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With R-E's EZ Shortwave ReceiverIf you're looking for a fun project that won't break the budget,here's a shortwave receiver that's not short on performance.

RODNEY A. KREUTER WA3ENK

IF YOUVE ALWAYS WONDERED WHAT

you might hear on shortwave ra­dio but didn't have the time orthe money to get involved with it,then our simple, inexpensiveshortwave receiver is the perfectproject for you. It will have youtuning in on shortwave in notime-as a matter of fact, the firstevening after the prototype wascompleted, stations from Ger­many, England, Cuba, Canadaand France were easily copied.

The shortwave receiver is a no­crystal set with an audio ampli­fier. It's a true superheterodynethat tunes 8.5 to 11 MHz in twobands and includes a 455-kHz IFfilter, automatic gain control(AGCl. tracking RF tuning, and avery sensitive detector. It's sen­sitivity of under a microvolt putsit in a class with some high-per­formance receivers .

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o Theory of operation~ The shortwave receiver takes ag: very straightforward approach to&3 the classic superheterodyne re­m ceiver. The basic block diagramo of our "superhet" is shown in Fig.is I, and the schematic is shown in<l:a: Fig. 2 .

56

The RF input tank, unlikemany simple designs, provides"tracking," in that the inputtuned circuit changes frequencywhen the oscillator is tuned. RFtuning is performed by 01, andoscillator tuning by 05. Both di­odes are Motorola MV209 varac­tors, wh ich act as voltage­variable capacitors. RF energy iscoupled into pins 1 and 2 of ICl,the Signetics NE602 double-bal­anced mixer.

The mixer combines the in­coming RF signal with the localoscillator and produces an inter­mediate frequency or IF of 455kHz. Both mixer and oscillatorfunctions are provided by ICL Ta­ble 1 shows its specifications.

To simplify construction andenhance performance, a ceramicIF filter, FLl, is used instead of amore common tunable IF trans­former. That results in a veryclean IF that never needs tuning.The filters are available withbandwidths from 4 to 12 kHz tosuit individual needs. The short­wave receiver will accept filterswith input and output imped­ances of 2000 ohms.

Thrning to the detector circuit,

02 and 03 provide a L2-volt b iasfor diode 04 and Q3 . The b iaskeeps both 04 and Q3 slightlyon, so only a small signal is nec­essary for detection, reducing thegain needed before the detectorand improving sensitivity.

The s ignal at the base of Q3contains two components. TheAC component is the dem odu ­lated audio, and the DC compo­nent is proportional to thestrength of the incoming signal.The DC component is filtered byR20 and C17 and is used to pro­vide an AGC signal to Q2 via AGCamplifier Q4 . That helps to re­duce fading that is so commonon the shortwave bands.

The audio output stage, IC2, isa Motorola MC34119 audio am­plifier. It provides about V4-watt ofaudio into speakers of 8 to 64ohms. No large output-couplingcapacitors are needed, but a largepower-supply decoupling capaci­tor provides excellent stability.

The prototype operates on a 9­volt battery and, if you listen atmoderate volumes, they give youreasonable service. For longerservice, use a pack of 6 or 8 "AA"cells, or an AC supply.

TUNINGVOLTAGE

IFAMPLIFIER

04

AUDIOBUFFER

03

AUDIOPOWER AMPLIFIER

VOLUME

SPKR

FIG. 1-BASIC BLOCK DIAGRAM of ou r superhet shortwave receiver. It's a true super­heterodyne des igned to tune 8.5 to 11 MHz in two bands.

