforapassiveprobearetoolowforthe

circuit under observation.Most passive probes used to be rated at 400 to 500V (think vacuum tubetransmitters),buttodayitisnotuncommontoseeaproberatedatonly200Vorevenlower,matchedtosolidstatecircuits.Thekey featureofahighvoltageprobe is safety—theprobebody isdesigned tokeepyourhandremovedfromthehighvoltage.

Occasionally theprobeconnectingcable ismademuch longer, for thesamereason—tokeepyourhandoutofthecabinetorchassiscontainingthehighvoltagecircuits.Mosthighvoltageprobesmatchaspecificmanufacturer’soscilloscopemodelandtheygenerallyarenotinterchangeable.

CurrentProbesCurrentprobesarealsogenerallymatchedtoaspecificmanufacture’soscilloscopemodelormodels.

The come in two types — ac (alternating current) only and dc (direct current). In both cases thebandwidthtendstobelimited.

AC-OnlyProbesTheac-onlyprobe is simply a transformer. InFigure4.8 the two sections shown are basically the

transformercore,withthefixedprimarywoundaroundtheuppersection.Therectangularslotiswherethe wire carrying the current is placed. Squeeze the handle and the two sections open in a scissorsmechanism.Insertthewireandthetransformernowconsistsofthefixedsecondarywindingandthewire(primary)ofthetransformer.

Almostanywirethatfitsintheprobejawscanbesensed,sothedistanceoftheconductorfromthemetalcorevaries.Thereforetheamplitudeofthecurrentseenonthescopeisnotveryaccurate,butwithintheprobebandwidthlimitationthewaveformisaccurate.

Oftenanamplifierisincludedintheprobehandlesotheconnectingcablecarriessignalinformationtothescopeandpowerfortheamplifiertotheprobe,alongwithcontrolsignalstotheprobeamplifier.

DCProbesAdcprobeusuallyusesaHallEffectsensor.Thisisasolid-statedevicethatdetectsamagneticfield

and produces a (generally)minute output voltage in response to the field.Figure4.9 is a generalizedcurveofaHallEffectdevice.Asyoucanseeithasalimitedrange,sothemagneticfieldfromthecurrentbeingsensedmustfallinthisrangeorthesensorwillgointosaturation,distortingthedisplayedcurrentwaveform.

AblockdiagramofthedccurrentsensorisinFigure4.10.Theelectronicsarebuilt intotheprobe,which resembles the ac-only current probe physically but uses the Hall Effect sensor instead of atransformerwinding.Again,theamplitudeofthecurrentsensedmaynotbeveryaccurate,duetovaryingwirepositioning,butthewaveshapeasseenonthescopeisaccurateaslongasthecurrentprobeisbeingusedwithinitsamplitudeandfrequencylimits.

OneMoreProbeTypeforHamsYears ago, when amplitude modulation was the most popular voice mode, amateurs built an RF

detector as an oscilloscope probe add-on. Shown in Figure 4.11, this is nothing more than a diode

detectorthatrecoverstheamplitudemodulationofatransmitter.Thisdiodegenerallyisgermanium—the1N34 being themost popular— since germanium diodes have a lower junction voltage than silicon.TodaysuchaprobeisstillusefulforanAMsignal,butnotveryusefulforSSB.HoweveritisveryhandytoseethekeyingenvelopeofaCWtransmitter.

Chapter5

ScopeSectionsinDetail

Years ago, if you wanted to apply for a US patent, you had to supply a drawing of the physicalmechanismofyourdevice.Infact,inthe19thcenturythepatentofficemightevenrequireyoutobringinaworkingmodel.As industrymovedmoreandmore into thedigitalworld, toagoodextentmoreandmoreinventionswerebasedonmechanismsthatdidnotexistphysically,butweremadeupofsoftwareprograms.

At first theUSpatentofficewouldsay“noway.”Butas their requirementsand theirunderstandingincreased,asdidthedigitalabilitiesofpatentapplicants,thelawsandruleschanged.TheofficestartedtograntUSpatentsondevicesthatwereatleastinpartconstructedinsoftware.

Wehaveasimilarsituationhere.Toexaminethecriticalsectionsofoscilloscopes,weusenamesofthesectionsthatrelatetohardware-onlyconstruction.Butjustabouteverysectioncanbeconstructedineitherhardware—withitsadvantagesandlimits—orinsoftwarewithcorrespondingadvantagesandlimits,orboth.

Asthischaptergoesthroughthescopesections,itwillgenerallydiscussahardwareversionofeachsectionandasoftwareversion.Hardwarescopesarestillbeingsoldinlargenumbers,especiallywherereasonablygoodperformanceisdesiredatalowercost.Bothhardwareandsoftwareimplementations—andcombinationsofboth—haveadvantagesanddisadvantages.

FunctionalBlockDiagramUsually, for a piece of electronic equipment, the block diagram thatmost people are familiarwith

shows how the various hardware blocks are interconnected. The functional block diagram of anoscilloscope inFigure5.1 shows how the various functions are interconnected.The diagram includesboth straight hardware-implemented oscilloscopes and the various configurations of digital signalprocessingbasedoscilloscopes.

Two input channels, labeledA andB, are shownon the left.On the right is a pictorial of a screenrepresenting both a straight cathode ray tube (CRT) display and a modern digitally driven flat paneldisplay.Theblockslabeled1and2 represent thepositionofeitheroneor twoanalog-to-digital (A/D)converters,asfoundinadigitalscope.TheadditionalA/Dconverter,labeled4,isdiscussedlaterinthischapter. The dotted section labeled MATH PROCESSOR is found in many digital scopes that use amicroprocessoror arePCbased.As longyouhave thecomputingpower, the ability todo simple andadvancedcalculationsandmeasurementsoninputwaveformscomesalmostfree!

TheotherboxshownindottedformrepresentsdataMEMORY.This isagainanalmost free function,availablewithdigitalprocessing.Atthebottomofthefigure,theconceptorideaoftheSYNCCIRCUITSblock remains the same for old and new analog oscilloscopes and digital oscilloscopes, but theHORIZONTALPROCESSOR/SWEEPCIRCUITSblockisverymuchdifferentbetweenthetwotechnologies.

VerticalFunctionsChapter 6 will discuss in detail the various input modes available in many oscilloscopes. The

correspondingfunctionsareshowninFigure5.2.Inastrictlyanalogoscilloscope,thetwomainblocksare a set of very precise — the degree of precision depending on the cost of the scope — analogamplifiers.Theyaredesignedtokeepanexactamplificationfactorandhavelowdcdrift.

This inputmodecontrolhas fourchoices—A,B,ALTandCHOPPED.DetailsofALT (alternate) andCHOPPEDarediscussedinChapter8.Belowthecontrolblockarepairsofcontrols,oneforeachinputchannel. POSITION moves the trace up and down and VERTICAL SCALE sets the gain — 10 mV/cm(millivoltspercentimeter),0.1V/cm,10V/cmandsoon.Butafterthesethereisavariablecalibrationcontrol(VERTCALCALIBRATE)thatallowsyoutosetascaleinbetweenthefixedvalues.Oftenthiscausesaproblem.Ifthiscontrolisnotturnedtooneend—itsCALIBRATEposition—theverticalscaleisnotcorrect.Toaddfurtherflexibility,theX10controlchangesthescalebyafactorof10—usuallyusedinconjunctionwiththeX10switchontheprobes.

Inadigitallyimplementedscope,thefirstblockisasetofanalogamplifiers,feedingeitheroneortwoA/Dconverters(blocks1and2).Withadualchannelscopeyouwouldexpecttoseetwoconverters,butif the scope bandwidth is low enough or the A/D converters fast enough (see Chapter 8 on scopespecifications) just one A/D may be shared, alternately converting channels A and B. After thisconversiontheverticalprocessingisstrictlydigital,andtheremaybeafeedbackloop(shownhereas3)tostabilizetheanalogamplifiers.

Ontherightofthefigureistheoutputtothevideoprocessorordisplaysystem.Thesecondfunctionaloutput (TOMATHPROCESSOR) represents theoutput to thevariousmeasurementandmath functions thatmaybedonebeforeoronthedisplayedwaveforms.

HorizontalProcessorandSweepCircuitsTheclassicoscilloscopeexplanation,datingbacktotheoriginalscopesinthe1930s,hadadiagram

forthehorizontalsectionconsistingofasweepwaveformsuchasthatshowninFigure5.3.Avoltageisapplied to thehorizontalplatesof aCRT,andby increasing thevoltage the traceon the screenmovesfromlefttoright.Attheendofthetracetheinputsignaltotheverticalsectionisblanked,andtheshortsectionofthewaveform,labeledRETRACE,bringsthetraceonthescreenbacktotheleftside.

Whetherdigitaloranalog, today’sscopes lookfunctionally like theblockdiagraminFigure5.4.Amultipositionswitchsetsthesweeprate(1µs/cm,10µs/cm,5ms/cmasexamples).Justasintheverticalsection,acalibrationcontrolmaybeusedtosetsweepratevaluesinbetweenthefixedsweepsettings.Butonceagain,whenacalibrationcontrol ispartofthescope, ithastobeset tothecalibratedsetting(usuallymarkedatoneendofthecontrol)forthefixedvaluestohold.Inaddition,forconvenience,oftenaswitchlabeledX5(times5)orX10(times10)issupplied.

Two inputs are shown from the SYNC section. The NORMAL input usually triggers— starts— thesweep.Morecapablescopesprovideadelayedsweep.AsshowninFigure5.5,abugappearson thescreen,intensifyingthevideoatthepointyoupickpastthestartofthedisplayedwaveform.Forexample,supposeyouwanttogetagoodlookatthefalltimeofyourMorsecodekeyerwaveformwhensendingastringofdots.

Youwouldsetthemainsweepatperhaps100ms/cmtoshowatleastonecompletedot,boththeriseand fall of the voltage.Nowyouwouldmove the bug to the falling edge (Figure5.6), select a fastersweep rate (perhaps 10µs/cm) for the delayed sweep, and put the delay sweep on.At this point youwouldseethefallingedge,atthefastersweeprateyouselected,thusallowingexaminationofthisedgeinmoredetail.

Some scopes provide an additional input to the HORIZONTAL PROCESSOR. There are severalmeasurementswhentwovoltagesarecompared(seeChapter7)againsteachother.OnevoltageinputissenttotheverticalaxisthroughChannelAorChannelBandtheotherintothehorizontalaxisthroughtheY-INPUT.Inthiscasenosweepvoltageisused.ThisY-INPUTgoesdirectlyintothehorizontalsectionofananalogscopebutmustgothroughanadditionalA/Dconverter(block4)inadigitalscope.

Mostoftheprecedingexplanationappearstoapplyonlytoanalogoscilloscopes,butitalsoappliestodigitaloscilloscopes.AsyouwillseeinafollowingsectionontheVIDEOPROCESSORANDDRIVER,thevoltagevalues sent to the vertical section are readout ofmemory starting at thememory location thatcorrespondstothetimeofthesweepstart,andreadoutataratecorrespondingtothesweeprateselected.Instead of controlling analog circuits, the controls described in Figure 5.4 are actually softwarecommandstosetthespeedsandfunctions.

SyncCircuitsPerhapsthemostimportantpartsofanyoscilloscopearethesynchronizationcircuits.Nomatterhow

highthebandwidth,nomatterhowgoodthedisplayresolution,nomatterhowaccuratelyawaveformispresented—ifyoucannotgetanstable,repeatabledisplayyoucannotseethewaveformundertest.TheSYNCCIRCUITS(Figure5.7)andcontrolsmake thepracticaldifferencebetweenbeingable toseewhatyouwanttoseeornot.

Theverticalsignals(ChannelsAandB)aresenttothesyncprocessor.Inananalogsystem,thisblockisasetofcomparators, triggercircuits, timedelaycircuits,andpulsegenerators.Inadigitalscopethesamefunctioniscarriedoutbyexaminingthestoreddata—inotherwordstheincomingwaveformhasbeenstoredinadigitalmemory,andthevaluesprocessed.

Whether the controls shown are actually physical switches and variable resistors or softwarecommands, theireffect is thesame.The input to theSYNCCIRCUITScomesfromtheverticalprocessingchannel.Aselectionismade—ChannelA,ChannelBoranexternalsignalthroughaseparateconnector.InadigitalsystemthisexternalconnectorwouldgotoaSchmidtTriggercircuitorothercircuittoresultinasquared-offdigitalpulse.

TheoperationoftheSYNCVOLTAGEandSYNCSLOPEcontrolsisillustratedinFigure5.8.Thevariouscombinationsshownare:

A—positivevoltage(valueselectable),positive(rising)slopeB—positivevoltage,negative(falling)slopeC—ZerocrossingvoltageD—negativevoltage,negativeslopeE—Negativevoltage,positiveslope.

Sincethevoltageselectionisvariable,thissetofcontrolsusuallyallowsyoutosetthesweeptriggerpointtoanypointontheincomingwaveform.

Theremainingcontrols—SYNCDELAYandDELAYTIME—werediscussed in theprecedingsection.ThelocationofthebugissetbytheDELAYTIMEcontrolandexpandingtothedelaypoint(turningitonandoff)iscontrolledbytheSYNCDELAYswitch.

Therearetwooutputs.TheNORMALoutputprovidesthestartpointintimeforthesweep,andDELAYoutputpositionsthebug.Whendelaysweepisturnedon,thedelayoutputprovidesthesweepstarttime.Keepinmindthatthisdescriptionisofthefunctions;howthehardwareinanyonescopeactuallydoesthisvarieswiththescopedesign.

VideoProcessorandDriverThereisaconsiderabledifferencebetweenthehardwareandoperationoftheVIDEOPROCESSORor

DRIVER in theclassicalanalogoscilloscopeand thehardwareandoperation inadigital scope.Analogscopes are still readily available and are often selected for the simplicity, lower cost and widerbandwidthforthecost.AversionoftheclassicalanalogvideosectionanddisplayissketchedinFigure5.9.ThevideofromtheVERTICALPROCESSINGblocksimplygoestoavideoamplifier,onethatprovidessymmetry—theability todrivebothpositiveandnegativewithrespect toareferencesuchasground.Thisamplifierisconnectedtotheverticalplatesandthusprovidestheverticaldeflectiononthescreen.

