Test Procedures 25.41

with a whip antenna at the probe inputconnector. Due to its sensitivity and accu-racy, the probe can be adapted for use inmany places around the shack.

The non-linear response of the probediode is compensated (improved) by a cir-cuit in the meter unit. The feedback loop ofa CA3160 op amp contains a diodematched to the one in the probe (see Note1). Fig 25.63 and its sequence of equationspresent a very simple sketch of howmatched diodes do this when dc is appliedthrough a diode. The final equation showsthat any difference between the op ampinput and output is due to a difference involtage drops across the two diodes. Whenthe diodes are matched, the error disap-pears. When RF is applied, the averagecurrents through the diodes must be equalto keep the voltage drops equal. Articlesby Kuzdrall (Note 1) Grebenkemper8 andLewallen9 are first-class descriptions ofthe principles used in this RF voltmeter.

Fig 25.64 shows three pots for calibrat-ing the meter unit. This should be per-formed at 400 Hz to avoid any effects dueto RF. The 100-kΩ pot is typically used tonull the offset of the CA3160, but is usedhere to initially set the offset to about0.5 mV to 1.0 mV with 100 mV input. Thenthe 1-MΩ pot sets the output to 100 mVwith 100 mV input, and the 10 kΩ pot setsthe output to 3.0 V when the input is 3.0 V.Finally, the three pots are alternately ad-justed until the 100 mV and 3.0 V set pointsoccur together. The 100-kΩ pot is helpfulin fine tuning the 100-mV point. A DVMwas used at the optional OUTPUT socketsduring calibration.

A CA3160 was selected for the input opamp because of its high input impedanceof 1.5 TΩ and high gain of 320,000. Al-though it has diodes that provide protec-tion, I think it could be sensitive toelectrostatic discharge, so handle it care-fully. Use IC sockets to permit easy re-placement of the ICs in case of damage.An LM358N IC follows the CA3160 (seeFig 25.64), primarily to drive the panel

Fig 25.65 — Inside view of the meterunit. RG-174 connects the INPUT andOUTPUT banana binding posts to thecircuit board. The rotary switch, one-lug tie strip and battery are also visible.

meter. If you use a DVM as the display,you can omit the LM358N circuit, meterand multi-position switch. The only func-tions lost are the battery-voltage check andpower on/off switch. Add an on/off switchto the circuit when the multi-positionswitch is omitted. The CA3160 op ampcircuit easily spans the range from 100 mVto 3.0 V without the need for a rangeswitch.

CONSTRUCTIONA 5×5×41/2-inch sloping-front instru-

ment case was used to house the compo-nents of the meter unit. The meter onthe front panel has a 2×2-inch face andrequires a 11/2-inch hole in the panel.Although a 50-μA meter from the junk boxis used here, a 1-mA movement will workas well, provided the series resistors arechanged accordingly.

The rotary switch has two poles and fivepositions for changing the meter range, test-ing the battery condition and switching thepower off. Two sets of double banana bind-ing posts are used, the INPUT pair acceptsthe dc signal from the RF probe. The OUT-PUT pair provides a voltage for a DVM dis-play, whether the panel meter is used ornot. On the rear of the case, a miniaturephone jack accepts 9-V power from eithera battery or a 9-V dc supply. The ICs andother parts are mounted on a RadioShackmultipurpose PC board that has very con-venient holes and traces. The board is boltedto the back of the case via standoff insula-tors and wired to the front panel compo-nents (see Fig 25.65).

Notes1J. A. Kuzdrall, “Linearized RF Detector

Spans 50-to-1 Range,” Analog Applica-tions Issue, Electronic Design, June 27,1994.

2Mouser Electronics, 2401 Hwy 287 N,Mansfield, TX 76063; tel 800-346-6873,fax 817-483-0931; E-mail sales@mouser.com; www.mouser.com.

3United States Plastics Corp, 1390Neubrecht Road, Lima, OH 45801-3196,

tel 419-228-2242, fax 419-228-5034.4If the power remains constant, load mis-

match multiplies the voltage by theSWR. A 2:1 SWR would produce 600 V,3:1 900 V and so on.—Ed

5ABCs of Probes, Tektronix Inc, Literaturenumber 60W-6053-7, July 1998. Tektronix,Inc. Export Sales, PO Box 500 M/S 50-255Beaverton, OR 97707-0001; 503-627-6877.Johnny Parham, “How to Select the ProperProbe,” Electronic Products, July 1997.

6Pomona Test and Measurement Accesso-ries catalog. ITT Pomona Electronics, 1500E Ninth St, Pomona, CA 91766-3835.

7A. Frost, “Are You Measuring Your Circuit orYour Scope Probe?” EDN, July 22, 1999.E. Feign, “High-Frequency Probes Drive50-Ω Measurements,” RF Design, Oct1998.

8J. Grebenkemper, KI6WX, “The TandemMatch—An Accurate Directional Watt-meter,” QST, Jan 1987, pp 18-26; and“Tandem Match Corrections,” QST, Jan1988, p 49.

9R. Lewallen, W7EL, “A Simplified and Accu-rate QRP Directional Wattmeter,” QST,Feb 1990, pp 19-23, 36.

10Ocean State Electronics, 6 Industrial Dr,PO Box 1458, Westerly, RI 02891; tel 800-866-6626, fax 401-596-3590.

Receiver Performance TestsComparing the performance of one re-

ceiver to another is difficult at best. Thefeatures of one receiver may outweigh asecond, even though its performance un-der some conditions is not as good as itcould be. Although the final decision onwhich receiver to purchase will more thanlikely be based on personal preference andcost, there are ways to compare receiverperformance characteristics. Some of themore important parameters are sensitiv-ity, blocking dynamic range and two-toneIMD dynamic range.

Instruments for measuring receiver per-formance should be of suitable quality andcalibration. Always remember that accu-racy can never be better than the tools usedto make the measurements. Common in-struments used for receiver testing include:• Signal generators• Hybrid combiner• Audio ac voltmeter• Distortion meter (FM measurements

only)• Noise figure meter (only required for

noise figure measurements)

• Step attenuators (10 dB and 1 dB stepsare useful)Signal generators must be calibrated ac-

curately in dBm or mV. The generatorsshould have extremely low leakage. That is,when the output of the generator is switchedoff, no signal should be detected at the oper-ating frequency with a sensitive receiver.Ideally, at least one of the signal generatorsshould be capable of amplitude modulation.A suitable lab-quality piece would be theHP-8640B, no longer manufactured, but agood item to scout on the surplus market.