+9V R13- [ +5V 10K +9V~ C4 C6 ~ R2

C9 R14 R1902 10K 10n1 1 0~F .1I 15K 1N4148

!03':' LJJ ':' 03

.1 1N4148 2N3904':'

sr-a 2 8 Rl 02

5 470n 2N3904

IClNE602 R8 R12 Cl0 C12

5.6K 470n .1 .01

':'R7 R9

C7.1 220 22K R21 R20

470K C17! 220K.1

':' AGC

+9VR22 IC2 5

1 4.7K MC3411 9si-e

12 C15 17~R23 R25 R26 BAND 18 .5-10 MHz + 4.7f1F + + C14':' C16

50K 10K lK BAND 210-11 .5 MHzl~F 470~F

TUNING FINETUNING

FIG. 2-SCHEP,'IATIC FOR THE SHORTWAVERECEIVER. The un it is powered from a 9-voltbattery, making it very portable. It's sensitivity of under a microvolt puts it in a class withsome very high-performance receivers.

Modifications and compromisesEvery engineer learns early on

that to design is to compromise.Usually performance is traded offfor reduced cost. This design isno exception. The basic designphilosophy was to produce a rea­sonable receiver at a reasonableprice. In tha t regard we 're veryhappy with th e outcome. We did,however, omit some features, as aresult .

Most modern shortwave re­ceivers include a beat-frequencyoscillator or BFO . The purpose ofthe BFO is exactly as its nameimplies. to beat a local oscillator(LO) signal against the incomingRF to produce a heterodyne fre­qu en cy in order to copy code (CW)or single side band (SSB). Thatcan be done at either the RF fre ­quency or the IF, although IFBFO 's are much more common.

The shortwave receiver's inputcoupling network provides tun­ing and impedance matchingfrom the 50-ohm antenna inputto the 1500-ohm input of theNE602 . A really good rece iverwould use double or even triple ~

tuning h ere. for better image re- Ejection and overload perfor- 53mance. -<

Images, which are produced in cDthe mixing of two signals . are (0

5~

TABLE 1- BASIC SPECIFICATIONS OF THE NE602 PARTS LIST

board u sing point -t o-point wir­ing.

1000pF 820pF 15QOpF

TgN%:~rLt;-:CEIVER6800

PFLJ 2200pF

3.4~H 4 . 8~IH

29 TURNS 30 AWG 35 TURNS 30AWGONT-37-2 GORE ";" ON T-37-2 GORE

FIG. 3-THIS HIGH-PASS FILTER will at­tenuate AM stations by 40 dB. Its low-fre­quency cutoff is about 2.2 MHz.

ConstructionEven though th is is a low-fre­

quency project, a PC board is rec­ommended ; you can make you rown from the provided foil pat­te rn or buy a finished vers ionfrom the so u rce mentioned in theparts list. Fi gure 4 s h ows theparts-placement di agram.

Inductors Ll, L2, an d L3 arewound on toroid cores, so they'remuch smaller than air-wo u n dcoils, an d can s till be "tu n ed" bys tretch ing or co m press ing thetu rns on the to ro ids. Rememberthat a turn is cou nted on a toroideve ry tim e the wire p a s s esthrough the cen ter of the core.After you tune the co ils , the wireca n be h eld in place with epoxy.

Any speaker from 8 to 64 oh mswill work wit h the MC34119. Ex­pect s ligh tly less a u dio outputwith higher-impedance s peak­ers. The speake r leads should betwisted tightly and kept sh ort .

If you us e stereo headphones,don't con nect the grou n d . J ustfeed the speaker output th roughresistors (you 'll n eed to experi­ment with the value) to the leftand right chan nels. Note that theMC34119 does not grou n d refer­ence the s peaker.

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Power supplyCurrent consumptionMaximum mixer frequencyMaximum oscillator FrequencyNoise figureMixer gainThird order interceptMixer input resistanceMixer input capacitanceMixer output resistanceMixer output capacitance

very hard t o e li m ina te . Re­member th at the output of a mi x­er is the sum an d di fference oftwo fr equenci es . For exam p le,suppos e we wanted to receiveWWV on 10 MHz us ing an IF of455 kHz. Usi ng low oscillator in ­jection, we wo uld generate a localoscilla tor of 10 MHz minus 455kHz , or 9.545 MHz.