The HORIZONTAL PROCESSOR/SWEEP CIRCUITS also requires a symmetrical amplifier, feeding thisdirect analog sweep voltage shown in Figure 5.9. This signalmoves the beamhorizontally across thescreen.Theothersignalisablankingpulse,forthedurationoftheretracetimeinFigure5.3.Duringthisperiod,asthetracemovesbacktotheleftsideofthescreen,theblankingpulseputsabiasonthegridshowninFigure5.9thatcutsoffthevideo—thusnoretraceisseenonthescreen.

Digitaloscilloscopes,althoughfunctionallyverymuchidentical,operatecompletelydifferentlyonahardwarebasis.Therearetwogeneralconfigurations—aself-containeddigitalscope,andonethatplugsintoandusesapersonalcomputerorotherdigitaldevicesuchasaniPadforcalculationsanddisplayofthetraces.

Figure5.10representsthisfunctionalconfiguration.Itisbasedonstoringtheincomingwaveformsindigitalmemoryaftertheyhavebeenconvertedfromanalogvoltagestodigitalwordsintheverticalinputsystem. How data is stored inmemory to be displayed is very dependent on the particular hardwaredevice.InWindowsPCs,videomemorycanbeapartofmainmemoryoritcanbeaseparate,highspeedmemory bank on the video card.Apple products have their own techniques, as doAndroid and otherdigitaldevices.HoweverthewaveformstorageconceptcanbeunderstoodbylookingatFigure5.11.

At the top isananalogwaveform thathasbeenconverted toadigitalnumberby the frontendA/Dconverters. Each of these converted numbers become a digitalword, and as seen at the bottomof thefigureeachcorrespondingstoredvaluegoesintoamemorylocation.Allmemorysystemshaveanaddressforeachpartofmemory,andinthefigureasetofaddressstartingataddress230throughaddress280isseentoholdthestoredvaluesforonechannel.

This is a very simple linear system, and various techniques for video compression and memorylocation selectionareused to speedupvideodisplay.Whatever the real storage techniqueused, thesevalues can be commanded to set a vertical position on the display screen as the horizontal processorprovidesahorizontalposition—thehorizontalprocessorprovidestheequivalentofasweep.Call thesequenceofvaluesquicklyoutofmemoryandyouhaveafastsweeprate—say10µs/cm.Moreslowlyandyouhaveaslowerrate,say100ms/cm.

Thus the storedvalueprovides theverticalpositionon the screenand rateandcall-out speed frommemoryprovidesthehorizontaltimingandposition.InFigure5.10,MEMORYisshowninarectangle—it

isactuallyintegraltothevideoprocessor.

WhatAboutMath?Earlierinthischaptertherewasastatementthatinadigitalscope,mathcalculationscomealmostfree

— that is, no additional hardware is required. Looking at Figure 5.11 you can see how the MATHPROCESSOR,alsoshowninFigure5.10,canbeusedtofindthepeakvalueorminimumvalue,ortoselectallpointsandcalculateanaverage—allbecausethedataexistsinmemory.IftheChannelAwaveformisstoredinmemorylocations230to280,andthechannelBwaveformisstoredinmemorylocations330to380, forming the function A+B now requires only a software command to add the value in memorylocation230 to thevalue in location330, thevalue in240 to thevalue in340,andsoon.Since largeamountsofincomingwaveformdatacanbestored,muchmorecomplexmathfunctions,suchasspectrumanalysis,canbedoneinsoftwarewiththeresultsdisplayedonthescreen.

DoestheDisplayTypeMatter?Plasma,LEDbacklight,thinfilm…ahostofdisplaytypesareavailable.Howeverthedisplaynow

hastobematchedtotheprocessorandnottotheoscilloscopefunctions.Thismeansthatthevideocard(or integralvideosection), forexampleonaPC, ismatched to thedisplay. Ifwhat isoften termed thenativeresolutionselected,theparticulardisplayusedwillnotaffecttheoscilloscopefunction.

Thereare,ofcourse,severalpossibleexceptions.Ifadigitaloscilloscopefrontendismatedwithanolder personal computer using640×480 resolution, your resultsmaynot be all they canbe.Anotherpossible exception is the set ofminiature oscilloscopes thatmate to smartphones or tablets. Here theproblemisnotsomuchdisplayresolution—youwon’tseethedifference—butthescreensize,evenwitha8or10inchtablet,mayrestrictwhatyoucansee.

StorageMostofthetimetheabilitytostoreawaveformonthescreenisnotofprimaryimportance.Thereis

one case, however, where it becomes very important. In general an oscilloscope is used to look atrepetitivewaveforms. For a digital oscilloscope,memory is built in. Looking again at Figure 5.11, ifthesememory locations represent a full screenof information,whenmemory location280 is filled theentiresetisnormally(andveryquickly)clearedandthenewincomingwaveformrefillslocations230to280.Theprocessrepeatsoverandover,andstorageofawaveformisnotimportant.

Thereisoneveryimportantcasewherestorageisrequired—whenyouwanttoexamineantransienteffectthatonlyhappensonce.Thenyouwouldliketofillup230thought280andfreeze—holdonto—theresult.Again,thisabilitycomesfreewithdigitalimplementation.

Therewereandstillareanalogscopeswithstoragecapability.TherethestorageisnotinthecircuitsbutintheCRT.Byaddingameshorscreenjustbehindthephosphorlayeronthefront totheCRTandsomeuniqueelectronflooding,theactualstorageisaccomplishedinthephosphorlayeronthescreen.Toseeanexampleofthistechnique,setyourInternetsearchenginetotypotronorSAGESystemDisplayandyoucanseethedetailsofoneapplication.Theseanalogstoragescopeswere,ofcourse,quiteexpensiveandveryrareoutsideofindustriallaboratoriesormilitaryhardware.

No-HardwareScopesSincesomedigitaloscilloscopesplug intoaPC,anduse thePCforall theprocessinganddisplay,

whybotherwiththedigitalfrontend?Youalreadyhaveaninputportforaudioonyoursoundcard;whynotusethisasthescopeinput?IfyousearchtheInternetyoucanfindseveralsuchsoftwarepackages—andforthemostparttheydowork.Butthereareseveraldisadvantages.

Firstofallyourinputisrestrictedtoaudiofrequencies—perhaps10Hzto20kHz.Next,youhavetoprotectyoursoundcard.ProtectionisdiscussedinChapter7ofthisbookandatypicalprotectioncircuitshown.Butaveryimportantrestrictiononthesesoftware-onlyscopesisdistortion.

Ifyouarelookingataplainsinewave,anditiswithinthebandwidthoftheaudiocard,yourresultswill be reasonable. If, however, you are looking at a squarewave, triangularwave or any repetitivewaveformotherthanasinewave,therewillprobablybeconsiderabledistortion.

Withoutgettingdeeplyintothemathtoexplainthis,atechniqueknownasFourieranalysisshowsthateveryrepetitivewaveformconsistsofasetofsinewaves,eachatamultipleofthewaveformfrequencyandwithacertainamplitude.Asanexample,supposeyouwanttolookataperfectsquarewaveat5kHz.Thissquarewaveisactuallymadeupoftheadditionofsinewavesat5kHz,15kHz,25kHzandotheroddmultiplesof5kHz—eachwithapreciseamplitude.

Since theno-hardware scopedoesnot showsinewavesabove20or25kHzaccurately, insteadofseeingaperfect5kHzsquarewaveyoucouldseeaverydistorted5kHzwaveformwithroundedtopandslopingsidesinsteadofthenice,rectangularsquarewaveyouexpect.No-hardwarescopesdohavetheirplaceinaudio-onlyapplications.Theyareinexpensive(oftenimplementedinfreesoftware)andfun—butdon’texpecttouseoninplaceofahardware-basedscope.

Chapter6

InputModes

What you can see on your oscilloscope is controlled to a large extent by what input modes areavailable,andtheinputmodesinturnarecontrolledby—asyoumightguess—bothcircuitdesignandthe oscilloscope controls available. In addition, the real-life design considerations that went into thescopeprovidefurthercapabilitiesandlimitations.

Inthischapter,toillustratevariouspointswewillusepicturesofvintageTektronixoscilloscopeinputmodules.This seriesof scopes consistedof abaseunitwith replaceable, often singlepurposeplug-inunitsforthefrontendofthescope.Wewillalsolookattoday’smoderndesignsthathavecombinedboththecapabilitiesandcontrolsoftheolderscopes—anobjectivewhichjustwasnotreachablebackinthevacuumtubedays.

As an example, suppose you need a single, very high gain input capability with very precisecalibration.Anaccurate,highgaincapabilityrequiresaccurateandlonglastingcalibration.Inaddition,extrabandwidthreductionmayberequiredtokeepsystemnoisefromcorruptingthewaveformyouwishtosee.Therefore, it isnotjustamatterofdesigningahighgaininputamplifier.Itmustbestable,haveaccurate and lasting calibration, and exhibit a degree of noise immunity. From this point of view, thedesignofanoscilloscopefrontendisnotverymuchdifferentfromthedesignofagoodpreampforyourAmateurRadioreceiver—lownoise,properbandwidth,stableamplificationandsoon.

AnumberofamateursdiscoveredthissimilaritywithlatermodelHeathkitoscilloscopes.Priortothedemiseofthecompany,Heathofferedasetofsolid-stateoscilloscopeswithbandwidthsrangingfrom5MHz to 35 MHz (and possibly higher with some models.) Hams who bought and built these kitsdiscoveredtheywerebuildingcircuits—andhavingtocalibratethem—justastheywouldanamateurreceiver.

InputModesAsmightbeexpected,inacompetitiveworld,differentmanufacturerspickandpublicizetheirscope

capabilities with names that often do not match those of their competitor’s capabilities for the samefunction.Here is a list of common inputmodes under generic—commonly accepted—names.Mostoftenwhentherearetwoavailableinputchannels,theyarereferredtoaschannelAandchannelB.

Ground(reference)AcCoupled(selectableindependentlyforeachchannel)DcCoupled(selectableindependentlyforeachchannel)InvertedDifferential(nogroundreference)HighGainFastRiseTime

DualTraceA/BAlternateA/BChoppedA+B(AplusB)A–B(AminusB)High-Z(highimpedance)Input50ΩInputWideBandDigitalInputsMulti-channelAudioOnlySomeoftheseinputmodesarefairlyobviousjustfromtheirtitles;otherrequireabitofexplanationas

tobenefitsandlimits.

CommonInputSelectionsIndependentoftheoscilloscopetype—hardwareonly,hardwareandsoftware,andsoftwareonly—

therearealmostalwaysthreeinputselections.ThesearepicturedinFigure6.1,wheretheleverswitchon the lefthandside isused toselectac inputsonly,dc inputsonlyoragroundreference.Thegroundreferenceisusedtosetabaselineof0Vinput,sothateverythingseenafterthatcanbereferencedtothisvalue(specificallythislineasseenonthescreen.)Theothertwoselections,accoupledanddccoupled,areexactlywhattheirnamesimply.

Supposeyouarelookingatawaveformthatconsistsofadclevelof30V,ontopofwhichisridinga1Vsawtooth.ByfirstselectingGND(thegroundreference)andpositioningitatthebottomofthescreenyouknowwheretheverticalpositionof0Vis.Next,byselectingDC(dconly)position, thewaveformwillshowupinitsentirety,assumingyouhavetheverticalvoltagescalesethighenough.Howeverthe1Vsawtoothwillbeveryhardtomeasure.

NextwhenyouselecttheAC(accoupled)position,thedcleveldisappearsandyoucanreadjustthevoltagescaletobetterseeandmeasuretheamplitudeofthesawtooth.

These inputselectionsareset independentlyforeachinputchannel—that is, there isanother leverswitch(notshown)forthesecondchannel.Figure6.2showstheon-screencontrolsofascopeusingapersonalcomputer,withthecouplingcontrolsatthelowerleft.Theinputcontrolsareduplicatedforeachofthetwochannels.

SomeoscilloscopesalsoprovideanINVERTEDselectionthatallowsyoutodisplayasignalwithlowvoltageatthetopofthescreenandhighvoltageatthebottom.Thisisgenerallyavailablewhenthescopealsohassomesortofintegralmathfunction.

SingleChannelModesDepending on the intended use, age of the scope, and cost, some oscilloscopes provide specific

specializedfeaturesoneitherasinglechannelinputormultiplechannelinputs.Notallofthesefeaturesarecompatiblewitheachother.

Previouslywementionedthathighgainfrontendsmaynotbecompatiblewithasystemlookingforhigh frequencies. Where the best frequency response is needed, manufacturers may refer to thischaracteristicasWideBandorFastRiseTime.Figure6.3isthefront-endplug-inofaTektronixscopeofafewgenerationsago,wherethedesignwasoptimizedforfastrisetime.

AlthoughWideBandandFastRiseTimeare similar, theyareusedandmeasureddifferently.WideBand refers to the highest frequency that displays (generally) at least 70.7% of the correct amplitudevalueofasinewave.FastRiseTimeisimportantfordigitalmeasurementsonly,andfastisincomparisontotheriseandfalltimeofadigitalpulseyouwanttomeasure.

Today the specifications of a scope would tell you its characteristics in these areas assuming themanufacturerincludesitinafulllistingofcapabilities.

Whilemostscopesaredesignedtohaveminimumloadingonacircuit—thatisthepresenceofthescope does not affect the circuit performance— some scopes are specifically labeledHigh-Z Input.Normallytheinputofamoderatevaluescopewouldlooklikea1MΩresistorinparallelwith10pF.AHigh-ZInputwouldbeseveral10sofmegohms,buttheinputcapacitanceisstillgenerallyatleast5pF.UsuallytheHigh-ZInputrequiresaspecialprobe,matchedtothescopedesign.

Forworkontransmissionlines,whereanyimpedancemismatchmayaffectthemeasurement,therearespecializedscopewhoseinputslooklikeapure50Ωresistor.Thustheycanactasamatchedload(forverylowpowerlevels)onaportorsplitterattachedtotheline.