Chapter 25.pmd 8/3/2007, 10:39 AM41

25.42 Chapter 25

While most signal generators are cali-brated in terms of microvolts, the real con-cern is not with the voltage from the genera-tor but with the power available. The unitthat is used for most low-level RF work isthe milliwatt, and power is often specifiedin decibels with respect to 1 mW (dBm).Hence, 0 dBm would be 1 mW. The dBmlevel, in a 50-Ω load, can be calculated withthe aid of the following equation:

( )[ ]2V20log 10 dBm 10= (22)

wheredBm = power with respect to 1 mWV = RMS voltage available at the output

of the signal generator

The convenience of a logarithmic powerunit such as the dBm becomes apparentwhen signals are amplified or attenuated.For example, a –107 dBm signal thatis applied to an amplifier with a gain of20 dB will result in an output increasedby 20 dB. Therefore in this example(–107 dBm + 20 dB) = 87 dBm. Similarly,a –107 dBm signal applied to an attenuatorwith a loss of 10 dB will result in an outputof (–107 dBm – 10 dB) or –117 dBm.

A hybrid combiner is a three-port deviceused to combine the signals from a pair ofgenerators for all dynamic range measure-ments. It has the characteristic that signalsapplied at ports 1 or 2 appear at port 3 andare attenuated by 3 dB.However, a signalfrom port 1 is attenuated 30 or 40 dB whensampled at port 2. Similarly, signals ap-plied at port 2 areisolated from port 1 some30 to 40 dB. The isolating properties of thebox prevent one signal generator frombeing frequency or phase modulated by theother. A second feature of a hybrid com-biner is that a 50-Ω impedance level ismaintained throughout the system.

Audio voltmeters should be calibratedin dB as well as volts. This facilitates easymeasurements and eliminates the need forcumbersome calculations. Be sure that thestep attenuators are in good working orderand suitable for the frequencies involved.A distortion meter, such as the Hewlett-Packard 339A, is required for FM sensi-tivity measurements and a noise figure

Fig 25.66 — A general test setup for measuring receiver MDS,or noise floor. Signal levels shown are for an examplediscussed in the text. Fig 25.67 — FM SINAD test setup.

meter, such as the Hewlett-Packard8970A, is excellent for certain kinds ofsensitivity measurements.

Receiver SensitivitySeveral methods are used to determine

receiver sensitivity. The mode under con-sideration often determines the bestchoice. One of the most common sensitiv-ity measurements is minimum discerniblesignal (MDS) or noise floor. It is suitablefor CW and SSB receivers.

This measurement indicates the mini-mum discernible signal that can be de-tected with the receiver. This level isdefined as that which will produce thesame audio-output power as the internallygenerated receiver noise. Hence, the term“noise floor.”

To measure MDS, use a signal generatortuned to the same frequency as the receiver(see Fig 25.66). With the generator outputat 0 or with maximum attenuation of itsoutput note the voltmeter reading. Nextincrease the generator output level until theac voltmeter at the receiveraudio-outputjack shows a 3-dB increase. The signalinput at this point is the MDS. Be certainthat the receiver is peaked on the generatorsignal. The filter bandwidth can affect theMDS. Always compare MDS readingstaken with identical filter bandwidths. (Anarrow bandwidth tends to improve MDSperformance.) MDS can be expressed inμV or dBm.

In the hypothetical example of Fig 25.66,the output of the signal generator is–133 dBm and the step attenuator is set to4 dB. Here is the calculation:

Noise floor = –133 dBm – 4 dB =–137 dBm (23)

where the noise floor is the power avail-able at the receiver antenna terminal and4 dB is the loss through the attenuator.

Receiver sensitivity is also often ex-pressed as 10 dB S+N/N (a 10-dB ratio ofsignal + noise to noise) or 10 dB S/N (signalto noise). The procedure and measurementare identical to MDS, except that the inputsignal is increased until the receiver out-put increases by 10 dB for 10 dB S+N/N

and 9.5 dB for 10 dB S/N (often called“10 dB signal to noise ratio”). AM receiversensitivity is usually expressed in thismanner with a 30% modulated, 1-kHz testsignal. (The modulation in this case iskeyed on and off and the signal level isadjusted for the desired increase in theaudio output.)

SINAD is a common sensitivity mea-surement normally associated with FMreceivers. It is an acronym for “signal plusnoise and distortion.” SINAD is a measureof signal quality:

distortion noise

distortion noise signal SINAD

+++

= (24)

where SINAD is expressed in dB. In thisexample, all quantities to the right of theequal sign are expressed in volts, and theratio is converted to dB by multiplying thelog of the fraction by 20.

⎟⎟⎠

⎞⎜⎜⎝

⎛+

++

=

(V)Distortion Noise(V)

(V)Distortion Noise(V) Signal(V)log 20

)dB(SINAD

Let’s look at this more closely. We canconsider distortion to be a part of the re-ceiver noise because distortion, like noise,is an unwanted signal added to the desiredsignal by the receiving system. Then, if weassume that the desired signal is muchstronger than the noise, SINAD closelyapproximates the signal to noise ratio. Thecommon 12-dB SINAD specificationtherefore corresponds to a 4:1 S/N ratio(noise + distortion = 0.25 × signal).

The basic test setup for measuringSINAD is shown in Fig 25.67. The level ofinput signal is adjusted to provide 25%distortion (12 dB SINAD). Narrow-bandFM signals, typical for amateur communi-cations, usually have 3-kHz peak devia-tion when modulated at 1000 Hz.

Noise figure is another measure of re-ceiver sensitivity. It provides a sensitivityevaluation that is independent of the sys-tem bandwidth. Noise figure is discussedfurther in the Receivers and Transmit-ters chapter.

Chapter 25.pmd 8/3/2007, 10:39 AM42

Test Procedures 25.43

Fig 25.68 — Receiver Blocking DR is measured with this equipment andarrangement. Measurements shown are for the example discussed in the text.

Fig 25.69 — Receiver IMD DR test setup. Signal levels shown are for the examplediscussed in the text.

Dynamic RangeDynamic range is the ability of the

receiver to tolerate strong signals outsideof its band-pass range. Two kinds will beconsidered:

Blocking dynamic range (blocking DR)is the difference, in dB, between the noisefloor and a signal that causes 1 dB of gaincompression in the receiver. It indicatesthe signal level, above the noise floor, thatbegins to cause desensitization.

IMD dynamic range (IMD DR) mea-sures the impact of two-tone IMD on areceiver. IMD is the production of spuri-ous responses that results when two ormore signals mix. IMD occurs in anyreceiver when signals of sufficient magni-tude are present. IMD DR is the differ-ence, in dB, between the noise floor andthe strength of two equal incoming signalsthat produce a third-order product 3 dBabove the noise floor.

What do these measurements mean?When the IMD DR is exceeded, false sig-nals begin to appear along with the desiredsignal. When the blocking DR is exceeded,the receiver begins losing its ability toamplify weak signals. Typically, the IMDDR is 20 dB or more below the blockingDR, so false signals appear well beforesensitivity is significantly decreased. IMDDR is one of the most significant param-eters that can be specified for a receiver. It

is generally a conservative evaluation forother effects, such as blocking, which willoccur only for signals well outside theIMD dynamic range of the receiver.