However, if a frequency of 9.09MHz was also present at the mi x­er input, we 'd also h ave an out­put fr equ en cy of 455 kH zbecause 9.545 MHz m inus 9. 09MHz equals 455 kHz. That otherundesired fre quency (9.09 MHz)is calle d the image .fr eq u en cy,Some sophisticated techniques ,s uch as image-reje ct mixers orup-converting receivers are avail­abl e, but almost all receivers re­ject the 9 .09 MHz at the inputtank. The tracking RF tan k onour s hor twave rec eiver helps agreat deal, but doesn 't elim inatethe problem.

Overload p erformance is an ­other impor tant aspect co n ­cern ing a shortwave receiver. Ifthe RF tan k is tu ned to 10 MHz, itwill le t lO-MHz s ign als pass an da tten uate- b u t not elim in a te­signals of all other frequencies. Ifa 50,000-watt AM s tatio n is lo­cated close to the tank, some ofthe s ignal w ill get through . Ifenough of it does , you 'll h ear theAM station as well as the short­wave.

Tes ts on ou r active ante nna(Radio-Electronics, February1989) proved that an AM-rejectfilter was necessa ry to "clean up"our ow n local 50-kilowatt s ta­tion. A h igh-pass filter that willattenuate AM s tations by 40 dB isshown in Fig. 3; its low-frequen­cy cu to ff is abou t 2. 2 MHz. Thefilter can b e co nst ructe d on apie ce of perforated construction

4.5V- 8V2.4 mao500 MHz200 MHz5 dB15 dB- 17 dBm1.5k3 pF1.5K3 pF

(typical)(typical)(typical)(typica l at 45 MHz)(typical at 45 MHz)(maximum)(typical)(typical )(typical)(typical)

All resistors are 1f4-watt, 5%.R1, R12-470 ohmsR2- 15,OOO ohmsR3, R11-3300 ohmsR4, R6, R1 Q--l 00 ohmsR5-2200 ohmsR7- 220 ohmsR8-5600 ohmsR9-22,000 ohmsR13, R14, R1 7, R24, R27- 10,OOO ohmsR15, R2Q--220,OOOohmsR16, R25-lO,OOO ohms, potentiometerR18, R21-470,OOO ohmsR19-10 ohmsR22-4700 ohmsR23-50,000 ohms, potentiometerR26-1000 ohmsCapacitorsC1-9-60 pF trimmerC2-82 pF, ceramicC3-100 fl.F, 16 volts, electrolyticC4, C21- 10 fl.F, 16 volts, electrolyticC5-C7, C9, C10, C13, C17---O.1 fl.F (poly-

ester or ceramic)C8, C11 , C12, C25-0.01 fl.F, ceramicC1~70 fl.F, 16 volts, electrolyticC15-4.7 flF, 16 volts, electrolyticC1-1 fl.F, 16 volts, electrolyticC18-C2Q--100 pF, NPOC22, C24---Q.001 fl.F, ceramicC23-47 pF, NPOSemiconductors01, 05-Motorola MV209 varactor02-04-1N4148 diodeFL1-455-kHz ceramic filter (8-kHz band-

width Toko HCFM2-455C)IC1- Signetics NE602N mixerIC2-Motorola MC34119 audio amplifierIC3-78L05 5-volt regulatorQ1-Q4-2N3904 NPN transistorOther componentsL1-3 turns #30 wire on L2's toroid coreL2- 21 turns # 30 wire tapped at 17 turns

on T-37-2 Micro Metals toroidL3-24 turns #30 wire tapped at 19 turns

on T-37-2 Micro Metals toroidS1-0POT switchS2, S3-SPST switchSPKR--8--64 ohm speakerJ1-BNC connectorMiscellaneous: PCboard,metal cabinet,

wire, solder, etc.Note: The following items are available

from a -Sat, P.O. Box 110, Boalsburg,PA16827.A PC board only (# EZSW­PCB) is $7 plus $1 shipping; a partialkit that includes a PC board (doesnot include potentiometers, switch·es, connectors, speaker, or case #EZSW-KIT) is $27 plus $2 shipping; akit for the AM reject filter (# AMREJ­KIT) is $3 plus S1 shipping; a kit forthe active antenna (# ACTANT-KIT,see text) is $15 plus $2 shipping.Pennsylvania residents must add6% sales tax. Allow 3-5 weeks fordelivery.