DualChannelInputsWhiletheearlyoscilloscopesdescribedinChapter1concentratedonallowingasinglesignaltobe

seen,scopecapabilityhasgrowntoinclude,forjustaboutallmodels,atwo-channelordual-traceinput.Again,usingaverypopularobsoleteTektronixplug-infrontendasanillustration,Figure6.4showstheidenticalcontrolsforthetwochannels.AfivepositionMODEswitchselectsthechannelorchannelstobedisplayed.

The first two positions allow you to see one channel only, either A or B. The third position,ALTERNATE,canbeverymisleading.Whatyouseedependsonthesweeptriggersection(asdescribedinChapter 5). If the sweep is triggered by one channel and that channel only, the horizontal position orrelativetimingofthesecondchannelmaynotbewhatyouobserve.

Theresultofmoderndual-channelinputsisillustrateinFigure6.5,whereasquarewaveisshowninchannelA, triggered by the squarewave,with a triangularwaveshape in the lower trace. Each traceprovidesitsowntrigger,soalthoughyouseebothinstablepositions,theyarenotnecessarilyoccurringintimerelativetoeachasshown.

Toensure that the twowaveformsaredisplayed incorrect timealignment, theCHOPPED function isoftenused.Hereeachwaveformissampledanddisplayed—onetinyincrementinproperpositionforchannelA,anincrementforB,backtoAandsoon.

Generally,aslongasthewaveformsareverymuchlowerinfrequencythanthechopped(orsampling)frequency,theswitchbackandforthisrelativelyinvisible.However,inallcases,whatyouseeisheavilydependentonwhatiseffectivelythesynchronizationofthesweepfunction.

ThefifthpositionontheMODEswitch,ADDED, isoneexampleofmathematicallycombining the twooutputs,inthiscaseasimpleaddition.

DualChannelwithArithmeticThesimplearithmetic function label shown inFigure6.4 (ADDED) impliesaddition.However,both

channelshavePOLARITY(NORMALorINVERTED)selectors,whichsimplyswitchininversionoftheeitherorbothwaveforms.Thisfunction,incombinationwiththeADDED(ALGEBRAICALLY)modepositionmeansthewaveformsAandBcanbecombinedinfourpossibleways:A+B,A–B,B–Aandfinally–A–B.

AnewerTektronixoscilloscope,showninFigure6.6,hasacontrollabeledMATHnearthecenterofthefigure.Hereanumberofcombinationsofsignal(andcalculations)maybeselected.

Oneofthemostvaluablemodesfordesignandtroubleshootingmodernelectronics,especiallywheredigitaldatalinesareused,isthedifferentialmode.Thearithmeticdescribedinthepreviousexamplehasan inherentcharacteristic.ForexampleA+Bis really (Aascompared toground)+(Bascompared toground).Whathappensifneithersignalisreferencedtoground,buttheyarereferencedtoeachother?Infact introducing the ground connectionmay shownoise and other extraneous signals that really do notexistonthelineundertest.

Figure6.7isablockdiagramofatypicalbalancedlinedriver—ithasfourindependentsections—A,B,C,andD.LookingatsectionAatthetopleft,adatawaveformisconnectedtopin1.Theoutputisbalancedandconnectedtopins2and3.Thisoutputisnotreferencedtoground,butasshownoneisthenominallylabeledpositivedataoutputlineandtheotheristhenegativeorreturn.

There are many such integrated circuits used with various configurations, but the commoncharacteristicisthelackofagroundreference.Attheotherendofthelineacomplementarydatareceiverhastwoinputsperchannel,andtheoutputofthecircuitisasingledatasteamforthechannel.

Inamodernpieceofelectronics,whereRFmaybefound,adesignermaychoosetosenddatafromoneprintedcircuitboardtoanotherusingadifferentialdriverandreceivertominimizeRFpickupwithrespecttoground.

AgainusinganoldTektronixscopeasanillustration,Figure6.8showsahighgaindifferentialfrontend.Tektronix actuallymade two such differential front ends, this one calledHIGHGAIN and a secondcalledWIDE-BAND.

SpecialModesWhenyouwanttolookatlogicsignals,adualchannelcapabilitymaynotbeenough.Thereareafew

manufacturersoffourchannelscopesandpossiblymodelswithevenmore.A newer approach is illustrated in Figure 6.9. As shown, the top two regular channels are not

connectedtoanysignals.Thisscopehasfourlogicsignalsonlyinputwires.Inthefigure,thefourlowertracesareinputsontheseconnections.Anyoneoftheseinputscanbeselectedasthesyncsource,whichgivesagooddegreeofflexibilityinchoosingwhattolookat.

One very specialized front end for a scope, which has no front end, is the software-only scopediscussed in Chapter 7. This scope uses your computer and audio card. As a result its bandwidth islimitedtotheaudiofrequencyrangeonly—perhapsafewtensofHzto20kHz.Severalversionsofthesoftware are available online,mostlywithout charge.Although a direct connection to a sound card ispossible,mostusersfollowtheonlinedirections tobuildasmallprotectioncircuit for thesoundcard.OnesuchcircuitisincludedinChapter7.

Chapter7

Let’sPutaScopetoWork

Oscilloscopes are generally used for one of three types of measurements: amplitude (voltage),duration or time (frequency), and calculations. Most older scopes did not have the ability to docalculations,butasdiscussedinprecedingchapters,aslongasyouhaveinformationindigitalformandare using a digital processor (in otherwords a small computer), the oscilloscope can do a variety ofcalculations.

AmplitudeMeasurementsWhenmostpeoplethinkofusinganoscilloscope,theyenvisionascreenwithaplotoftheamplitude

orvoltageofasignalontheverticalaxisofascreen,alongwithtimeonthehorizontalaxis.Tomakethisprimaryplotonthedisplay,thereareseveralstepsyouhavetotake.Itisverypossibletooverlookoneofthesesteps,inwhichcasethedisplayedwaveformmaymisleadyouandnotrepresentthetruepictureofthesignal.

First pick a voltage scale, and if there is a variable scale control, make sure the voltage scaleselectionswitchisinthecalibratedposition.LookingatFigure7.1youcanseescaleselectionswitchesfor channel 1 and channel 2. Notice that the center knob (labeled VAR) for channel 1 is in the fullclockwiseposition,correspondingtothecalibrationpositiononthepanel.TheVARknobforchannel2isnot in thesameposition,soasa result thevoltagescaleselectedforchannel1 iscorrectbut thescaleselectedforchannel2isincorrect.Itisveryeasyandverycommontooverlookcheckingthissetting.

Checkyour probe compensation.Compensation, as noted in a prior chapter, only holds for theX10settingontheprobeswitch.IfyouarelookingprimarilyatsinewavesandRF,itisdoubtfulthat(exceptforextrememaladjustmentof thecompensationscrew) thisadjustmentwillmakemuchofadifference.Digitalsignalsareadifferentstory—justlookatFigure7.2!Withcompensationmaladjustment,squarewaves,pulses,sawtoothandotherlinearwaveformsmaybeshownfarfromtheirtruewaveshape.

Setupazeroreference.MostscopeshaveaninputsettingmarkedGROUNDorGND; it isdifficult tomakeameasurementifyoudonothaveareference.

Nowyouareinapositiontomakeavoltagemeasurement.Countthenumberofcentimeters(cm)ontheverticalscale.Youmayhave tomove thehorizontalpositioncontrol (if there isone)orchange thetriggerpointtogetagoodmeasurementonthesubdivisionsofthegraticule.Multiplythenumberofcmbythevoltagescaleandyouhaveyournumber—almost!

Nowgobackandlookatyourprobe.DoyouhaveitintheX10position?Ifso,multiplyyourpreviousreadingby10!

Awordofcaution:Itiseasytogetconfusedwhenyoucompareareadingfromavoltmeterwiththemeasurementyoujusttookonascopeandseethattheyaredifferentandassumethateitherthescopeorthevoltmeter is inerror.Remember thatmostvoltmetersmeasureRMS(root-mean-square)values,butyoumostprobablyjustmeasuredapeak-to-peakvalueonthescope.Togetanequivalentvalue,takethepeak-to-peaknumber,divideby2togetthepeakvalue,andthenmultiplythepeakvalueby0.707togetRMS.Thissimpleconversionholdsonlyforsinusoidalwaveforms.

Another thing to keep in mind when measuring waveforms near the upper limit of the scope’sbandwidthisthatthegenerallyacceptedfigureforbandwidthisthefrequencywherethemeasurementisoffby3dBinvoltage.Thatis,thewaveformmayshow3dBlowerthanitactuallyis.

AlignmentwithaSweepGeneratorOneverycommonvoltagemeasurementusingascopeisthealignmentofanIFstripinareceiveror

the measurement of an RF or audio filter. That requires a signal generator produces a signal at thefrequencyorfrequenciesofinterest,orasweepgenerator(asignalgeneratorthatsweepsitsoutputsignaloverarangeoffrequencies).

Asanexample,assumetheOBJECTUNDERTESTinFigure7.3isafilterwithacenterfrequencyof1MHz.Ifwewantedtoseethefilterresponsefrom900kHzto1.1MHz,thesweepgeneratorwouldbesetup for its output frequency to vary from the lower 900 kHz limit to the upper 1.1MHz limit. As thefrequency changes, the sweep output voltage also changes, so every point on the horizontal axiscorrespondstoafrequencyinthesweptrange.TheRFvoltagegoesintothefilterandtheoutputofthefiltergoestotheverticalinputofthescope.

Thenetresultisaplotseenonthescopedisplaythatshowsthefiltercharacteristicsoverfrequency.SeeFigure7.4foranexamplefilterresponseplot.IftheactualtestwererunatRF,asmalldiodedetector(Figure7.5)wouldbeplacedinserieswiththeVERTICALINPUTsothattheactualverticalinputwouldbedc.Thedcvoltagevalueatanyfrequencywoulddependonthefiltercharacteristic.

AmplifierLinearityForcheckingthe“goodness”ofanaudioamplifier,itslinearitycanbeobservedbythesystemshown

inFigure7.6.TheinputtotheamplifierisconnectedtochannelAandtheoutputtochannelB.Theusualtestwouldusesinewaveinputs,butafurther,morerigoroustestwoulduseasquarewave,triangular,orsawtooth input.Thesinewavecouldbeatanyfrequencywithin thecapabilityof theamplifier,but thesquarewavefrequencyshouldbelimitedtoapproximately1/3theupperfrequencylimitoftheamplifier.

Althoughitiscustomaryjusttocomparetheinputandoutput“byeyeball,”amoreinterestingtestistoset thevoltage scaleonboth channels so that theybothgive the samevertical deflection.Then set thescopetotheA–B(AminusB)position.Onthescreen,ifthereisnophasedelaythroughtheamplifier(animpossibility) the two sinewaveswould subtractout, but in thepractical case,withnodistortion, thedisplaywouldshowthephaseshiftintheamplifier.

TransmittedAMwaveformFormany years the classic use of an oscilloscope in the ham shackwas to observe (and set) the

modulation level of anAM transmitter.Most oldARRLHandbooks and reference volumes devoted atleastonepagetotheoutputRFwaveformasseenonascopeundervariousconditions.Asanexample,Figure 7.7A shows a properly modulated AM transmitter and Figure 7.7B shows one withovermodulation.

Thesepicturesstillholdforsimple,olderAMmodulatedtransmitters.ManynewhamrigsincludeanAMcapability,withtheabilitytoapplyvariousdegreesofspeechprocessingandcompression.Often,asseenonanoscilloscope,thetransmittedsignalwilllookfine—suchasinFigure7.7A—butontheairreportswillbeverymixedandnegative,becauseitisoftenverydifficulttoseetheresultofprocessingin

thissimplescopetest.

To see theRFoutput froma transmitter, it is usually not necessary to build a sampling circuit thatphysicallyconnectstothesignalonthetransmissionline;usually—sincetheSWRonacoaxiallineisrarely1:1—youcansimplyeitherwindaloopofwirearoundtheantennafeedlineinthestationortapea 6 or 12 inch piece of wire along the feed line. Usually there will be sufficient pickup to see thetransmittedRFwaveform

SSBTestingObserving a single-sideband signal can also be interesting but misleading. A bit of a voice

conversationon40metersiscapturedinFigure7.8.AmoremeaningfulviewisFigure7.9,whichshowsawell-adjusted75meterSSBtransmittermodulatedbyasingletone.Forcomparison,Figure7.10showswhatthetransmitteroutputlookslikewithasingletonebutwithacarrierthatisnotfullysuppressed.Inotherwords,thetransmitterisnotputtingoutasingle-sidebandsignalbutasidebandwithsomecarrier.

Againawordof caution—manypictureshavebeenpublished that showwhat agoodSSBsignalshouldlooklike.However,justaswithanAMtransmitter,itisquitepossibletohaveabeautifulpicturebutgetverypooron-the-airqualityreportsduetomaladjustmentofthespeechprocessor.

AlsoseethesectiononmeasuringPEPoutputpowerlaterinthischapter.

CWTESTINGThereareseveralmeasurementsthatanoscilloscopecanmaketocheckthequalityofyourCWsignal.

The first, andmost basic, is to look at the amplitude of the transmitted dots and dashes.Do they stayconstantwithin theperiodof adotordash?While this seems likea fundamental requirement,didyoueverhearsomeonetransmittingasignalthat,ifanhonestreportweregiven,wouldwarrantatonevalue(the T in RST, 1 to 9) of less than T9? Older transmitters, especially with poorly regulated powersupplies,oftensufferedfromthisproblem.Ifthetransmittedwaveformwereexaminedonascopewhileaseriesofdotsordasheswerekeyed,youcouldeasilyseeachangeinamplitudeduringtheperiodofadotordash.

AgoodkeyingwaveformisshowninFigure7.11.Thetoptraceisthekeyclosureandthebottomthetransmittedwaveform.Notice thewaveformlags thekeyclosurebyafixedperiod; this isnormal.Thewaveform has a gradual build-up and decay. Compare this with Figure 7.12, where the transmittedwaveformishassharpedgeswhichresultinabroadsignal—inotherwordskeyclicksupanddowntheband.

Figure7.13showsanotherproblemthatcanbeseenwithanoscilloscope:shorteningofthefirstdotordashwhenusingbreak-in.Sometimesthiseffectinnotnoticeableandsometimesithasthereceivingoperator scratching his or her head and asking “Whatwas that first letter?”This effect is particularlynoticeablewithhigh-speedCWandfastbreak-in.