Both dynamic range tests require twosignal generators and a hybrid combiner.When testing blocking DR (see Fig 25.68),one generator is set for a weak signal ofroughly −110 dBm. The receiver is tunedto this frequency and peaked for maximumresponse. (ARRL Lab procedures requirethis level to be about 10 dB below the1-dB compression point, if the AGC canbe disabled. Otherwise, the level is set to20 dB above the MDS.)

The second generator is set to a fre-quency 20 kHz away from the first and itslevel is increased until the receiver outputdrops by 1 dB, as measured with the acvoltmeter.

In the example shown, the output of thegenerator is –7 dBm, the loss through thecombiner is fixed at 3 dB and the step at-tenuator is set to 10 dB. The 1-dB com-pression level is calculated as follows:

dBm 20 dB 10 dBm 3 dBm 7

level Blocking

−=−−−

= (25)

To express this as a dynamic range,the blocking level is referenced to thereceiver noise floor (calculated earlier).Calculate it as follows:

Block DR =noise floor – blocking level =–137 dBm – (–20 dBm) = –117 dB

This value is usually expressed as anabsolute value: 117 dB.

Two-Tone IMD TestThe setup for measuring IMD DR is

shown in Fig 25.69. Two signals of equallevel, spaced 20-kHz apart are injected intothe receiver input. When we call theserequencies f1 and f2, the so-called third-order IMD products will appear at frequen-cies of (2f1 − f2) and (2f2 − f1). If the twoinput frequencies are 14.040 and 14.060MHz, the third-order products will be at14.020 and 14.080 MHz. Let’s talk througha measurement with these frequencies.

First, set the generators for f1 and f2.Adjust each of them for an output of–10 dBm. Tune the receiver to either ofthe third-order IMD products. Adjust thestep attenuator until the IMD product pro-duces an output 3 dB above the noise levelas read on the ac voltmeter.

For an example, say the output of thegenerator is –10 dBm, the loss through thecombiner is 3 dB and the amount of at-tenuation used is 30 dB. The signal level atthe receiver antenna terminal that just be-gins to cause IMD problems is calculatedas:

dBm 43

db 30 dB 3 dBm 10 level IMD

−=

−−−= (27)

To express this as a dynamic range theIMD level is referenced to the noise flooras follows:

( )dB 94

dBm 43 dBm 137

level IMD floor noise DR IMD

−=

−−−=

−=

Therefore, the IMD dynamic range ofthis receiver would be 94 dB.

Third-Order InterceptAnother parameter used to quanti-

fyreceiver performance is the third-orderinput intercept (IP3). This is the point atwhich the desired response and the third-order IMD response intersect, if extendedbeyond their linear regions (see Fig 25.70).Greater IP3 indicates better receiver perfor-mance. Calculate IP3 like this:

IP3 = 1.5 (IMD dynamic range in dB) + (MDS in dBm) (29)

For our example receiver:

IP3 = 1.5 (94 dB) + (–137 dBm) = +4 dBm

The preferred method for the third-order input intercept, however, is to use S5

(26)

(28)

Chapter 25.pmd 8/3/2007, 10:39 AM43

25.44 Chapter 25

signal levels instead of MDS levels. Thisis due to the tendency for a receiver toexhibit non-linearity at the noise floorlevel (i.e., near the level where MDS istypically defined).

Thus, Third Order Intercept =(3 × (S5 IMD Level) – (S5 Reference))/2

The results of this method should showclose correlation with the intercept pointdetermined by the MDS test. If not, thetest engineer determines (from further in-vestigation) which method provides amore accurate result.

In the same way (using the S5 method),

Second Order Interrupt = 2 × (S5 IMDLevel) – (S5 Reference)

The example receiver we have discussedhere is purely imaginary. Nonetheless, itsperformance is typical of contemporarycommunications receivers.

Fig 25.70 — A plot of the receivercharacteristics that determine third-order input intercept, a measure ofreceiver performance.

Fig 25.71 — Performance plot of the receiver discussed in the text. This is a goodway to visualize the interaction of receiver-performance measurements. Note that–147 dBm is considered the lowest possible noise level in a 500 Hz receiverbandwidth.

Fig 25.72 — A complex signal in thetime and frequency domains. A is athree-dimensional display of amplitude,time and frequency. B is anoscilloscope display of time vsamplitude. C is spectrum analyzerdisplay of the frequency domain andshows frequency vs amplitude.

Evaluating the DataThus far, a fair amount of data has been

gathered with no mention of what the num-bers really mean. It is somewhat easier tounderstand exactly what is happening byarranging the data as shown in Fig 25.71.The base line represents power levels witha very small level at the left and a higherlevel (0 dBm) at the right.

The noise floor of our hypotheticalreceiver is at −137 dBm, the IMD level(the level at which signals will begin tocreate spurious responses) at −43 dBm andthe blocking level (the level at which sig-nals will begin to desensitize the receiver)at −20 dBm. The IMD dynamic range issome 23 dB smaller than the blocking dy-namic range. This means IMD productswill be heard long before the receiver be-gins to desensitize, some 23 dB sooner.

SPECTRUM ANALYZERSA spectrum analyzer is similar to an

oscilloscope. Both visually present anelectrical signal through graphic represen-tation. The oscilloscope is used to observeelectrical signals in the time domain (am-plitude as a function of time). The timedomain, however, gives little informationabout the frequencies that make up com-plex signals. Amplifiers, mixers, oscilla-tors, detectors, modulators and filters arebest characterized in terms of their fre-quency response. This information is ob-tained by viewing electrical signals in thefrequency domain (amplitude as a func-tion of frequency). One instrument thatcan display the frequency domain is thespectrum analyzer.

Time and Frequency Domain

To better understand the concepts oftime and frequency domain, see Fig 25.72.The three-dimensional coordinates showtime (as the line sloping toward the bottom

right), frequency (as the line rising towardthe top right) and amplitude (as the verti-cal axis). The two discrete frequenciesshown are harmonically related, so we’llrefer to them as f1 and 2f1.

In the representation of time domain atB, all frequency components of a signalare summed together. In fact, if the twodiscrete frequencies shown were appliedto the input of an oscilloscope, we wouldsee the solid line (which corresponds to f1+ 2f1) on the display.

In the frequency domain, complex sig-nals (signals composed of more than onefrequency) are separated into their indi-vidual frequency components. A spectrumanalyzer measures and displays the powerlevel at each discrete frequency; this dis-play is shown at C.