All receivers n eed a good anten­na; th is one is n o exception . Al­though the first field trials werecon ducted in a s tate park with 30

feet of wire thrown over a treelimb, a good antenna will greatlyimprove reception. A dipole willgive good results, but if you'recramped for space, try an activeantenna (see Radio-Electronics,February 1989). A good rgrou n dalso helps.

The receiver should be in­stalled in a metal cabinet to re­duce the effects of hand capaci­tance and provide some shield­ing from strong local AM sta­tions . Figure 5 shows theprototype receiver. Note that theactive antenna and the 2 .2-MHzhigh-pass filter are used in theprototype, although they are notmandatory. The holes for thespeaker were made using a neattrick: Draw the outline on a pieceofperforated construction board,and tape the board to the cab­inet. Then use the board as a drillguide.

Table 2 is a gutde to let youmodify the receiver for frequencyranges other than 8.5 to 11.5MHz (actually 8 .5-10 MHz forband 1 and 10-11.5 MHz for band2) used in the prototype. Don'tthink of L2 and L3 as tappedcoils, but rather as "selectable"coils. For example, L3 is specifiedas a 24-turn coil with a tap at 19turns. What that really means isthat a coil of either 19 turns or 24turns is switch-selectable. Youcould even wind a 45-turn coilwith taps at 14, 15, 17, 19, 24, 2~,and 34 turns for L3. With theright switch (good luck findingone), you could tune 5 to 16 MHzin 8 bands. Remember that it hasto switch the capacitors, as well .

Since the coils must be hand­wound, there will be some varia­tion. Wire size was calculated forno. 30 wire. Other wire sizes maybe used but you will find it hardto get as many as 45 turns on aT-37-2 core with larger wire. Thespacing of the wire on the corewill also change the tuning fre­quency. The values are given asreasonable starting points. If youwish to build the receiver forsome frequency other than theprototype, follow these steps:1) Build the unit completely ex­cept for the two coils .2) Using Table 2 , wind the os­cillator coil. Tack the coil into thecircuit from ground to the junc­tion of C20, C22, and C23. (Thatway you won't need the bandswitch.)

TABLE 2

Frequency C2 C18, C19 C23 L1, L2 (Ant) L3 (Osc)(MHz) (pF) (pF) (pF) (# of turns on T-37-2 core)

5 100 120 68 5,41 456 100 120 68 4,30 347 82 100 47 4, 26 298 82 100 47 3,22 2410 82 100 47 3,17 1912 82 100 47 2, 15 1714 68 82 33 2, 14 1515 68 82 33 2, 13 14

TABLE 3

NE602 MC34119

Pin 1 1.27 V OVPin 2 1.27 V 4.15 VPin 3 OV 4.11 VPin 4 3.64 V 3.97 VPin 5 3.59 V 4.14 VPin 6 4.99 V 9.09 VPin 7 4.33 V OVPin 8 5.05 V 4.20 V

02 03 04 '-01 »zEmitter 0.95 V 0.80 V 0.27 V OV c»Base 1.61 V 1.45 V 0.82 V 0.58 V :x:

-<Collector 2.56 V 3.30 V 9.17 V 7.41 V ~

(C(C~

5!

FIG. 5-THE INSIDE OF THE PROTOTYPE RECEIVER. Note that the active antenna andthe 2.2-MHz high-pass filter are used in the prototype, although they are not mandatory.