Toexaminethetransmittedwaveformcloselyatthebeginningandendofadotordashyouwillwanttosetupaveryfasthorizontalscopescanspeed.Thisrequiresyoutouseadelayedtriggerasexplainedin Chapter 5. The horizontal scan speed is set to show one or more dots or dashes and the bug,representing the position of the delayed sweep, or theSWEEP DELAY TIME,moved onto the leading ortrailingedgeofthedotordash.Thetimebaseforthedelayedsweepissettoahighspeedandthedelaysweepturnedon.Youwillthenbeabletoexaminetheleadingortrailingedgeofthedotordashingreatdetail.

DigitalSignalsWith the popularity of digital equipment, some oscilloscope manufacturers have added input

connections specifically to allow viewing of digital signals, and usually a binary number equivalentnumber(4,8,and16)ofthemsimultaneously.AsanexampletheOsciumscopeshowninFigure2.14inChapter2hasfourdigitalinputs.Theremaybe,dependingonthespecificscopedesign,advantagesanddisadvantagestotheseinputs.

Certainly having a number of simultaneous inputs (more than two) is an advantage. If the designconnects your signals directly into digital logic elements, without using an A/D (analog-to-digital)converter,higherspeedlogiccircuitsmaybemonitored.However,thisdirectconnectionmeansthattheactualwaveformisnotshown;justlogic1sand0sareonthescreen.

Apossiblefurtherlimitationofadirectdigitalconnectionistheallowableinputvoltagerange.Whilemostcommondigitalcircuitsoperatebetween0V(orperhapsminusafewvolts,downto–12V)andupto+5V(orpossiblyashighas+20V),theallowableinputvoltagerangeofdirectdigitalconnectionsareverymuch restricted as compared to the allowable voltage range using the usual dual-channel, probeconnectedinputs.

Theoscilloscopebandwidthcanalsoprovideamisleadingbut important limitation,when theusualChannel A/Channel B inputs are used. As discussed in Chapter 5, signals such as square waves andrepetitivepulsesaremadeupofsumsofsinewaves.Figure7.14Ashowsasquarewavesuperimposedonasinewaveofthesamefrequency.Figure7.14Bshowsthesamesquarewave,butthistimethesinewaveisaddedtoasecondsinewavethatisatthreetimesthefrequency.Thisthirdharmonicsinewaveamplitudehasbeenmultipliedbyanumberlessthan1.

Figure7.14Cshowstheresultiftheoriginalsinewave,asecondoneofthreetimesthefrequencyandathirdoneoffivetimesthefrequency.Finally,Figure7.14Dshowstheresultofaddingtheoriginal,threetimes, five timesand seven times theoriginal frequency—eachharmoniccomponenthasacalculatedmultiplier.Thusasyouaddmoreandmoreoddharmonicsinewaves,theresultlooksmoreandmorelikearectangularsquarewave.

Nowsupposeyouhaveasquarewaveinputat,forexample,10MHz.Ifyourscopebandwidthisonly40MHz, the thirdharmonicof the10MHzinput(30MHz)willget through,but thefifthharmonic(50MHz)andseventhharmonic(70MHz)componentswillbeattenuated.ThenthesquarewaveyoumightseecouldlookmoreliketheoneinFigure7.14B—thatisthesquarewavewouldberoundedattheriseandfall,andnotflatatthetopandbottom.

Fortunatelythesituationisnottotallylikethis,becauseascope’sbandwidthdoesnotfalloffacliff.A45MHzbandwidthmeanstheresponsetoa45MHzsinewavewouldbeabout3dBdowninvoltage,sosomeofthe5thharmonicenergywouldgetthrough.

Thenet result is this:Whileascopebandwidthspecification tellsyou justhowhigh in inputsignalfrequencyyoucangosufferingonlya3dBloss,thesamebandwidthisalimitationofthefidelityofthedigitalsignals;theymaynotbeseenwiththeirproperwaveshape!ThisphenomenonisdiscussedfurtherinChapter8,wherespecificationsforascope’sverticalchannelriseandfalltimearenoted.

Anotherconsiderationthatcanbeagreatadvantagewhenviewingdigitalsignalsistheabilitytoseeone-timeaction.Sometimes,forunanticipatedreasons,somethingwillhappenveryquickly,andyoureyewillnotcatchit.Heretheabilitytosetanoscilloscopeintoastoragemode,wherethetraceistriggeredjustonceandtheresultleftonthescreen,canbeinvaluable.Again,aspointedoutbefore,digitalstoragescopes retain the input data in memory, so that the ability to store one or more traces is built-in forexaminationatleisure.

RFDetectionAs mentioned in the preceding section, oscilloscope bandwidth is not a cliff. A bandwidth

specification such as 50MHzdoesnotmean that youwill seeno signal above this frequency. It doesmeanthatthisisthefrequency,commonlynotedas3dBdown,abovewhichwaveformswillnotbeseenwith their true amplitude. It is not unusual to be able to see signals at twice or three times the ratedbandwidth,buttheamplitudewilllikelynotbeaccurateabovetheratedfrequency.

Ifyouaretryingtodothis,remembertochecktheprobesettingandperhapsplaceitintheX1positioninsteadofX10.Asecond importantpoint,especiallywithRFatany time, is thatyoumaynotknowtheactualvoltageofanRFsignal.Somecautionisrequiredsotheoscilloscopeortheprobeisnotdamagedbyovervoltage.

Asnotedpreviously,topickupRFfromatransmissionlineoftenitisnotnecessarytoconnecttothetransmissionline.Aloopofseveralturnsaroundacoaxialline,especiallywheretheSWRisnotcloseto1:1,maybesufficient.A6or12inchpieceorwiretapedtothetransmissionlinemayalsowork.

WhenyouwanttoseethemodulationenvelopeofanRFsignal,asimplediodedetectorconnectedtotheRF pickup devicewill suffice.Figure 7.15 shows one such circuit. Youmay have to increase ordecrease the value of the 50 pF output capacitor, depending on the bandwidth of the modulation. Agermanium diode, the venerable 1N34 is shown. A germanium diode here has the advantage ofmoresensitivitysincethevoltagedropacrossitislessthanasilicondiode.

Time(Frequency)MeasurementsInthenormaluse,thehorizontalaxisofanoscilloscopeissetsothetracemovesfromlefttorightat

the set time base rate— but as you can see from the title of this section this time base or scan raterepresents both time and frequency. For any periodic wave, the relationship is f = 1/T, where f isfrequencyandTistime.Asanexample,a1000Hzsinewave—thatis,1000alternationspersecond—traces out 1000 sinewaves per second. The duration or period of a single sinewave is 1/1000 of asecond. To see the waveform of interest, you have to set the time base so the waveform is visible.Fortunatelyyoudon’thavetoknowthewaveformfrequencyaccurately;youcanputthewaveformonthescreenandvarythetimebaseuntilyougetasatisfactorypicture.

UsingtheTimeBaseAll oscilloscopes have a TIMEBASE control,which is calibrated in units of time per division. For

example, inFigure 7.16 the TIME BASE control (which is labeled TIME/DIV) is set to 0.5ms (one halfmillisecond)perdivision.Therefore,ifthegraticuleonthescreenhas10majordivisionsacross,ittakes10times0.5mstosweepacrossthescreen,or5ms.

As another example, a 400Hz sinewavehas a periodof (T = 1/f) or (T = 1/400). Therefore onecompletesinewavetakes1/400ofasecondor2.5ms.Sincethehorizontalscanrateortimebaseissetto5msforacompletescanacross,5/2.5=2,sotherewillbetwocompletesinewavesshowing.

However,justaswithsettingtheverticalchannelsensitivity,itisveryeasytomakeamistakehere.LookjusttotheupperleftoftheTIME/DIVcontrolinthefigure.Thereisavariablesweepcontrolonthisoscilloscope, and unless it is turned fully clockwise to theCALIBRATE position the time basewill notreflect themaincontrolsetting.Inaddition,someoscilloscopeshaveanX5 (5times)multiplierswitch.HavingthisswitchintheONpositionalsowillmakeanydirectreadingoftheTIME/DIVcontrolwrongbyafactorof5.

AswillbediscussedinChapter8,theaccuracyofthetimebasemaybefrom5%to10%forolderanalogscopesandwithinafewpercentfornewerdigitalscopes.Ifmoreaccuracyisneeded,anexternaltimebaseormarkergeneratorcanbeused.Therearegenerallytwowaysthegeneratorcangiveyouabettertimebasereading.Thefirstistoputthewaveformyouaremeasuringintoonechannelandthetimegeneratorintoasecond,superimposethemanddoyourmeasurement.

ThesecondwaytoistousetheVARIABLESWEEPcontrolmentionedbefore.PutthemarkergeneratorintoonechannelandusetheVARIABLESWEEPcontroltomakethemarkerpulseslineupwiththedivisionsof the graticule on the screen. Now you can count the divisions directly, because they have beencalibratedbythemarkergenerator.

LissajousPatternsAnolderwaytouseascopetomeasurefrequencyusesaknown,calibratedsinewavegenerator.As

seeninFigure7.17,theknownfrequencysourceisconnectedtoeithertheverticalinputorthehorizontalinputandtheunknowninto theother input.Whentheknowngenerator isset toanexactmultipleof theunknownfrequency,oneof thepatternsshowninthefigureresults.Thenumberoflobesisequal tothefrequency ratio. This technique dates back to before inexpensive digital frequency counters wereavailable,butiscostfreeandisanadditionaluseforyourscope.

GadgetstoAddThere are any number of small (and not-so-small) circuits you can build to extend the use of your

oscilloscope.Someareveryquick tobuildanduse—forexample, ifyouhaveasinewavegeneratorwithaknownfrequencycalibration,justaddaresistorinserieswithanunknowncapacitororinductorandyoucanmeasurethevoltageacrosstheunknowncomponentandthencalculateitsvalue.

DiodeTesterAverypopularaccessoryisshowninFigure7.18.Itisadiodetester,andwhenthediodeisoperating

properly the pattern ofFigure7.19 results.Notice the approximately 0.6V flat horizontal line on thedisplay.Thiswasasilicondiodeundertest;ittakesapproximately0.6Vforwardbiasbeforeitconducts.The same circuit canbe used to check the base-emitter andbase-collector junctions of transistors andseveralothertypesofdiodes.

HighFrequencyAmmeterAn oscilloscope can be very useful as an ammeter. At frequencies above 60 Hz power line ac,

ammeterperformancefallsoffinagenerallyunknownmanner.Chapter6includedadiscussionofusingadual channel scope in a differential mode. By placing a very small resistance in series with the linecarryingthecurrent,andplacingthetwoprobesatoppositeendsoftheresistor,Ohm’sLawwillgiveyouthe current as the voltage difference divided by the resistance. The frequency limit here is the scopeperformance,whichisobviouslyalothigherthan60Hz!

SoundCardProtectionChapter8 includesabriefdiscussionof an interestingno-hardwareoscilloscope thatusesyourPC

andthecapabilitiesofthesoundcard.Specifically,aversionofoscilloscopesoftwareishostedonthePC.TheinputisnotthroughananalogverticalamplifierandexternalA/D,butthroughtheA/Dsfoundonthecomputer’ssoundcard.

Obviouslytheperformanceofthistypeofscopeislimitedtothebandwidthofatypicalsoundcard,perhaps20–20,000Hz.Exposingyoursoundcardtoanoutsidesignalcanresultinthelossofthecard.AbasicprotectioncircuitforthisscopeisinFigure7.20.Usingtwosilicondiodesinserieslimitsthevoltage into the sound card to about 1.2V, positiveor negative.Anoptional voltagedivider (a singlevariableresistor)isshowntoadjusttheinputlevel.Inadditiontothebandwidthlimitation,thistypeofsystemdoesnothaveahighinputimpedanceandcaneasilyloaddownthecircuitundertest.Foranaudiofrequencyscopetocompareamplifierinputandoutputsignals(inotherwords, lookfordistortion), the

priceisright.

ScopeBandwidthExtenderIfyouhaveanoscilloscopewith,forexample,abandwidthof10MHzandwouldliketoexaminea28

MHz signal, youmight be able to do it. If you could convert the 28MHz signal down to a frequencybelow10MHz,thatwouldbeideal.Thissolutionisnotmuchdifferentthantheapproachusedinmanyreceivers—heterodyningthesignaldowntoalowerfrequency.OnesuchcircuitisshowninFigure7.21(reproduced fromChapter 25 of the 2010ARRLHandbook). This circuit uses three transistors and asingleMiniCircuitsSRA-1mixer.

Althoughthisprojectmaybemorethanyouwanttoundertake,itdoesservetoremindyouthatyoucantakeanoldreceiver,makingsurenottooverloaditwithtoohighaninputsignal,andtuneittowhateversignalyouwanttoexamine.ConnectyouroscilloscopetotheIFamplifierandyouwillgettoviewthehigher frequency signal. Remember, however, that the signal you see is now limited by the receiverbandwidthandnotthescopebandwidth.

MeasureYourPEPOutputOneof themostperplexingquestionsmanyhamsask,after they thinkabout theFCCrequirementof

“nomorethan1500wattsPEP,”is“HowcanImeasureit?”JusthowmuchPEP(peakenvelopepower)ismytransmitterputtingout?Eventhoughyoumaybeusingoneofthemany100wattoutputtransceivers(perthemanufacturer’sspecification)socommononthebands,justhowmuchpoweriscomingout?

You can use a scope to get a fair measurement of this number, using the calculation technique inChapter2ofTheARRLHandbookanddescribedhere,butyouwillhavetoexercisecautionsothatyoudon’tdamageyourself,yourscopeoritsprobe.WithhigherpowertransmittersyoucouldbeexposedtoRFvoltagesinthe600to1000Vrange.

First connect your transmitter to 50Ω dummy load through a coax T adapter. The load has to beresistive,withnoinductanceorcapacitance.

Yourscopeprobeisprobablylimitedtoafew10sofvolts,sonextyouwillhavetobuildavoltagedividerwitharatioofperhaps50to1(seeFigure7.22).Forexample,youcouldusea49kΩresistor(R1)anda1kΩresistor(R2).Theexactvaluesarenotimportant,justaslongasyouknowthedividerratio.