The frequency domain contains infor-mation not apparent in the time domainand therefore the spectrum analyzer offersadvantages over the oscilloscope for cer-tain measurements. As might be expected,some measurements are best made in thetime domain. In these cases, the oscillo-scope is a valuable instrument.

Spectrum Analyzer Basics

There are several different types ofspectrum analyzers, but by far the most

Chapter 25.pmd 8/3/2007, 10:39 AM44

Test Procedures 25.45

most useful, it should display signals ofwidely different levels. As an example,signals differing by 60 dB, which is a thou-sand to one difference in voltage or a mil-lion to one in power, would be difficult todisplay. This would mean that if powerwere displayed, one signal would be onemillion times larger than the other (in thecase of voltage one signal would be a thou-sand times larger). In either case it wouldbe difficult to display both signals on aCRT. The solution to this problem is to usea logarithmic display that shows the rela-tive signal levels in decibels. Using thistechnique, a 1000:1 ratio of voltage re-duces to a 60-dB difference.

The conversion of the signal to a loga-rithm is usually performed in the IF ampli-fier or detector, resulting in an outputvoltage proportional to the logarithm ofthe input RF level. This output voltage isthen used to drive the CRT display.

Spectrum Analyzer PerformanceSpecifications

The performance parameters of a spec-trum analyzer are specified in terms simi-lar to those used for radio receivers, inspite of the fact that there are many differ-ences between a receiver and a spectrumanalyzer.

The sensitivity of a receiver is oftenspecified as the minimum discernible sig-nal, which means the smallest signal thatcan be heard. In the case of the spectrumanalyzer, it is not the smallest signal thatcan be heard, but the smallest signal thatcan be seen. The dynamic range of thespectrum analyzer determines the largestand smallest signals that can be simulta-neously viewed on the analyzer. As with areceiver, there are several factors that canaffect dynamic range, such as IMD, sec-ond- and third-order distortion and block-ing. IMD dynamic range is the maximumdifference in signal level between theminimum detectable signal and the levelof two signals of equal strength that gen-erate an IMD product equal to the mini-mum detectable signal.

Although the communications receiveris an excellent example to introduce thespectrum analyzer, there are several dif-ferences such as the previously explainedlack of a demodulator. Unlike the commu-nications receiver, the spectrum analyzeris not a sensitive radio receiver. To pre-serve a wide dynamic range, the spectrumanalyzer often uses passive mixers for thefirst and second mixers. Therefore, refer-ring to Fig 25.73, the noise figure of theanalyzer is no better than the losses of theinput low-pass filter plus the first mixer,the first IF filter, the second mixer and theloss of the second IF filter. This often re-

Fig 25.73 — A block diagram of a superheterodyne spectrum analyzer. Inputfrequencies of up to 300 MHz are up converted by the local oscillator andmixer to a fixed frequency of 400 MHz.

common is nothing more than an electroni-cally tuned superheterodyne receiver. Thereceiver is tuned by means of a ramp volt-age. This ramp voltage performs two func-tions: First, it sweeps the frequency of theanalyzer local oscillator; second, it de-flects a beam across the horizontal axis ofa CRT display, as shown in Fig 25.73. Thevertical axis deflection of the CRT beamis determined by the strength of the re-ceived signal. In this way, the CRT dis-plays frequency on the horizontal axis andsignal strength on the vertical axis.

Most spectrum analyzers use an up-con-verting technique so that a fixed tunedinput filter can remove the image. Onlythe first local oscillator need be tuned totune the receiver. In the up-conversiondesign, a wide-band input is converted toan IF higher than the highest input fre-quency. As with most up-converting com-munications receivers, it is not easy toachieve the desired ultimate selectivity atthe first IF, because of the high frequency.For this reason, multiple conversions areused to generate an IF low enough so thatthe desired selectivity is practical. In theexample shown, dual conversion is used:The first IF is at 400 MHz; the second at10.7 MHz.

In the example spectrum analyzer, thefirst local oscillator is swept from400 MHz to 700 MHz; this converts theinput (from nearly 0 MHz to 300 MHz) tothe first IF of 400 MHz. The usual rule ofthumb for varactor tuned oscillators is thatthe maximum practical tuning ratio (the

ratio of the highest frequency to the lowestfrequency) is an octave, a 2:1 ratio. In ourexample spectrum analyzer, the tuningratio of the first local oscillator is 1.75:1,which meets this specification.

The image frequency spans 800 MHz to1100 MHz and is easily eliminated using alow-pass filter with a cut-off frequencyaround 300 MHz. The 400-MHz first IF isconverted to 10.7 MHz where the ultimateselectivity of the analyzer is obtained.The image of the second conversion,(421.4 MHz), is eliminated by the first IFfilter. The attenuation of the image shouldbe great, on the order of 60 to 80 dB. Thisrequires a first IF filter with a high Q; thisis achieved by using helical resonators,SAW resonators or cavity filters. Anothermethod of eliminating the image problemis to use triple conversion; converting firstto an intermediate IF such as 50 MHz andthen to 10.7 MHz. As with any receiver, anadditional frequency conversion requiresadded circuitry and adds potential spuri-ous responses.

Most of the signal amplification takesplace at the lowest IF; in the case of theexample analyzer this is 10.7 MHz. Herethe communications receiver and the spec-trum analyzer differ. A communicationsreceiver demodulates the incoming signalso that the modulation can be heard or fur-ther demodulated for RTTY or packet orother mode of operation. In the spectrumanalyzer, only the signal strength isneeded.

In order for the spectrum analyzer to be

Chapter 25.pmd 8/3/2007, 10:39 AM45

25.46 Chapter 25

sults in a combined noise figure of morethan 20 dB. With that kind of noise figurethe spectrum analyzer is obviously not acommunications receiver for extractingvery weak signals from the noise but ameasuring instrument for the analysis offrequency spectrum.

The selectivity of the analyzer is calledthe resolution bandwidth. This term refersto the minimum frequency separation oftwo signals of equal level that can be re-solved so there is a 3-dB dip between thetwo. The IF filters used in a spectrum ana-lyzer differ from a communications re-ceiver in that the filters in a spectrumanalyzer have very gentle skirts androunded passbands, rather than the flatpassband and very steep skirts used on anIF filter in a high-quality communicationsreceiver. This rounded passband is neces-sary because the signals pass into the filterpassband as the spectrum analyzer scansthe desired frequency range. If the signalssuddenly pop into the passband (as theywould if the filter had steep skirts), thefilter tends to ring; a filter with gentleskirts is less likely to ring. This ringing,called scan loss, distorts the display andrequires that the analyzer not sweep fre-

Fig 25.74 — Alternate bench setups for viewing the output of a high powertransmitter or oscillator on a spectrum analyzer. A uses a line sampler to pickoff a small amount of the transmitter or amplifier power. In B, most of thetransmitter power is dissipated in the power attenuator.