3) Lightly cou ple a high-imped­ance scope or frequency cou nterto pin 7 of IC1 ; note that theNE60 2 will not drive a 50-ohminpu t withou t a buffer. A lO-pFseries capacitor is therefore rec­om men de d.4 ) Turn the tu n ing and fin e tu n­ing, if you are u sing one, com­pl etel y co u nte r cl ockw ise andmeasure the frequency. Now turnthe tuning and fine tuning all theway clockwise an d measure thenew frequency. If it's lower thanthe first frequ en cy, you've got the

TroubleshootingIf you have any problems, the

DC voltages shown in Table 3should help. All voltages weretaken with a new 9-volt alkalinebattery powering the receiver.The volume con trol is about V3with no s ign al input. Total cur­rent is 22 mill iamps. R-E

provide a separate, stable tuningvoltage of up to 20 volts. Sincethe cu rren t drawn by the diodesis in the microamp range, a sepa­rate 9-volt battery may be used.Just remember that as the bat­te ry ages, the tuning range willchange.

· 6 ) If you are satis fied with thetuning range, wind the antennacoil with a tu rn or two less thanthe oscillator coil. This is neces­sary becaus e the input tuned cir­cu i t operates a t 0.455 MH zhigher in frequency that that ofthe oscillator.

The varactors used in this re­ceiver only need to vary by about15 pF to cover 8. 5 to 10.0 MHz or10.0 MHz to 11.5 MHz. That canbe from 25 to 40 pF, or 0 to 15 pF,or any combination that gives achange of about 15 pF. When thebias voltage is changed from 1 to5 volts, the capac itan ce reallychanges from about 40 pF to 26pF. If a well-regulated supply ofhigher than 5 volts but less than20 volts is available, it may beused to increase the tuningrange. Since we're running it ona 9-volt battery, we decided to reg­ulate down to 5 volts. If you de­cide to operate the varactor on ahigher voltage, remember thatthe NE602 is rated at a maximumof 8 volts. The high side of thetuning potentiometer can be con­nected to a higher voltage as longas the connection from the PCboard to the high side of the po­tentiometer is left disconnected.

Tuning 1500 kHz with a single­turn potentiometer can be tricky.A "poor mans ten turn" can bemade by putting a 10K potenti­ometer in series with the normal50K potentiometer for fine tun­ing. Be careful with the leads go­ing to the potentiometers; any ACsignal will "modulate" the os­cillator with disastrous results.Since the tuning of a varactorisn't linear with voltage, you maywant to experiment with dif­fer ent potentiometers, such aslinear, log, or audio.

J1EXT.ANT.

RECEIVERBOARD

potentio meter in backwards .5) Add 0.455 MHz to the two fre­quencies that you h ave jus t mea­sured. This is your tuning range.

If you a re building the unit fora higher frequency range, say onthe orde r of 14 or 15 MHz, you willfin d th at the tu n ing range is 2 or3 MHz. On the other h and, unitsbuilt for 3 or 4 MHz will tu ne onlyabout 0 .5 MHz. That is caused bythe r a t her s mall capac i tancechange of the MV209. Typically;capacitan ce vs. (revers e) voltageof the MV209 is 40 pF at 1volt, 26

pF at 5 volts, 14 pF at10 volts , and 9 pF at 20volts.

L o w - fr equ en c ytuned circu its requiremore capacit ance thanhigh-frequency tunedcirc u its . S ince thechange in capacitanceof the MV209 is fixed,it becomes a s malle rpe rcen tage c ha ngewit h low- frequen cytanks than wit h h igh­frequency tan ks. Andyou can forget about aseries or pa rallel com­bination of MV209's.The percen tage worksout the same as a s in ­gle one. If you requ irem ore tu n ing rang e ,the best method is to

VOLUMES2

POWER

ACTIVEANTENNA

BOARD

HIGH-PASSFILTER

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