DividerRatio=R2/(R1+R2)=1000/(49,000+1000)=0.021/0.02=50

This is not the place for 1/4W resistors, unless you like to seeRF arcing! Physically large, non-inductiveresistorsareneeded,perhapsofthe2Wvarietyorphysicallylarger.Theresistorpowerratingisnotcritical.WhenyouconnectthefreeendofthelargerresistortotheremainingcenterportoftheTadapter,doitsuchawaythatneitheryourhandnoranythingelsewillcomeincontactwiththehighRFvoltagethispoint.Thescopeprobeisconnectedtothejunctionofthetworesistorsandtheprobereturnisconnectedtoground.

Nowyoucanturnonyourtransmitteron,talkintothemicrophone,andmeasurethehighestpeak-to-peakvoltageseenonyourscope—forexample,yousee4Vpeak-to-peakonyourscope.

TakethatmeasuredvoltageandmultiplyitbythevoltagedividerratiotogettheactualvoltageattheTadapter.Inthisexample,4V×50=200V.

Thisisnowyour“true”transmittedpeak-to-peakvoltage.Divideby2.Younowhaveyourtransmittedpeakvoltage(peakenvelopevoltage,orPEV,asshowninFigure7.23).Inthisexample,

PEV=200V/2=100VYouarealmostthere.FollowingtheHandbookmethod,multiply thispeakvalueby0.707toget the

RMSvoltage(VRMS).Inthisexample,

VRMS=100V×0.707=70.7V.Then,witha50Ωload,yourPEP=VRMS2/50.Inthisexample,PEP=70.72/50=100W.Figure7.24isaplotofPEPversusmeasuredpeak-to-peakvoltageintoa50Ωload.

OtherOscilloscopeCapabilitiesBecauseneweroscilloscopescontainamicroprocessor,memory,input/outputconnections,program

controllers and storedprograms, theymeet thedefinitionof a computer. It’s no surprise that this samehardwarewithadditionalprogrammingcanbeusedtosupplyothermathandtestfunctions.

For example, to measure the peak-to-peak value of a waveform, the program just looks for themaximumstoredvalueoftheinputandtheminimumstoredvalue,subtracts,andthatitthepeak-to-peakvalue.Tomeasureperiodsandthereforefrequency,itlookseitherforzerocrossingsorpointsofsimilarinflection(goesfrompositiveslopetonegativeslopeorvice-versa).

Thefollowinglistisasummaryofthefunctionssupportedbymodernoscilloscopes.Ofcourse,theseextrafeaturesareavailableforaprice—youarepayingforthecapabilityandsoftwareratherthanextrahardware.

Digitalbusmonitor/analyzerDigitalpatterngenerator(multichannel)Logicanalyzer(multichannel)Mathfunctions:FFT(fastFouriertransforms),derivatives,integrals,statistics,frequencyanalysis

NetworkanalyzerPowersupplies:±5Vand/or±12VSignalgeneratorsSpectrumanalyzerVoltmeterThis is, of course, only a partial list just to illustrate the computing power available in modern

oscilloscopes.

Chapter8

IfYouAreGoingtoBuyOne—Specifications

Thepreviouschaptershavediscussedoscilloscopecharacteristicsandapplications.You’rethinkingthatascopemightbeausefuladditiontoyourworkshop.Howdoyouchoose?

WhatAreYouGoingtoUseItFor?Thisisthesamequestionmanyofushavefacedwhenplanningtobuyanewpersonalcomputer.There

are a wide variety of oscilloscopes (and personal computers!) available, with a wide variety ofcapabilities.Butasusual,thefirstquestionisnot“Whatscope?”but“Whatuse?”

Pricesofnewunitsrangefrom$150or$200andup—veryup.Usedscopescanbehadforaslittleas$25atfleamarkets.Therearemany1950s,1960sand1970sunitsstillbeingsoldandtraded.Buttheyoftenhaveonlyone inputchannel,maynotusehigh impedanceprobes(and thereforecan loaddownacircuit)andareprimarilyvacuumtubedesigns.

Youcanalso find1980sand later laboratory-qualityscopes for$100ormorewithcapabilitieswecouldonlydreamofyearsago.Thereare,howeverseveralproblemswiththesetypesofusedscopes.

Forscopesthatusevacuumtubes—wheredoyougetthemandatwhatprice?Notonlydoyouhaveto plan to replace tubes from time to time (if not immediatelywhen you buy the unit) but these olderdesignsneedperiodiccalibration,especiallywhenyoureplacetubes.Thelab-qualityunitswerebuilttobeasaccurateandstableaspossible,butyoucangetacluefromfindingastickeronthecasefromtheoriginalowner.Thisstickeroftensays“CalibrationDue_____”withahandwrittendate.Iftheyarenotrecalibrated—andcalibrationisnotalwaysaneasytaskwithoutaccuratestandardsequipment—youcannotbesurejusthowgoodyourmeasurementsare.

Theotherproblemwith theusedequipment isheat.Althoughthe1950sunitsweredesignedforTVrepairshopsandbeforeuniversalairconditioning,thelab-qualityusedequipmentwasprimarilyintendedforairconditioned,lowhumidityenvironments.Puttingoneoftheminadampbasementworkshopmaynotworkoutverywell.

There are three general types of oscilloscopes you can consider tomeet your needs.The first is acompletelyself-containedunit.Itmaybeanalogordigital,withtheanalogunitstendingtowardthelowerendofthepriceandperformanceranges.Today’sscopesarenowherenearaslargeandheavyasthoseofpastgenerations,buttheycantakeupmeasurablebenchroom.

The second type is adigital pluggable unit matingwith a laptop or personal computer. They arerelatively small, usually have no panel controls and accept commands by way of the PC mouse andkeyboard.Theyareveryportable,andcanbepluggedintoalmostanypersonalcomputerorlaptop—butfirsttheirmatchingsoftwaremustbeinstalledinthePC,usuallyfromaCD.

Thethirdtype,ultra-minis,plugintotablets,smartphonesandotherhand-helddigitaldevices.TheirsoftwareishostedinthedigitaldeviceandmustbedownloadedfromtheInternet.Generallytheyhavealargenumberofavailable functions,but theirperformance specifications (suchasbandwidth)areveryminimal.

Thereisafourthtype,softwareonlyscopes,thathavelimitedcapabilitiesataudiofrequencies.Thesearediscussedlaterinthischapter.

Don’tOverbuyYes,itwouldbeverynicetoownamoderndigitalstoragescopewithabandwidthof500MHzanda

high-precisionspectrumanalyzerbuiltin.Buthowoftenareyougoingtousethemaximumcapabilities?Aspointedoutbefore, toseea6meter(50MHz)signal,a30MHzscopemaybegoodenough—theamplitudeofthedisplayedwaveformwillbelowerthantheactualvalue,butyoucanprobablyseethesignal.Itisnicetohaveaninputsensitivityof3µV,butoftenambientnoisewillbegreaterthanthatsoyourmeasurementsinthisrangewillbehighlyquestionable.Whypayforthiscapability?

The control section of a recent model analog oscilloscope is pictured inFigure 8.1. By contrast,Figure8.2 shows someof the controls on adigital scope.Manyof the functions are selectedbypushbuttonorrotarycontrolofmenuitemsdisplayedonthescreen.

Moderndigitalscopesoftenusetheirdigitalmemoryandcomputingpowertogiveyouahostofextrafunctions.But someof these functionsmaynot be available simultaneouslywhen the unit is used as ascope.Whatisthepointofusingthesignalgeneratorfunctionastheinputtoanamplifierwhenyouwanttolookattheoutput,butthescopefunctionisnotavailablewhenusingthesignalgeneratorfunction?

The following sectionswill first look at scope specifications in general, then some of the specialfeaturesofadigitalscope.Youmayfindthatnotallofthespecificationsyouwanttoknowarelistedonamanufacturer’swebsiteordatasheet.

SpecificationsCommontoAnalogandDigitalScopes

BasicCapabilitiesoftheVerticalChannelsUnlessyouare looking tobuyanantiqueoscilloscope (from the1950s to1970s), thereare certain

basiccapabilitiesandfeaturesfoundonjustaboutalloscilloscopes.Theseinclude:Dualchannelinputs.Allscopesnowhavetwoanalogchannels.Acanddccouplingplusground.Selectingtheaccoupling(input)positionallowyoutoseeanac

waveformindependentofanydcvoltageitmayberidingon.Thisisveryhandywhenyouhaveasmallripplevoltageyouwant to lookat,but it isontopofa50Vdcline.Ifyouselect thedcposition,youwouldhavetoset theverticalscale toallowa50Vsignal tobeseen—thus thesmallripplewillbealmost invisible.Selectaccouplingandnowyoucanpickagoodscale tosee the ripple.TheGroundposition gives you a reference level to set the vertical position of the sweep. Discussion of dcmeasurementsfromthispointonwillbewithrespecttotheselectedGroundposition.

Verticalchannelaccuracy.This is anumber,usually expressedas apercentage, suppliedby themanufacturer.There isone importantdifferenceherebetweenananalogandadigitaloscilloscope.Ananalogscopemustkeepcircuitaccuracyandstabilityfromtheinputthoughanentirecircuitchainuptothedisplay.Adigitalscopeonlyhastheanalogaccuracyproblemthroughitsinputstages.Fromthereon,theaccuracydependsprimarilyontheinputA/D(analogtodigital)converter.A/Dconverteraccuracyisdiscussedlaterinthischapter.Asyoumightsurmise,theanalogscopemayrequireperiodiccalibrationoftheverticalchannelswhereasthedigitalscoperarelyrequiresit.

Selectionofeitherchannelalone,both(alternate)andboth(chopped).Thechoppedmode,whereasmallpieceofeachchannelissampledanddisplayedalternately,isfrequencydependent.Forexample,ifthechopfrequencyis500kHz,thismeansthatapieceofeachinputwaveformisshownapproximatelyevery2µs.Ifyouarelookingatawaveformwithinformationthatcouldappearwithina2µsperiodonchannelA,itmayormaynotshowupifthechopselectionisonchannelBduringitsoccurrence.

Algebraiccombinations (AplusB,AminusB,and soon). Some scopeswill offerA+Band aninvertfunctiononB.Byswitchingprobes,allfouralgebraiccombinationsarethenpossible.

High input impedance inputs. Generally 1MΩ and 10 to 25 pF are the normal valueswithoutprobes.

Fixed and variable input scales (sensitivity). The ability tomeasure down to a few tenths of amillivoltsoundslikeagoodidea,butinpracticesystemnoiseandgeneralpick-uplimitsitsusefulness.The upper range is important, since applying overvoltage can have a simple effect — if the inputmaximumratingis150Vandyouapply500V,youcaneasilyburnouttheverticalamplifier.Theresult,simplystated,isadeadscope.Normally,themaximuminputvoltageratingislimitedbytheprobeused.WhenthevariablesensitivitycontrolisintheCALIBRATEposition,thenumberontheselectionscalegivesyouthevoltagevaluepermajorgraticuledivision.Thusifyouselect0.5V,thismeansa0.5Vpeak-to-peaksinewavewillbeexactlyonegraticuledivision(usuallymeasuredincentimeters)high.

Probessupplied.Mostcommercialscopes,purchasednew,comewithasetofsimpleprobesthatcanbesettoeitherX1(straightvoltagemeasurement)orX10 (attenuatestheinputvoltagebyafactorof10).

Frequencyresponse(bandwidth).This is thenumber thatmosthamsfocuson.It is important,butnottheonlyimportantnumber.Asmentionedinearlierchapters,thebandwidthnumbermeansthisisthefrequencywhereaninputsinewavewillbedisplayed3dBlowerthanitsactualvalue.Attheupperlimitofthebandwidthspecification,a10Vsinewavewillbedisplayedas7.07V.

Thebandwidthspecificationcanbeverymisleadingwhenyouwanttoknowthescopeperformanceatfrequencies higher (and lower) than the specified bandwidth number. InFigure8.3 the horizontal linerepresentsconstantgain.Lineashowsthegainfallingoffatafrequencymuchlowerthanthebandwidthfrequency,butithashighergainthantheothersbeyondthebandwidthfrequency.

Linecistheotherextreme.Itshowsthegainfallingoffmuchclosertothebandwidthfrequency,butshows lowergainpast thebandwidth frequency.Youcansee thatall threedesignsmeet thebandwidthspecification, but if you are looking for performance beyond the bandwidth point, there can be bigdifferencesamongscopemodels.Inpracticetheslopinglinewouldnotbeasstraightasshownbutratherasmoothcurve.

Frequencyresponse(riseandfalltimes).No,thisisnotanaccidentalduplicationofthefrequencyresponseparagraphsabove.Ifyouarelookingatadigitalpulse—assumingithasperfectverticalrise

andfalltimes—thebandwidthalsolimitshowwellthe“perfect”pulsewillbedisplayed.Thegenerallyacceptedformula—anapproximation—is

t(rise)=0.35/bandwidth(inMHz)

Thus,asseeninFigure8.4,aperfectsquarewaverise timewillbeseenasa35nsrise timeonascopewitha10MHzbandwidth.Although“risetime”hasbeenusedinthisexplanation,thesameideasandformulasholdforfalltimes.

Since perfect square waves, with perfect rise and fall times do not exist, a real question is whatexactlyareyouseeing?Theanswerisgivenbythisformula:

wheret(rise)isthescoperisetimenumberfromtheprecedingparagraphandp(actual)istheactualpulserisetime.

If you want to visualize it, the answer is rather like the value of the hypotenuse of a right-angletriangle,wherethescoperisetimeandthepulserisetimearethetwoorthogonalsides.

BasicCapabilitiesoftheHorizontalChannel Horizontal sweep selection. This control is usually marked in units of time (microseconds,

milliseconds and seconds) As an example, selection 1 µs translates into a sweep rate of 1 µs perhorizontalgraticulemarking.Withtheusual1markpercentimeter,itwouldtake10µstosweepacrossthe screen.A2MHzsinewave (acompleteperiodequals5µs)wouldbe seenas twocomplete sinewavesacross,assumingthefirstonestartedattheleftedgeofthescreen.