Fig 25.75 — A notch filter is another way to reduce the level of a transmitter’sfundamental signal so that the fundamental does not generate harmonics withinthe analyzer. However, in order to know the amplitude relationship between thefundamental and the transmitter’s actual harmonics and spurs, the attenuation ofthe fundamental in the notch filter must be known.

quency too quickly. All this means that thescan rate must be checked periodically tobe certain the signal amplitude is not af-fected by fast tuning.

Spectrum Analyzer ApplicationsSpectrum analyzers are used in situa-

tions where the signals to be analyzed arevery complex and an oscilloscope displaywould be an indecipherable jumble. Thespectrum analyzer is also used when thefrequency of the signals to be analyzed isvery high. Although high-performanceoscilloscopes are capable of operation intothe UHF region, moderately priced spec-trum analyzers can be used well into thegigahertz region.

A spectrum analyzer can also be used toview very low-level signals. For an oscillo-scope to display a VHF waveform, the band-width of the oscilloscope must extend fromzero to the frequency of the waveform. Ifharmonic distortion and other higher-fre-quency distortions are to be seen the band-width of the oscilloscope must exceed thefundamental frequency of the waveform.This broad bandwidth can also admit a lot ofnoise power. The spectrum analyzer, on theother hand, analyzes the waveform using a

narrow bandwidth; thus it is capable of re-ducing the noise power admitted.

Probably the most common applicationof the spectrum analyzer is the measure-ment of the harmonic content and otherspurious signals in the output of a radiotransmitter. Fig 25.74 shows two ways toconnect the transmitter and spectrum ana-lyzer. The method shown at A should notbe used for wide-band measurements sincemost line-sampling devices do not exhibita constant-amplitude output over a broadfrequency range. Using a line sampler isfine for narrow-band measurements, how-ever. The method shown at B is used in theARRL Lab. The attenuator must be capableof dissipating the transmitter power. It mustalso have sufficient attenuation to protectthe spectrum analyzer input. Many spec-trum analyzer mixers can be damaged byonly a few milliwatts, so most analyzershave an adjustable input attenuator that willprovide a reasonable amount of attenuationto protect the sensitive input mixer fromdamage. The power limitation of the attenu-ator itself is usually on the order of a wattor so, however. This means that 20 dB ofadditional attenuation is required for a100-W transmitter, 30 dB for a 1000-Wtransmitter and so on, to limit the input tothe spectrum analyzer to 1 W. There arespecialized attenuators that are made fortransmitter testing; these attenuators pro-vide the necessary power dissipation andattenuation in the 20 to 30-dB range.

When using a spectrum analyzer it isvery important that the maximum amountof attenuation be applied before a mea-surement is made. In addition, it is a goodpractice to start with maximum attenua-tion and view the entire spectrum of a sig-nal before the attenuator is adjusted. Thesignal being viewed could appear to be ata safe level, but another spectral compo-nent, which is not visible, could be abovethe damage limit. It is also very importantto limit the input power to the analyzerwhen pulse power is being measured. Theaverage power may be small enough so theinput attenuator is not damaged, but thepeak pulse power, which may not bereadily visible on the analyzer display, candestroy a mixer, literally in microseconds.

When using a spectrum analyzer it isnecessary to ensure that the analyzer doesnot generate additional spurious signalsthat are then attributed to the system undertest. Some of the spurious signals that canbe generated by a spectrum analyzer areharmonics and IMD. If it is desired tomeasure the harmonic levels of a transmit-ter at a level below the spurious level ofthe analyzer itself, a notch filter can beinserted between the attenuator and thespectrum analyzer as shown in Fig 25.75.

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Test Procedures 25.47

This reduces the level of the fundamentalsignal and prevents that signal from gen-erating harmonics within the analyzer,while still allowing the harmonics fromthe transmitter to pass through to the ana-lyzer without attenuation. Use cautionwith this technique; detuning the notchfilter or inadvertently changing the trans-mitter frequency will allow potentiallyhigh levels of power to enter the analyzer.In addition, use care when choosing fil-ters; some filters (such as cavity filters)respond not only to the fundamental butnotch out odd harmonics as well.

It is good practice to check for the gen-eration of spurious signals within the spec-trum analyzer. When a spurious signal isgenerated by a spectrum analyzer, addingattenuation at the analyzer input will causethe internally generated spurious signalsto decrease by an amount greater thanthe added attenuation. If attenuationadded ahead of the analyzer causes all ofthe visible signals to decrease by the sameamount, this indicates a spurious-freedisplay.

The input impedance for most RF spec-trum analyzers is 50 Ω; not all circuits haveconvenient 50-Ω connections that can beaccessed for testing purposes, however.Using a probe such as the one shown in Fig25.76 allows the analyzer to be used as atroubleshooting tool. The probe can beused to track down signals within a trans-mitter or receiver, much like an oscillo-scope is used. The probe shown offers a100:1 voltage reduction and loads the cir-cuit with 5000 Ω. A different type of probeis shown in Fig 25.77. This inductivepickup coil (sometimes called a “sniffer”)is very handy for troubleshooting. The coilis used to couple signals from the radiatedmagnetic field of a circuit into the ana-lyzer. A short length of miniature coax iswound into a pick-up loop and soldered toa larger piece of coax. The use of the coaxshields the loop from coupling energyfrom the electric field component. Thedimensions of the loop are not critical, butsmaller loop dimensions make the loopmore accurate in locating the source ofradiated RF. The shield of the coax pro-vides a complete electrostatic shield with-out introducing a shorted turn.

The sniffer allows the spectrum ana-lyzer to sense RF energy without contact-ing the circuit being analyzed. If the loopis brought near an oscillator coil, the oscil-lator can be tuned without directly con-tacting (and thus disturbing) the circuit.The oscillator can then be checked for re-liable starting and the generation of spuri-ous sidebands. With the coil brought nearthe tuned circuits of amplifiers or fre-quency multipliers, those stages can be

Fig 25.77 — A “sniffer” probeconsisting of an inductive pick-up. Ithas an advantage of not loading thecircuit under test. See text for details.

Fig 25.76 — A schematic representation of a voltage probe designed for use witha spectrum analyzer. Keep the probe tip (resistor and capacitor) and ground leadsas short as possible.

tuned using a similar technique.Even though the sniffer does not contact

the circuit being evaluated, it does extractsome energy from the circuit. For this rea-son, the loop should be placed as far fromthe tuned circuit as is practical. If the loopis placed too far from the circuit, the sig-nal will be too weak or the pick-up loopwill pick up energy from other parts of thecircuit and not give an accurate indicationof the circuit under test.

The sniffer is very handy to locatesources of RF leakage. By probing theshields and cabinets of RF generat-ing equipment (such as transmitters) egressand ingress points of RF energy can be

identified by increased indications on theanalyzer display.