Sweepmagnifier.Manyscopeshaveasweepmagnifiercontrol,either5Xor10X.Whenselected,thesweepratechosenisdividedbythisnumber—themagnifierexpandsthedisplaybythe5or10factor.

Direct horizontal input. This is, perhaps, a rarely used feature (see the section ofChapter 7 onLissajousfigures).However,itdoescomeinhandyfromtimetotime.Unfortunatelythesensitivitycontrol

fordirect input (voltspercm) isoftenhidden—sometimes it ispiggybackedon thehorizontal sweepsettingcontrol—butunmarked!

Accuracy and stability. Since the horizontal sweep is used to measure time or frequency, itsaccuracyandstabilityarecritical.Analogscopes,usingpurelyanalogcircuits,usuallyhavemuchpoorerspecifications than digital scopes. Not only do the analog circuits making up the sweep have to bedesignedtobeaccurate,buttheyalsohavetobestablewithtemperatureandtime.Digitalscopesusuallyuseacrystal-controlledclockandthusare lesspronetocircuitdrift.Whileanalogsweepcircuitsmayrequireperiodiccalibration,digitalscopesrarelyifeverrequirethissortofalignment.

Delayedsweep.Thereareseveralnamesandtechniquesfordisplayingasectionofawaveformthatoccurslaterthanthepointwherethehorizontalsweepistriggered.YoucanseesuchtermsasDelayTimePosition,HoldoffandSegmentMagnification.Thisfunctionwasdescribedinanearlierchapter,butinsummarythesefunctionsallowyoutopickthepointyouwanttoexamineandexpandthesweeparoundthe selected pointwithout changing the trigger point.Youwould use this feature if, for example, yourscopeissettodisplaya10mssegmentofwaveformacrossthescreen,andyouwanttoexaminecloselyapointonthewaveformthatoccurs2msfromthestartofthesweep.Youcanexamineanyselectedpartoftheentire10ms longwaveforminasmallselectedareawithasweeprateof1µs/cmin thisselectedarea.

BasicCapabilitiesoftheTriggerSectionWhile many people focus on the bandwidth specification, the triggering features are of equal

importance.Ifyoudonothaveastablesweep,startedbyatrigger,youcannotseewhateveryouwanttolookat.Whenitcomesusinganoscilloscope,settingthetriggeristheonethingthatgivesmanypeopleaproblem.Notalloftheselectionslistedfollowingwillbefoundoneveryscope,buttheyarecapabilitiesyoushouldconsider.

Triggersources.ChannelAalone,ChannelB alone, alternate channelsAandB ,Line (60HZ),external(thoughatriggerinputconnector).ThealternateselectiontriggersthesweeponchannelAfortheinputonchannelA,andthen(alternately)triggersonchannelBfortheinputwaveformonchannelB.

Triggermodesand selections.There are generally at least twomodes.AUTO triggers the sweepeven if there is no discernible signal for triggering, but when a signal appears the mode changes toNORMAL.NORMALallowsyoutoselectmanuallythetriggeringparameters:positiveslope,negativeslope,selectedpositivevoltage,selectednegativevoltage,zerovolts,zerovoltcrossingandothers.

Digitalscopeshaveadditionalmodessuchassinglesweep,tobediscussedlater.Olderscopes,suchas seen often at fleamarkets,may also have positions corresponding to the “old”NTSCTV standardwaveform.

DigitalScopesThereareanumberof specifications thatapply, someunexpectedly, todigitaloscilloscopes.Asan

example,lookatFigure8.5.Theinputsofthesescopesaresettosampletheincomingwaveformandthenconvertthemeasuredvaluetoadigitalnumber.Simpleinconcept,butitcancauseproblemssuchasinthefigure.ApulseisshowninAasitappearsonananalogscope.Bisthesamewaveform,butsampled.If the sampling frequency is not high enough, you will lose detail on the waveform even though thespecifiedbandwidthishighenoughforthewaveform.

Howhigh should the sampling frequencybe?There isno standardanswer.Certainly five times the

highestfrequencyyouwanttoseeisnotenough.Tentimes?Ahundredtimes?Thatmightbeenough,butrememberthatyouarelookingatthehighestfrequencyyouwanttosee,soforrectangularpulsesyouneedtoseeharmonics.

Agoodcompromisewoulduse thebandwidthas thebasenumber,and thenperhapsahighmultiple(500?1000?)ofthebandwidthwouldbeenough.Ifyoubelievetheseparagraphshaveavoidedanexactanswer,youarecorrect.Somemanufacturersusedatapredictionandsmoothingtechniquestoatleastfillinthegapsartificially,ifnotreallyimprovetheresponsetoveryfasttransitions.

Another technique used by some digital oscilloscope designers moves the horizontal trigger onsuccessivesweeps(Figure8.6).Ifthewaveformyouwanttoseeisabsolutelyrepetitive,withlittleornojitter,onesweep ismade in thenormal fashionstartedby thesweep trigger.Asecondsweep ismade,delayed by perhaps 1/10 of the sample rate in time. A third sweep is made, delayed by twice thisincrement,andsoon.Attheendof10sweeps,10sampleshavebeentakeninplaceoftheonethattheusualsamplingwouldhaveproduced.Thusthewaveformissampledat10timesthenormalsamplerate,butitrequired10sweepstodoso.Againthewaveformmustbeabsolutelyrepetitiveandvirtuallyjitter-freeforthistechniquetowork,butwhenitdoesitisveryeffective.

This just one of themore interesting and complex problems that occurwhen using a digital scope.Certainly they have advantages, but an understanding of the functions is very necessary to be able tounderstandwhatyouaremeasuring.

NumberofbitsoftheinputA/Dconverter.Againwehaveasimpleideathatgetscomplicatedveryquickly. Suppose theA/D converter has 12 bits.Thismeans the input voltage can be divided into 212parts, or 4096 parts. However system noise and uncertainty will knock out one or more bits. Butobviously,bettertooptformore,evenifseveralarenoteffective.

Memory size. Ready to do somemultiplications? Try this: Thememory needed is equal to (thenumberofbitsofasinglepointonthewaveform)times(thenumberofsamplespersecond)times(theamountofwaveformyouwanttoseeinsecondsorfractionsofasecond).Nowconsiderthat therearetwochannels—doyouwanttodoublethememorysizeorcutthenumberofsamplespersecondinhalf,givinghalftoeachchannel?

Processorcapability.Okay,nowthedataisinmemory.Cantheprocessorhandleit?Certainly,ifyouhaveaself-containeddigitalscope,thedesignersmadesurethereisenoughhorsepowertoprocessthisdataandgeneratethedisplayyouwant.Butremember,someoftheprocessorcapabilityisgoingtocomputerbookkeeping,backgroundoperationsanddisplaygeneration.

With a self-contained oscilloscope, such as the one inFigure 8.7, the manufacturer has sized thememory and processor to match the specifications. The controls on the right side are actual menuselectors,andthecurrentsettingsareshownontherightsideofthescreen.

Now if you are using an outboard scope that plugs into theUSBport, is there a bottleneck? Is thehardware capable or the operating system fast enough?Were you planning to plug into an old, slowcomputer?Figure8.8 is froma scope that plugs into an iPador iPhone. Its bandwidth is 5MHz, andwhileithasagoodsetoffunctionsandcontrols,itshostdevicesdonothavemuchmemoryorcomputinghorsepower.Thingsdoindeedgetinteresting,andnotallthequestionshavediscreteanswers.

Processortype.UntilnowtherehasbeenanassumptionthattheexternaldigitalscopeplugsintoaWindows computer. Is the unit you are planning to buy availablewith software compatible with non-Windowsoperatingsystems?HowaboutAppleorLinuxcompatibility?

Convenience. Many new scopes offer convenient features, such as automatic setup. Connect awaveform, and the scope sets the vertical voltage scale, horizontal sweep rate and trigger settings tonumbersitmeasuresasapplicable.Thenyoucantweakthesesettinganywaywant.Howaboutone-buttonmeasurements?Select fromamenu toget readoutofpeak-to-peakvoltage,maximumvoltages, averagevoltage, frequency, periods and othermeasurements. Is there a cost?Of course, but if you aremakingrepetitivemeasurementsitmightwellbeworthit.

Waveformstorage.Inanearlierchapteritwasnotedthatdigitalscopesstoretheinputdata,andthuscanprovideastoredpicture.Onenumberthatcanbespecifiedishowbigthispicturecanbe;thatishowmanysecondsormillisecondsofdatacanbestored.

Timescale.Does thescopeofferboth linearsweepand logarithmicsweep?Alogsweepcanbeinvaluablewhenlookingatalongsequenceofevents.

Direct multichannel digital inputs. As described earlier in this book, some scopes offermultichannel(atleastfour)inputsforlookingatfourdigitalsignalssimultaneously.UsuallytheallowablevoltagerangefortheseinputsislessthanallowedforanalogchannelsAandB,butsufficientfornormaldigitalsignals.

Anythingelse?Isthereanyendtothislist?Ofcoursenot.Whenanoscilloscopeisbasedonadigitalprocessor,anythingthatcanbeimaginedasafunctionforadigitalprocessorcanbeprogrammed.Thereisacost,butaswasnotedinthefirstparagraphofthischapter.Yourfirsttaskistodecidewhatyouwanttodowiththescope.

OtherFeaturesandTypes

LimitationsAsdiscussedinChapter7,someadvanceddigitalscopesincludetestinstruments,takingadvantageof

thememoryandprocessorusedinthebasicoscilloscope.Usually,eachofthesefeaturesarespecifiedinsomedetail.Asanexample,whereanFFT(FastFourierProcessor)isincluded,thenumberofpointsandspeedarealsospecified.Thereare,however,severalcautionstobenoted.First,iftheA/Dandanalogfront end are limits on the scope bandwidth, the included other functions will be limited to thesebandwidths.Don’texpecta30MHzbandwidthscopetoshowsignalspectrumto500MHz!

Next,whiledigitalprocessorsandtheirassociatedmemoryareverycapableofbeingprogrammedassignalfunctiongenerators,manyofthesecapabilitiesandotherscannotbeusedsimultaneouslywiththeoscilloscopefunctions.Thusifthereisasignalgeneratorthatyouplantousetotestacircuit,selectingthesignalgeneratorfunctionmaydisablethescopefunction.

Interfaces,Connections,andAdditionsNot listed inChapter 7, but of some importance, is the ability of some scopes to be connected to

analysis or simulation programs such asMATLAB, or to other hardware— perhaps a large capacityrecorder.Sincethesefunctionsandthegeneralcomplexityofsomemodernoscilloscopescanberatherhigh, youmight look for detailed user instructions and video demonstrations on the CD or DVD thatcomeswiththescope.UsuallytheCDcontainstheusermanualandotherinformation.

JustOneMoreTypeofOscilloscopeAvailable for download, often free and very interesting, is the set of hardware-free oscilloscopes.

Theseuseapersonalcomputersoundcardasthechannelconnections.Thesoftware,similartothatusedwithhardwarescopesthatconnecttoaPCviaaUSBconnection,ishostedonthePC.

Thereare,ofcourse,cautionsandlimitationsassociatedwiththisapproach.Sincethesoundcardisinterfacedwith (sometimes) unknown signals, protection for your sound card is strongly suggested.AprotectioncircuitforthissituationisgiveninChapter7.

Figure 8.9 is a screen shot from one such scope.Downloading and setting it up takes only a fewminutes.However,thebandwidth,inthiscase,islimitedtothecapabilityofyoursoundcard.Generally,forsinewaves,thismeansalowerfrequencylimitofperhaps20Hzandanupperfrequencylimitof20kHz.

For pulses and other non-sinusoidal signals where Fourier analysis predicts frequency content ofmultiplesof thebasewaveshape, thebestyoucanobtainwithout seriousdistortion isperhaps20kHzdividedby5,sothatthe5thharmonicenergywillbepassedandthedisplayedwaveformrecognizable.Since the source of these scopes is the Internet, and names, addresses and cost (if any) are constantlychangingitwillprobablytakeyouasmuchtimetofindoneofthesesoftwarescopesonlineasitwilltodownloaditandgetitworking!

Appendix1

SoftwareOscilloscopes—CapableandFree!

Overtheyears,Ihaveseenandusedawidevarietyofoscilloscopesinmyhomeworkshop.Theyallhadonethingincommon—acost.

Asdiscussedinearlierchapters,therearestillquiteafewold-lineanalogoscilloscopesforsale—self-contained with a large CRT facing out of the front panel. The newer generations of scopes aredigitallybased,whereaftersomesmallanaloginputcircuits thesignalgoesintooneortwoanalog-to-digital(A/D)converters.Fromthispointonthesignalsyouwanttoseeareinacomputerofsortsbuiltinto the scope. In fact somemodern scopes don’t include this computation ability but connect to yourpersonalcomputer.

More recent is a new set of oscilloscopes that are tiny enough to fit in your shirt pocket.Digilentmakes a unit that interfaces though aUSB port to a PC.Another unitmade byOscium connects to aniPhone, iPod or iPad (see Appendix 3). Unfortunately these ultra-miniature scopes have a limitedfrequencybandwidth.

Another option is a scope requiring no hardware! Since today’s scopes are primarily software,runninginsomesortofcomputer,isseemsthatallyouneedisafrontendconsistingofanA/Dconverter.YoualreadyhaveA/DconvertersbuiltintoyourPC—thesoundcard.Whynotusethat?

Thisisexactlywhatothersrealized,andthereareprobablyadozen,orperhapsevenmore,software-only scopes available for download on the Internet.Many of them are free for ham use.Aswith anydownloadprogram,anactivevirusormalwarechecker is anabsolutenecessitybefore running suchaprogram.

What’sTheCatch?Therearereallytwomajorlimitationstousingasoftware-onlyscope.Thefirstisthatthatthesignal

tobeseenisconnecteddirectlytothesoundcard.Ifthissignalvoltageistoohigh—poofgoesthesoundcard (or even worse, the sound card circuitry on the mother board!). Therefore a simple protectioncircuit,suchastheonedescribedinChapter7,isneededtoprotectthesoundcard.Besuretotakethetimetobuildanadequateprotectioncircuit!