One very powerful characteristic of thespectrum analyzer is the instrument’s ca-pability to measure very low- level signals.This characteristic is very advantageouswhen very high levels of attenuation aremeasured. Fig 25.78 shows the setup fortuning the notch and passband of a VHFduplexer. The spectrum analyzer, beingcapable of viewing signals well into the lowmicrovolt region, is capable of measuringthe insertion loss of the notch cavity morethan 100 dB below the signal generatoroutput. Making a measurement of this sortrequires care in the interconnection of theequipment and a well designed spectrumanalyzer and signal generator. RF energyleaking from the signal generator cabinet,line cord or even the coax itself, can getinto the spectrum analyzer through similarpaths and corrupt the measurement. Thisleakage can make the measurement lookeither better or worse than the actualattenuation, depending on the phaserelationship of the leaked signal.

Extensions of Spectral AnalysisWhat if a signal generator is connected

to a spectrum analyzer so that the signalgenerator output frequency is exactly thesame as the receiving frequency of thespectrum analyzer? It would certainly ap-pear to be a real convenience not to have to

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25.48 Chapter 25

Fig 25.78 — Block diagram of a spectrum analyzer and signal generator beingused to tune the band-pass and notch filters of a duplexer. All ports of theduplexer must be properly terminated and good quality coax with intact shieldingused to reduce leakage.

Fig 25.79 — A signal generator (shown inthe figure as the “Tracking Generator”)locked to the local oscillator of a spec-trum analyzer can be used to determinefilter response over a range of frequencies.

Fig 25.80 — A network analyzer is usuallyfound in commercial communicationsdevelopment labs. It can measure boththe phase and magnitude of the filterinput and output signals. See text fordetails.

continually reset the signal generator tothe desired frequency. It is, however, morethan a convenience. A signal generatorconnected in this way is called a tracking

generator because the output frequencytracks the spectrum analyzer input fre-quency. The tracking generator makes itpossible to make swept frequency mea-

surements of the attenuation characteris-tics of circuits, even when the attenuationinvolved is large.

Fig 25.79 shows the connection of atracking generator to a circuit under test.In order for the tracking generator to cre-ate an output frequency exactly equal tothe input frequency of the spectrum ana-lyzer, the internal local oscillator frequen-cies of the spectrum analyzer must beknown. This is the reason for the intercon-nections between the tracking generatorand the spectrum analyzer. The test setupshown will measure the gain or loss of thecircuit under test. Only the magnitude ofthe gain or loss is available; in some cases,the phase angle between the input andoutput would also be an important andnecessary parameter.

The spectrum analyzer is not sensitiveto the phase angle of the tracking genera-tor output. In the process of generating thetracking generator output, there are noguarantees that the phase of the trackinggenerator will be either known or constant.This is especially true of VHF spectrumanalyzers/tracking generators where a fewinches of coaxial cable represents a sig-nificant phase shift.

One effective way of measuring thephase angle between the input and outputof a device under test is to sample the phaseof the input and output of device under testand apply the samples to a phase detector.Fig 25.80 shows a block diagram of thistechnique. An instrument that can measureboth the magnitude and phase of a signal iscalled a vector network analyzer or simplya network analyzer. The magnitude andphase can be displayed either separatelyor together. When the magnitude andphase are displayed together the two canbe presented as two separate traces, simi-lar to the two traces on a dual-trace oscil-loscope. A much more useful method ofdisplay is to present the magnitude andphase as a polar plot where the locus of thepoints of a vector having the length of themagnitude and the angle of the phase aredisplayed. Very sophisticated networkanalyzers can display all of the S param-eters of a circuit in either a polar format ora Smith Chart format.

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Test Procedures 25.49

Transmitter Performance TestsThe test setup used in the ARRL Labo-

ratory for measuring an HF transmitter oramplifier is shown in Fig 25.81. As can beseen, different power levels dictate differ-ent amounts of attenuation between thetransmitter or amplifier and the spectrumanalyzer.

Spurious EmissionsFig 25.82 shows the broadband spectrum

of a transmitter, showing the harmonics inthe output. The horizontal (frequency) scaleis 5 MHz per division; the main output of thetransmitter at 7 MHz can be seen about 1.5major divisions from the left of the trace.Although not shown, a very large apparentsignal is often seen at the extreme left ofthe trace. This occurs at what would be zerofrequency and it is caused by the first localoscillator frequency being exactly the firstIF. All up-converting superheterody ne spec-trum analyzers have this IF feed-through; inaddition, this signal is occasionally accom-panied by a smaller spurious signal, gener-ated within the analyzer. To determine whatpart of the displayed signal is a spurious re-sponse caused by IF feed-through and whatis an actual input signal, simply remove theinput signal and observe the trace. It is notnecessary or desirable that the transmitter bemodulated for this broadband test.

Other transmitter tests that can be per-formed with a spectrum analyzer includemeasurement of two-tone IMD and SSBcarrier and unwanted sideband suppres-sion.

Two-Tone IMDInvestigating the sidebands from a

modulated transmitter requires a narrow-band spectrum analysis and produces dis-plays similar to that shown in Fig 25.83. Inthis example, a two-tone test signal is usedto modulate the transmitter. The displayshows the two test tones plus some of theIMD produced by the SSB transmitter. Thetest setup used to produce this display isshown in Fig 25.84.

In this example, a two-tone test signalwith frequencies of 700 and 1900 Hz isused to modulate the transmitter. Set thetransmitter output and audio input to themanufacturer’s specifications. Each de-sired tone is adjusted to be equal in ampli-tude and centered on the display. The stepattenuators and analyzer controls are thenadjusted to set the two desired signals 6 dBbelow the 0-dB reference (top) line. TheIMD products can then be read directlyfrom the display in terms of “dB belowPeak Envelope Power (PEP).” (In the ex-ample shown, the third-order products are30 dB below PEP, the fifth-order products

0 5 10 15 20 25 30 35 40 45 50–80

–70

–60

–50

–40

–30

–20

–10

0

Frequency (MHz)

Reference Level: 0 dBc

(A)

HBK05_25-82

–10 –8 –6 –4 –2 0 2 4 6 8 10–80

–70

–60

–50

–40

–30

–20

–10

0

Frequency Offset (kHz)

Reference Level: 0 dB PEP

HBK05_25-83

Fig 25.81 — These setups are used in the ARRL Laboratory for testingtransmitters or amplifiers with several different power levels.

Fig 25.82 — Comparison of two different transmitters on the 40-m band astypically seen on a spectrum analyzer display. The display at the left shows arelatively clean transmitted signal but the transmitter at the right shows morespurious signal content. Horizontal scale is 5 MHz per division; vertical is 10 dBper division. According to current FCC spectral purity requirements bothtransmitters are acceptable.