Thesecondlimitationisfrequency.Youcangenerallygetreasonableresultsonsinewavesuptothesoundcardlimitof20kHzorso,butasthefiguresinthisAppendixshow,non-sinusoidalwaveformscanbebadlydistortedastheirrepetitionfrequencygoesup.

WhatDoYouGetFree?As noted before, there are many software scopes available. Soundcard Oscilloscope, V1.41 by

ChristianZeitnitz(www.zeitnitz.eu/scope_en)wasusedheretoshowthefeaturesandlimitationstypicalofthesescopes.Makenomistake,thisisafullfeatured,stableandinfactfun-to-usetestinstrumentwithinitslimitations(primarilyfrequencylimitations).

The tests and resulting pictures here were made using both a three-year-old Pentium PC runningWindows 7 and a five-year-old Asus Netbook, running Windows XP. There was no noticeableperformancedifferencewitheithercomputer.

SoundcardOscilloscopecomeswitha16-pagemanualinPDFformatandbuilt-inhelpfiles.Asusualwithsoftwaredocumentationandhelpfiles,manyquestionsgounanswered,butalittleexperimentationoftenanswersthequestion.

ThescopeoutputplugsintotheLINEINPUTjackofthesoundcard.Becausethescopeisdualchannel,adualchannel(orstereo)inputisneeded.OnmostPCsthemicrophoneinputismonauralandwillnotbesuitable.

Whenyoubringupthesoftware,afterthelicensingnotice,asetoftabsontopareusedtoselectwhatyouwanttodo.FigureA1.1showstheSETTINGStab.AsyoucanseetheinputselectedforthePCisLINEIN.NextyoucangototheOSCILLOSCOPEtabandmaketherequiredtimebaseandvoltagesettings.ThenbacktotheSETTINGStabandsaveyoursettingsforfutureuse.AlsocheckthatthesoundcardsettingsinyourPChavethegainforLINEINturnedup.

FigureA1.2showsthescreenwhentheOSCILLOSCOPE tabhasbeenselected.Voltagecalibrationisnot exact; thevoltage scale is arbitraryand its range is a16-bitnumber.However ifyouput aknownwaveforminonechannel,youcangetreasonablemeasurementsbyadjustingthecursorandcomparingtheunknownwaveformwiththeknownwaveform.Therelativeaccuracyisonepartin32,768.Betweentheamplitudecontrolsandtheresistorsinthesoundcardprotectioncircuit,youhavetoworkabittoadjustthesystemtoshowyourwaveform.

Thehorizontalsweeprateiscalibratedintotalrange,presumablyacrosstheentirescreen—nottimepercmasinmostscopes.Slightlyconfusingisthe1CHANNELmode(secondcolumn,nearthebottom).1CHANNEL does not mean single channel; it means two separate channels as contrasted to the otheravailablesettingsofaddingandsubtractingthetwochannels.

FigureA1.2showsaroughly2kHzsquarewaveandthesamefrequencytriangularwaveonthescopeface.Notice that thesquarewaveisrelativelysquare—comparethis toFigureA1.3wherea10kHzsquarewaveisshown.

Mathematically,byatechniqueknownasFourieranalysis,asquarewavecanbeshowntobemadeupofthesumofasinewaveatthefundamentalfrequencyaddedtoothersinewavesatthethirdharmonic,fifthharmonic, seventhharmonicandsoon—inotherwordsalloddharmonics.Eachharmonichasaspecificamplitude.ThisisdiscussedinChapter4andChapter7ofthisbook.Theresultisadisplayofaverydistorted squarewavewhen these harmonics fall outside the sound card’s 20kHzbandwidth.Atleastonesoftwarescopeclaimsperformancerelatedtothesoundcardsamplingrate—perhapsupto96kHzormore.

FigureA1.4 shows the display when the FREQUENCY tab is selected. This is actually a spectrumanalyzer.Herea1kHzsquarewaveshows theexpectedharmonicsat3kHz,5kHz,7kHzandsoon.Unfortunately,theamplitudesherearenotshownaccurately.

TheSIGNALGENERATOR tab,FigureA1.5providesasetofselectedoutputwaveforms, justaswithexpensive self-contained scopes. Another included capability is an audio recorder in the EXTRAS tab(FigureA1.6),whichyoucouldusetorecordthewaveformunderexamination.Finally,theXYGRAPHtaballowsyoutoseeLissajousfigures,whichcouldbeveryhandyincomparingrelatedaudiofrequencies.

OtherFreeSoftwareScopesMany of the other available software scopes are quite similar to the one just described. A short

Internet searchwill bring up both scope downloads andmore information about these devices.A fewversionsincludeon-screenmultipurposevoltmeters,frequencymeters,ohmmeters,ammeters,andalargevariety of other test equipment. Some require some external circuitry. A few use special samplingtechniquesthattheyclaimallowusebeyondthecapabilityofthesoundcard—althoughprobablyonlyforalimitedsectionofawaveform.

To see a fraction of what is available set your search engine to find the exact phrase “softwareoscilloscope”andthewords“free”and“download.”Whateverthetruecapabilitiesofthesescopesarethepriceisright.Allyouhavetodoisspendyourtimeexploringtheseveryusefulprograms.

Appendix2

QSTProductReview:RigolDS1052EandTektronixTBS1042Oscilloscopes

Thisreview,byPhilSalas,AD5X,originallyappearedinOctober2013QST.Most hams have basic test equipment consisting of at least a digital multimeter, SWR meter and

dummy load. These three instruments provide the ability to do basic troubleshooting. AdditionalequipmentoftenincludesanaccurateRFpowermeter,afrequencycounterandanoscilloscope.Ofthese,historicallytheoscilloscopehasbeenthemostexpensive,leadinghamstoexplorethesurplusequipmentmarket. Used analog oscilloscopes can be quite good, but they are also large and inconvenient forrecordingdata.Ifsomethinggoeswrong,theymaybedifficultandexpensivetorepair.

Digital sampling oscilloscopes (DSOs) have become available at prices justifiable for many hamexperimenters. The two reviewed here provide features and capabilities that will satisfy most homeusers.

TheOscilloscopeDecisionProcessIn the past, factors to be considered when choosing an oscilloscope included the number of

simultaneous signals that you might need to measure, the bandwidth necessary, on-screen digital datareadoutsalongwiththewaveformdisplay,andspectrumanalysiscapability.Withtoday’sDSOs,theonlydecisionyoureallyneedtomakeisthebandwidthrequired.Asyouincreasethebandwidthrequirement,though,thecostoftheoscilloscopecanincreasesignificantly.

With thesefactors inmind, this reviewfocusesonRigolandTektronixDSOswitha40 to50MHzbandwidth,asthisissufficienttopermitmostmeasurementsdesiredatthelowestcost.Asyoucanseein

TableA2.1,thesetwoinstrumentshaveverysimilarbasicspecifications.Detailedspecificationscanbefoundonthemanufacturers’websites.

Let’sMakeaFewTestsToseehowtheoscilloscopesperform,IselectedseveralteststhatIthoughthamswouldfinduseful

and interesting. For the first test, I looked at themeasured frequency response of the oscilloscopes. ImeasuredRFpowerwithacalibratedsetupandthencheckedtheamplitudeofthefrequencyon7,28and50MHz.TheRigolhas a50MHzbandwidthand theTektronixhasa40MHzbandwidth, so Iwouldexpecttoseesomerolloffon10and6meters.Thisdoesn’tmeanyoucan’tlookatsignals,justthattheamplitudeofhigherfrequencysignalsmaynotbecompletelyaccurate.

Mynexttestinvolvedmeasuringtransmitterovershoot.Whenatransceiver’soutputpowerisreduced,often the transceiveroutputwillovershoot (behigher than) thesetpoweron the firstCWcharacterorspeechsyllable.Thishappensbecauseafinitetimeisrequiredforthetransceiver’sALCtocontrolthesignal.Ifovershootishighenough,itcantripprotectioncircuitryinanexternalRFpoweramplifieroreven damage it. For this test I set my Icom IC-706MKIIG transceiver to 25W output, as this is theapproximatedrivepowerneededforfulloutputfrommyElecraftKPA500amplifier.

NextIwantedtolookattheamplifierenable/disabletimingversustheRFsignaloutput.Thistimingisimportantwhendrivinganamplifiertoensurethatnohotswitchingoftheamplifierortransceivertakesplace.(Hotswitchingmeanstransmittingasignalbeforerelaycontactshaveclosed.)Theampkey-to-RFsignalandRFsignal-to-ampunkeytimingsarebothimportantbecauseyouwanttomakesurethatthereisno chance of hot switching on either amplifier keying or amplifier unkeying. I fed the IC-706MKIIGtransceiver’sHSENDoutputtochannel2ontheoscilloscopeandsettheoscilloscopetotriggeronchannel2.Afallingedgetriggershowstheamp-enabletiming,andarisingedgetriggershowstheamp-disabletiming. I couldhave fedHSEND into theEXTERNALTRIGGER input on the oscilloscope, but Iwanted todisplayHSENDalongwiththeRFsignaltobetterclarifythetiming.

My final test involved two-tone testing of my transceiver. A two-tone test is a standard test of atransceiver’s linearity thatnormally requiresa spectrumanalyzer.However,bothoscilloscopeshaveafastFouriertransform(FFT)mathfeaturethatshouldpermitdisplayofsignalsinthefrequencydomain.For this test,a two-toneaudiosignal is fed into the transceiver’smicrophone input,and thecompositelevel adjusted for 25 W peak output. After displaying the normal modulated RF signal on theoscilloscope, select theFFTmodeandmake sureyouaredisplaying in thedB scale.Use theverticalknobtoselectdB/division,thehorizontaltimingknobtoselecttheHz/division,andcenterthesignalonyourdisplaywiththehorizontalpositioncontrol.

TektronixTBS1042As with many computers and test instruments today, only a condensed version of the manual was

enclosedwiththeTBS1042.Thefullmanual(159pages)isdownloadableonline.Theonlyaccessoriesprovidedwiththeoscilloscopearethe120Vacpowercordandapairof10:1probes(notswitchableto1:1).Formostmeasurements,you’llwant tousea10:1probebecause thecapacitive loadingofa1:1probewillbeaproblemforhigherfrequencyRFsignals.Also,a10:1probeprovidesbetteroverloadprotectionshouldyouaccidentallyconnecttoahighvoltagesource.A1:1probeismostusableforaudiomeasurementsatverylowsignallevels.

Animportant featureofany instrument is itseaseofuse.ThereforeIattempted touse theTBS1042without reading themanual,other thanreadingabouthowtocompensate theprobes.As it turnedout, Iwasabletoquicklysetupandmeasureeverythinginallthetestswithoutcrackingthebook!WhatmakesthiseasyistheAUTOSETbuttonthatsetsuptheunitforyou.JustapplyasignalandpressAUTOSET.Withinafewsecondsyou’llhaveadisplaythatwillbeveryclosetowhatyouwant.Fromthispoint,youcansimplychangetheverticalsensitivityandhorizontaltimingtorefinethedisplaytoyourliking.

TheUSBportonthefrontoftheunitprovideseitherprintorsavefunctions.TheTBS1042determinesiftheconnecteddeviceisaprinterormemorystick,andwilleitherprintorsavethescreendatawhenthePRINTbuttonispushed.

Thefrequencyresponsetestresultedinameasuredrolloffof0.57dBon10meters,and1.67dBon6meters,muchbetterthanthemanufacturer’s3dBspecificationforthe40MHzbandwidth.

For theovershoot test, I setmy transceiveroutput toanominal25Woutput leveland triggered theTBS1042onthechannel1input.Youhaveachoiceofenablingeithertwohorizontalcursorstomeasureamplitude, or two vertical cursors to measure time. I enabled the horizontal cursors to display theovershootamplitude.TheresultsareshowninFigureA2.1.Notethatwithasetoutputpowerof25W,theoutputpeaksat72Vpeak(100W)onthefirstdit

NextI lookedat theamp-keyenable(FigureA2.2) timingwith the transceiverset for fullbreak-in.

The bottom trace is the amp-enableHSEND line from the radio. The results are interesting. The amp-enable-to-RFoutputtimeof15msisfineforvacuumrelaysandPINdiodes.Itisprobablyokayforopenframe relaysusedonmanyampsnotdesigned for full break-in (QSK)operation, but it ismarginal.Atypicalenabletimeforopen-framerelaysis12-20ms.

The amp disable timing (FigureA2.3) shows a problemwith QSK-switched amplifiers. The ampdisable linegoeshighabout4msbeforeRFdrops tozero(theverticalcursorswereenabled tobettershow this). So youmay hot switch an amplifier that is operating inQSK.To be on the safe side, IC-706MKIIGusersshouldonlyoperatesemibreak-in.

Mylasttestwasatwo-tonetestofthetransceiveroutput.FiguresA2.4andA2.5comparethedisplayofaspectrumanalyzer(aRigolDSA815-TG)withtheFFTdisplayontheTBS1042.Asyoucansee,theTBS1042 frequencydisplay is virtually identical to the spectrumanalyzer, very useful for this type ofmeasurement.

Manufacturer: Tektronix Inc, 14150 SW Karl Braun Dr, PO Box 500, Beaverton, OR 97077;www.tek.com.

TheDS1052didnotincludeabbreviatedinstructions,butthefullmanual(166pages)isdownloadableonline.TheDS1052Eincludesa120Vaclinecord,apairofswitchable10:1/1:1oscilloscopeprobes,andaUSBcableforinterfacingtoyourcomputer.

Again,Iattemptedtousetheoscilloscopewithoutreadingthemanual,otherthanthesectiononprobecompensation.AndagainIhadnoproblems.TheAUTObuttonontheRigoloscilloscopeisequivalenttotheAUTOSETbuttonontheTektronixoscilloscope.Afterapplyingasignal,pressAUTOandthenadjusttheverticalsensitivityandhorizontaltimingtorefinethedisplaytoyourliking.

The only thing I had problems with was saving the display to a USB memory stick. The SAVEprocedureisveryflexible,permittingyoutosavedifferentformatsandevenpermittingyoutonamethefiles.TheSAVEprocesswasn’tintuitive,requiringmetorefertothemanual.

Thefrequencyresponsetestwasinteresting.Thespecificationisfora3dBrolloffat50MHz,butIfoundnorolloffatallon6meters.