Fig 25.83 — An SSB transmitter two-tone test as seen on a spectrum analy-zer. Each horizontal division represents2 kHz and each vertical division is 10 dB.The third-order products are 30 dB belowthe PEP (top line), the fifth-order productsare down 37 dB and seventh-orderproducts are down 40 dB. This representsacceptable (but not ideal) performance.

0 5 10 15 20 25 30 35 40 45 50–80

–70

–60

–50

–40

–30

–20

–10

0

Frequency (MHz)

Reference Level: 0 dBc

(B)

are 37 dB down, the seventh- order prod-ucts are down 40 dB.)

Carrier and Unwanted SidebandSuppression

Single-tone audio input signals can beused with the same setup to measure un-wanted sideband and carrier suppressionof SSB signals. In this case, set the singletone to the 0-dB reference line. (Once thelevel is set, the audio can be disabled forcarrier suppression measurements inorderto eliminate IMD and other effects.)

Phase NoisePhase/composite noise is also measured

with spectrum analyzers in the ARRL Lab.This test requires specialized equipmentand is included here for information pur-poses only.

The purpose of the Composite-Noise

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25.50 Chapter 25

Fig 25.84 — The test setup used in the ARRL Laboratory to measure the IMDperformance of transmitters and amplifiers. 2 4 6 8 10 12 14 16 18 20 22

–140

–130

–120

–110

–100

–90

–80

–70

–60

Frequency Sweep: 2 to 22 kHz from Carrier

Reference Level: - 60 dBc/HzVertical Scale: dBc/Hz

HBK05_25-85

Fig 25.85 — The spectral-displayresults of a composite-noise test in theARRL Lab. Power output is 100 W at14 MHz. Vertical divisions are 10 dB;horizontal divisions are 2 kHz. The logreference level (the top horizontal lineon the scale) represents –60 dBc/Hzand the baseline is –140 dBc/Hz. Thecarrier, off the left edge of the plot, isnot shown. This plot shows compositetransmitted noise 2 to 22 kHz from thecarrier.

Fig 25.86 — CW keying waveform test setup.

Fig 25.87 — Typical CW keying wave-form test results. This display is for theICOM IC-707 (semi-break-in mode)reviewed in April 94 QST. The uppertrace is the actual key closure; the lowertrace is the RF envelope. Horizontaldivisions are 10 ms. The transceiverwas being operated at 100 W output at14 MHz.

Fig 25.88 — PTT-to-RF-output test setup for voice-mode transmitters.

test is to observe and measure the phaseand amplitude noise, as well as any close-in spurious signals generated by a trans-mitter. Since phase noise is the primarynoise component in any well-designedtransmitter, almost all of the noise ob-served during this test is phase noise.

This measurement is accomplished inthe lab by converting the transmitter out-put down to a frequency band about 10 or20 kHz above baseband. A mixer and asignal generator (used as a local oscilla-tor) are used to perform this conversion.Filters remove the 0-Hz component as wellas any unwanted heterodyne components.A spectrum analyzer (see Fig 25.85) dis-plays the remaining noise and spurioussignals from 2 to 22 kHz from the carrierfrequency (in the CW mode).

Tests in the Time DomainOscilloscopes are used for transmitter

testing in the time domain. Dual-trace in-struments are best in most cases, provid-

ing easy to read time-delay measurementsbetween keying input and RF- or audio-output signals. Common transmitter mea-surements performed with ’scopes includeCW keying wave shape and time delay andSSB/FM transmit-to-audio turnaroundtests (important for many digital modes).

A typical setup for measuring CW key-ing waveform and time delay is shown inFig 25.86. A keying test generator is usedto repeatedly key the transmitter at a con-trolled rate. The generator can be set toany reasonable speed, but ARRL tests areusually conducted at 20-ms on and 20-msoff (25 Hz, 50% duty cycle). Fig 25.87shows a typical display. The rise and falltimes of the RF output pulse are measuredbetween the 10% and 90% points on theleading and trailing edges, respectively.The delay times are measured between the50% points of the keying and RF outputwaveforms. Look at the Receivers andTransmitters chapter for further discus-sion of CW keying issues.

For voice modes (SSB/FM), a PTT-to-RF output test is similar to CW keyingtests. It measures rise and fall times, aswell as the on- and off-delay times just asin the CW test. See Fig 25.88 for the testsetup.

“Turnaround time” is the time it takesfor a transceiver to switch from the 50%fall time of a keying pulse to 50% rise ofaudio output. The test setup is shown inFig 25.89. Turn-around time measure-ments require extreme care with respect totransmitter output power, attenuation, sig-nal-generator output and the maximuminput signal that can be tolerated by thegenerator. The generator’s specifications

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Test Procedures 25.51

Fig 25.89 — Transmit-receive turn-around time test setup.

must not be exceeded and the input to thereceiver must be at the required level, usu-ally S9. Receiver AGC is usually off forthis test, but experimentation with AGCand signal input level can reveal surpris-ing variations. The keying rate must beconsiderably slower than the turn-around

time; rates of 200-ms on/200-ms off orfaster, have been used with success inProduct Review tests at the ARRL Lab.

Turn-around time is an important con-sideration with some digital modes.AMTOR, for example, requires a turn-around time of 35 ms or less.

OSCILLOSCOPE BIBLIOGRAPHYR. vanErk, Oscilloscopes, Functional

Operation and Measuring Examples,McGraw-Hill Book Co, New York,1978.

V. Bunze, Probing in Perspective—Appli-cation Note 152, Hewlett-Packard Co,Colorado Springs, CO, 1972 (Pub No.5952-1892).

The XYZs of Using a Scope, Tektronix,Inc, Portland, OR, 1981 (Pub No. 41AX-4758).

Basic Techniques of Waveform Measure-ment (Parts 1 and 2), Hewlett-PackardCo, Colorado Springs, CO, 1980 (PubNo. 5953-3873).

J. Millman, and H. Taub, Pulse Digital andSwitching Waveforms, McGraw-HillBook Co, New York, 1965, pp 50-54.

V. Martin, ABCs of DMMs, Fluke Corp,PO Box 9090, Everett, WA 98206.

GLOSSARYAlternating current (ac) — The polarity

constantly reverses, as contrasted to dc(direct current) where polarity is fixed.

Analog — Signals which have a full set ofvalues. If the signal varies between 0and 10 V all values in this range can befound. Compare this to a digital system.

Attenuator — A device which reduces theamplitude of a signal.

Average value — Obtained by recordingor measuring N samples of a signal, add-ing up all of these values, and dividingthis sum by N.

Bandwidth — A measure of how wide asignal is in frequency. If a signal covers14,200 to 14,205 kHz its bandwidth issaid to be 5 kHz.

BNC — A small bayonet-type connectorused with coax cable.