Fortheovershootandamp-key/unkeytimingtests,IfoundIcoulddisplayeverythingatthesametime.When Iwent to theTRIGGERmenu I found that oneof theoptionswas triggeringonbothnegative andpositive going slopes of the triggering signal. This let me look at the amp-key HSEND going low ontransmit and high on un-key, and the resultant RF signal— including overshoot. The resulting timingwaveformisshowninFigureA2.6.Thebottomtraceistheamp-enable/disable(HSEND)lineoutoftheIC-706MKIIG.

Ialsotestedtheovershootandamp-disabletimingseparatelysoastoprovidedetailsimilarwiththeTektronixTBS1042 tests. I enabled thevertical cursors inbothcases so as tomoreeasilydisplay thetime.Fromthedetailedview,Ifoundthatthefirst-ditovershootlastslessthan2ms(FigureA2.7)andonun-key,RFisstillbeingoutputabout4msaftertheampkeylinehasgonehigh(FigureA2.8).

ForthefinaltestIattemptedatwo-tonetestasI’ddonewiththeTektronixunit.FigureA2.9showsthetime-domaintwo-toneRF-modulatedsignal.ApparentlytheRigolDS1052Edoesn’thaveenoughbuffermemorydepthforthenecessaryresolutionfortwo-tonetesting(thebuffermemoryiswherethecapturedsamplesarestored).Thereisplentyofresolutiontoshowthemainsignalanditsharmonics,butclose-insignalresolutionisnotpractical.

Manufacturer: Rigol Technologies Inc.,7401 First Place, Suite N, Oakwood Village, OH 44146;www.rigolna.com.

SomeFinalObservationsIdidnoticeafewotherdifferencesbetweenthesetwooscilloscopesthatareworthwhiletopointout.Bothoscilloscopeshavea5.7 inchdiagonalcolordisplay.However,youcanturnoff theright-side

menu on the Rigol, which provides a little more display area than on the Tektronix. The Tektronixoscilloscope takes about 30 seconds to boot up,whereas the Rigol is up and running in less than 10seconds.Also,theTektronixtakesabout30secondstosaveafiletoaUSBmemorystick,whereastheRigoltakesabout1second.AndIdidliketheRigol’sabilitytotriggeronbothapositiveandnegativetriggeronthesamedisplay.However,theTektronixoscilloscope’sabilitytodisplayafrequencydomaintwo-tonetestspectrumisimportanttome.

Somewhatoff-topic, Iwould like toencourage theARRLLab to include transceiverovershoot andtransceiveramplifierenable/disableoutputtimingmeasurementswithreviewsofHFtransceivers.Theseparameters are becoming increasingly important when interfacing a transceiver to an amplifier —especiallywhentheamplifierissolid-state.

ConclusionForhamswhowanttostepuptothenextleveloftesting,troubleshootingandunderstandingequipment

performance,anoscilloscopebecomesmoreofanecessity.Fortunately,digital samplingoscilloscopeshave become surprisingly affordable. The two oscilloscopes discussed herewill providemost of thecapabilitiesdesiredbythemoresophisticatedhamatapricethatiseasilyjustifiable.

Appendix3

QSTProductReview:OsciumiMSO-204PortableOscilloscope

Thisreview,byPaulDanzer,N1II,originallyappearedinJune2014QST.Someyearsago(OK,manyyearsago)Iwasayoungengineer,assignedtoconducttestsonaUSNavy

shipusedtoevaluatevariouselectronicdevices.WhenIdroveuptothepierlateoneSundayevening,themenondutyatthegangplankhadanordertoallowmeontotheship,buttherewasnooneelsearoundtohelp me. The test was supposed to start early Monday morning and I had a 70+ pound Tektronixoscilloscope thathad tobe carriedup three flightsof stairs to the test room.Thenavycalls the stairsladders;Icalledthemtorture—70+pounds,threeflightsofstairs!

IhadforgottenaboutthisincidentuntiltheOsciumiMSO-204portableoscilloscopewasdeliveredtomy door. And portable it is! It’s shirt pocket size, and the weight is obviously not a problem— theproblemistonotloseitinyourpocket.

TheoscilloscopeconnectstoanAppledevice—iPad,iPhoneoriPod—suppliedbytheuser.TheApple device runs the control software and provides the oscilloscope display and user interface. Ofcourse, thedisplay isbest seenwhen the scope isplugged intoan iPad,whichhasanominal9.7 inch(diagonal)display,ortheslightlysmaller7.9inchiPadmini.BepreparedtolookcloselyifyouusethescopewiththesmalldisplayonaniPhoneoriPod.

Generally speaking, thedisplayscalesarecontrolledby the touchscreen techniquesyouexpect formobiledevices.Onceyoupickthecorrectmenuitem,youcanpinchtomakethescalesmallerorslideyourfingerwidertomakeitbigger.Givenalist,slideyourfingerupordownfromthecurrentsettingtothenewsetting.

TherearetwoOsciumoscilloscopemodelsavailable.ThisreviewfeaturestheiMSO-204,whichhastwoanalogandfourdigitalchannelinputs.Thelesscapable(andlessexpensive)iMSO-104hasasingleanalogchannelanddifferencesinthedetailedspecifications.

Given its size, can the iMSO-204 really replace a scopewith abuilt-in6 inchdisplay and a frontpanelfullofknobs?Willitreallyreplacealargercomputer-controlledunitthatrequiresaPCorlaptoptowork?

WhyisItSoSmall?The answer, of course, is that with the increasing speed and accuracy of analog-to-digital (A/D)

converters,lessanalogcircuitryisneededupfront.Otherthansomesignalswitching,selectionofacordccouplingandperhapssomeprotectioncircuitry,youcouldgodirectlyintoanA/Dconverterandsendthe output to an external digital processor.Voltage scaling, triggering, level shifting and formatting fordisplayisalldoneinsoftware.

Thisisparticularlytruefordigitalinputs,wheretheA/Dconvertersarenotneeded.Osciumspecifiesthedigital inputs for–0.5 to+7V.Dependingon theprobe settings (×1or×10), theanalog inputs arelimitedto–8to+13Vor–40to+40V—certainlysufficientfortoday’ssolidstatecircuits.

GettingStartedFirst you have to download the software (free, of course) from the Apple store (go to

www.oscium.com/oscilloscopes/imso-204andclickthedownloadbuttononthispage).Next,connecttheprobes.Theprobesforthetwoanaloginputchannelsslidein,noproblem.Touse

thefourdigitalchannelinputs,thedigitalconnectors—fourwiresplusaground—havetobepluggedintoasetofprobes.Lookcarefully.Thematingpinsontheprobesareatanangle—thewiresleevesslideinatthatangletoconnect.Amagnifyingglassmighthelp!Theseconnectionsarefrictionfits,soahardpullmaydisconnectaprobefromitsconnectingwire.

Tofireupthescope,turnoffyourAppledevice(fromnowonIamjustgoingtorefertothedeviceasan iPad). Then plug the scope into the iPad connector. In the last year or so Apple has changedconnectors.Thescopehasanolder30pinconnector,butOsciumoffersanoptionaladaptertoconnectthescopetodeviceswiththenewerLightningconnector.

Finally, turnonyour iPad, select theOSCIUM icon, andyouare set.Perhaps the first thingyouwillwanttodoisexplorethefunctions.Bytouchingtheiconinthelowerrightcornerthescopegoesintoademomode,wheremost,butnotallfunctionsareavailable.

AnalogandDigitalScopeFunctionsThescopecomesequippedwithtwoconventionalanalogprobesandtwoanalogchannels.Thetrigger

iconatthebottomallowsyoutoselectthetriggeringsourceandmode.FigureA3.1showsthewaveformsfromtwoanalogsignals.

Somewhat unusual are the four digital channels. They do not use anA/D converter— the probesconnectdirectlytodigitalcircuitryinthescope.Whenworkingwithdigitalcircuits,oftenyouwouldliketolookatmorethantwosignalsatatime.FigureA3.2showsthewaveformsfromfourstagesofa4790digitalcounterchip,wiredasadivide-by-10circuit(tracesD1toD4atthebottomofthescreen).Muchtomysurprise—althoughnotcapturedinthescreenshot—oneofthestagesshowed“snivits”—short,narrow spikes due to slightmistiming built into the circuit. Since the snivitswere on an intermediatestage,theydidnotaffectthedivide-by-10feature.

AGood,ClearPictureWith older analog scopes, to get a good, clear picture you have to carefully adjust intensity (or

brightness),focus,andtriggering.WiththeiMSO-204,thefirsttwoarenolongeraconsiderationbecausethedisplayissynthesizedinthedigitalrealm.Triggeringisstillacriticalfactor.Thisscopedoesnothaveanexternaltriggerinput,butdoesofferawidevarietyoftriggercapabilitiesfromtheselectedinputs.

When using the digital inputs, any of the four can be selected as the trigger. This is a very handyfeaturewhenlookingatseriescircuitssuchascounters.

Thetwoanaloginputs,AandB,maybecombinedorselectedinvariousways—either,alternate,orbothaswellaspositive,negative,dclevel,upslopeanddownslope—inotherwordsafullassortmentof trigger capabilities.By touching theTRIGGER iconon thebottomof the screen, the trigger selectionmenu inFigureA3.3 comesupandyoucan select theactual trigger level.Your selection immediatelygoesintoeffectafteryounexttouchablankareaofthescreenandyourwaveformthenbecomesvisible.

TheDisplayAt the riskofupsetting touchscreen lovers, remember that the screenonmanydevicescanbemore

sensitivethanyouexpect.Somesettingsrequiremorethanonetaporacombinationoftapsandswipes.Onoccasion,thedevicewillbringupmenusandfunctionsyoudidnotexpect.Osciumhasdesignedtheirsoftwaresothatifyoutouchanunoccupiedportionofthescreen,usuallythesoftwarewillreverttothepreviousstateordisplay.

Thescopetechnologyusedisoneofsamplingtheinputsanddigitallystoringtheresult,soonscreenstorage in a built-in function. In the lower right corner ofFigureA3.4 is aPLAY icon that sets eithernormalscopedisplaysorastop-storeddisplay.Youcanlookataneventeitherwithamanualfreezeoranautomaticallytriggeredfreeze—veryhandywhenchasingtransients.

TheOscium software offers two functions for capturing and saving oscilloscope screen shots.Onecaptures the screen so it canbee-mailed (Ididn’t try this).Theother stores a screen shot to the iPadphotostorage,butthemanualsuggestsyouusetheiPad’sbuilt-inpicturestorefunction,whichishowthescreenshotsinthisreviewwerestored.TheiPad’sscreencapturefunctionrequirespressingtwobuttons,andIfoundit“touchy,”sometimesrequiringacoupleoftries.Justblameitontheoperator!

Built-InSpectrumAnalyzerMany, if not most, of today’s digital oscilloscopes include both an FFT (fast Fourier transform)

analysis/display function and a signal/function generator. This scope does not include a generator, butdoes offer FFT, integration and differentiation. Admittedly it is not clear just how applicable theintegrationanddifferentiationfunctionsaretomosthamprojects,butifyouhavethem,youwillprobablyfindauseforthem.

Thereisalsoasetof15veryhandyautomaticmeasurementroutines.Theseinclude,toquotefromthemanual:Minimum,Maximum,Mean,Peak toPeak,RMS,DutyCycle (+),DutyCycle (–),PulseWidth(+),PulseWidth(–),CycleMean,CycleRMS,Frequency,Period,RiseTime,andFallTime.Selectingthesefunctionsisquitesimple,butunderstandingexactlywhatyouaremeasuringtakesabitofpractice.

IsItCapableEnough?Theanswertothisquestiondepends,ofcourse,onwhatyouwanttodotheiMSO-204.Thebandwidth

isspecifiedas5MHz,butyoucaneasilyseesinewaves—RF—to14MHzandbeyondwithoutknownamplitudecalibration.However,itisasamplingscope.Thehigherthefrequency,thefewerthesamplespersinewave,soperhapsthewaveformshownathigherfrequenciesislessrepresentativeoftheactualsignal.InFigureA3.4youcanseetheresultofthesamplingprocesswheretheactualinputwaveformisasmooth sinusoid but the display shows some increments. As you increase the input signal frequencybeyondthe5MHzbandwidthnumber—wellbeyond—thiseffectbecomesmoreandmoreobvious.

FigureA3.5isascreencaptureofa7.15MHzSSBsignal,takenatlowpoweratadummyload.Thisispasttherated5MHzbandwidth,andthescopewastriggeredtoshowsyllables.Similarresultswereseenat14MHz,butofcoursetheverticalcalibrationdoesnotholdpasttheratedbandwidth.

Oneimportantfactor—keeptheinputwaveformamplitudedownwithinthespecifiedlimits.Evenifyoudonotdamageanything,thewaveformwillbedistortediftheinputA/Disnearitsamplitudelimit.

InSummaryIfyouneedsomethingveryportable—forexampleonARRLFieldDayorwhengoingtoarestricted

spacetotrackdownatransientonthelocalrepeater—andyoualreadyownaniPadorsimilardevice—thismightbeagoodchoice.The5MHzbandwidthiscertainlyalimitation,butitdoesshowhigherinputsignalfrequencies.

SomeskillandpracticeisneededtoconqueramultifunctioniPadwiththeOsciumsoftware.It’snotassimpleas,say,selectingaphototoview.Aswithmostcomplexequipment,theinstructionbookdoesnot cover everything. Some features are self-explanatory as seen on the screen; others not in the bookrequiretrialandexploration.

Therearesimilarproducts,andatleastoneaboutthesamesizeandwithsamebandwidth.Howevertheyrequirealaptop(orPC)andthusarenotasportableandconvenient.

Asareplacementforyournormalbenchoscilloscope,thisisprobablynotthebestchoice.Butforitsuniquecharacteristics—sizeandportability—itworks,andwellenoughtoincludeinyourproverbialportabletoolbox.

To get a better idea of how the iMSO-204works, you can download themanual from theOsciumwebsiteand the software from theApple store.Themanual explainshow to turnonanduse thedemomodewithouthavingthescopehardware.

Manufacturer:Oscium, A Dechnia LLC, 5909 NW Expressway, Suite 269, Oklahoma City, OK73132;www.oscium.com.