Bridge circuit — Four passive elements,such as resistors, inductors, connectedas a pair of voltage dividers with a meteror other measuring device across twoopposite junctions. Used to indicate therelative values of the four passive ele-ments. See the chapter discussion ofWheatstone bridges.

CMOS — A family of digital logic ele-ments usually selected for their lowpower drain. See the Electrical Signalsand Components chapter of this Hand-book.

Coaxial cable (coax) — A cable formed oftwo conductors that share the same axis.The center conductor may be a singlewire or a stranded cable. The outer con-

ductor is called the shield. The shieldmay be flexible braid, foil, semirigid orrigid metal. For more information, lookin the Transmission Lines chapter.

Combiners — See Hybrid.D’Arsonval meter — A common me-

chanical meter consisting of a perma-nent magnet and a moving coil (withpointer attached).

Direct Current (dc) — The polarity isfixed for all time, as contrasted to ac(alternating current) where polarity con-stantly reverses.

Digital — A system that allows signals toassume a finite range of states. Binarylogic is the most common example.Only two values are permitted in a bi-nary system: one value is defined as alogical 1 and the other value as a logical0. See the Handbook chapter on Elec-trical Signals and Components.

Divider — A network of components thatproduce an output signal that is a frac-tion of the input signal. The ratio of theoutput to the input is the division factor.An analog divider divides voltage (astring of series connected resistors) orcurrent (parallel connected resistors).Digital dividers divide pulse trains orfrequency.

DMM (digital multimeter) — A test in-strument that usually measures at least:voltage, current and resistance, and dis-plays the result on a numeric digit dis-play, rather than an analog meter.

Dummy antenna or dummy load — A

resistor or set of resistors used in placeof an antenna to test a transmitter with-out radiating any electromagnetic en-ergy into the air.

DVM (digital volt meter) — See DMM.FET voltmeter — See also VTVM. An

updated version of a VTVM using fieldeffect transistors (FETs) in place ofvacuum tubes.

Flip-flop — A digital circuit that has twostable states. See the chapter on Elec-trical Signals and Components.

Frequency marker — Test signals gener-ated at selected intervals (such as25 kHz, 50 kHz, 100 kHz) for calibratingthe dials of receivers and transmitters.

Fundamental — The first signal or fre-quency in a series of harmonically re-lated signals. This term is often used todescribe an oscillator or transmitter’sdesired signal.

Harmonic — A signal occurring at someintegral multiple (such as two, three,four) of a fundamental frequency.

Hybrid (hybrid combiners) — A deviceused to connect two signal generators toone receiver for test purposes, withoutthe two generators affecting each other.

IC (integrated circuit) — A completecircuit built into a single electronic com-ponent.

LCD (liquid crystal display) — A low-power display device utilizing the phys-ics of liquid crystals. They usually needeither ambient light or backlighting tobe seen.

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25.52 Chapter 25

LED (light emitting diode) — A diodethat emits light when an appropriatevoltage (usually 1.5 V at about 20 mA)is connected. They are used either astiny pilot lights or in bar shapes to dis-play letters and numbers.

Loran — A navigation system using very-low-frequency transmitters.

Marker — See Frequency marker.Multiplier — A circuit that purposely

creates some desired harmonic of itsinput signal. For example, a frequencymultiplier that takes energy from a3.5-MHz exciter and puts out RF at7 MHz is a two times multiplier, usuallycalled a frequency doubler.

N — A type of coaxial cable connectorcommon at UHF and higher frequencies.

NAND — A digital element that performsthe not-and function. See the ElectricalSignals and Components chapter.

Noise (noise figure) — Noise is generatedin all electrical circuits. It is particularlycritical in those stages of a receiver thatare closest to the antenna (RF amplifierand mixer), because noise generated inthese stages can mask a weak signal. Thenoise figure is a measure of this noisegeneration. Lower noise figures meanthat less noise is generated and weakersignals can be heard.

NOR — A digital element that performs thenot-or function. See the Electrical Sig-nals and Components chapter.

Null (nulling) — The process of adjustinga circuit for a minimum reading on a testmeter or instrument. At a perfect nullthere is null, or no, energy to be seen.

Ohmmeter — A meter that measures thevalue of resistors. Usually part of amultimeter. See VOM and DMM.

Peak value — The highest value of a sig-nal during the measuring time. If a mea-sured voltage varies in value from 1 to10 V over a measuring period, the peakvalue would be the highest measured,10 V.

PL-259 — A connector used for coaxialcable, usually at HF. It is also known asa male UHF connector. It is an inexpen-sive and common connector, but it is notweatherproof, nor is its impedance con-stant over frequency.

Prescaler — A circuit used ahead of acounter to extend the counter range tohigher frequencies. A counter capableof operating up to 50 MHz can count upto 500 MHz when used with a divide-by-10 prescaler.

Q — The ratio of the reactance to the re-sistance of a component or circuit. Itprovides a measure of bandwidth.Lower resistive losses make for a higherQ, and a narrower bandwidth.

RMS (root mean square) — A measureof the value of a voltage or current ob-tained by taking values from successivesmall time slices over a complete cycleof the waveform, squaring those values,taking the mean of the squares, and thenthe square root of the mean. Very sig-nificant when working with good ac sinewaves, where the RMS of the sine waveis 0.707 of the peak value.

Scope — Slang for oscilloscope. See theOscilloscopes section of this chapter.

Shunt — Elements connected in parallel.Sinusoidal (sine wave) — The nominal

waveform for unmodulated RF energyand many other ac voltages.

Spectrum — Used to describe a range offrequencies or wavelengths. The RF

spectrum starts at perhaps 10 kHz andextends up to several hundred gigahertz.The light spectrum goes from infraredto ultraviolet.

Spurious emissions, or spurs — Un-wanted energy generated by a transmit-ter or other circuit. These emissions in-clude, but are not limited to, harmonics.

Thermocouple — A device made up of twodifferent metals joined at two places. Ifone joint is hot and the other cold a volt-age may be developed, which is a mea-sure of the temperature difference.

Time domain — A measurement tech-nique where the results are plotted orshown against a scale of time. In con-trast to the frequency domain, where theresults are plotted against a scale of fre-quency.

TTL (Transistor-transistor-logic) — Alogic IC family commonly used with5 V supplies. See the chapter on Elec-trical Signals and Components.

Vernier dial or vernier drive — A me-chanical system of tuning dials, fre-quently used in older equipment, wherethe knob might turn 10 times for eachsingle rotation of the control shaft.

VOM (volt-ohm-meter) — A multimeterwhose design predates digital multi-meters (see DMM).

VTVM (vacuum tube voltmeter) — Ameter that was developed to provide ahigh input resistance and therefore lowcurrent drain (loading) from the circuitbeing tested. Now replaced by the FETmeter.

Wheatstone bridge — See Bridge circuit.

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