FM Packet Deviation MeterPut your packet station on the money for 20 bucks.

by Steven R. Sampson N50 WK

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I've really enjoyed watching all the sig­nals as much as listening to them, and it too ka bit of analysis to figure out the best way touse the meter. After several days of monitor­ing signa ls over the air, I found that thewhole range of the meter is used by various

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Figurr I. Schematic f or 'M FM Packet Deviation Metu .

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Photo A. Tbe FM Packet Deviation Mf!ter makes this station complete.

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causing a positi ve output to the third stage.Bill recommends that you not change thevalues of the second-stage resistors. The cir­cuit is based on the LM-324 op amp chip. IIdraws about 1 rnA total and will last forev eron a 9 volt banery. It's very simple to put to­gether on a perfboard. See Figure I .

T here are a lot of "plug-and-play" ama­teurs today, and many are working FM

packet. While many traditional amateurs candraw a Bessel function chan with their eyesclosed. this new breed of ham is a lot lesstechnical. Many have a hard lime digestingthe concepts of bandwid th and frequencydrift. never mind deviation. This article willhelp. It shows how 10 bui ld a useful Insuu­meru. explains exactly why it is needed. andchallenges the less-than-technical ham to ell­pand his or her electronic experti se .

Like most newcomers to VHF packet ra­dio, I SCI my system up by connecting all thecables and getting on the air. It wasn't 100long before I checked my audio levels. Un­like voice. there aren' t a lot o f people whocomplain if your packet audio is 100 hot ortoo weak. Actually, I don't think anyone lo­cally listens to the packet tones because Iwas hotter than a two dollar pistol. First J setmy rece ive audio level , and this was simplyan increase in vol ume until the TNC DataCarrier Detect (DCD) light illuminated, fol­lowed by a squelch adjustment (some TNCscan operate without squelch, and this is thebetter way to go). You can make a pre ttygood judgement about setting the transmitteraudio level by listening with another radio,but the CO!TCC1 method is to use a deviationmeter. You won't find inexpensive deviationmeters at any rad io store. so you're going tohave to build one. Thi s article presents a de­v ia tio n mete r based o n Wi ll iam C ro w lN6MWS's design from the January 1990 is­sue of 73 Amateur Radio Today. The circuituses pans available at Radio Shack, and willrun about $20. Bill 's ci rcuit featured manyother useful functions which I de leted fromthis design 10 make it a simple one-eveningproject.

Figure I shows the sc hematic. This meteris based on simple AC voltmeter principles.It picks up the j\C voltage from the receiv­er 's FM detector. amplifie s, recti fie s, anddrives the meter movement. The first stagetakes the AC volt age from a scanner or yourham rig 's discri minator output, b locks theDC , and amplifies it wi th a gain of three.The nex t two op amp stages form a cleverfu ll-wave recti fier fun ction. The positivehalf of the input wave form passes aroundthe second op amp to the third. while thenegative half is inverted by the second stage.

22 73 Amateur Radio Today. August, 1993

Photo B. The FM Packet Deviation Meter with cover removed. Photo C. The fin ished product.

Figure 2. PC board pattern and parts place­ment diagram.

and the predicted bandwidth using 5 kHz de­viation is a steady 22 kHz. Transmitting asignal wit h this wide a bandwidth is certainto fai l with distant packet stations, and likelyeven to fail across town. There are two rea­sons: First, most rigs will clip thc audio tolimit the deviation, which causes distortionThe second reason is crystal stabili ty. Onerig may be tuncd 1.4 kHz higher in freque n­cy, and the other 1.4 kHz lower, and still bewithin crystal tolerance on 145 MHz. Thisgives us abou t 12 kHz of worse-case usab lereceiver bandwidth.

Using 3 kHz deviation results in a modu­lation index of 1.36, and the chart showsabout four side band pairs, or 8 times 2.2kHz for a 17.6 kHz bandwidth on the more

oFH ~CKL DEV. MET

Deviation BasicsWhether an FM receiver has a discrimina­

tor, ratio detec to r, quadratu re detector, o rone of the modem phase detectors makes lit­tle difference as long as the output of the de­tector is proportional to the amplitude of themodulating torte. When a signal is fed to theFM modu lator, it varies the frequency di­rectly. The modulated PM signal is a vari­able set of sidebands whose total bandwidthdepends both on the frequency of modula­tion and the amount of deviation. The limitsset by the typical narrow band FM receiverIF stage is about 15 kHz.

The best method of determining the band­width of an FM signal is to use a Besselfunction chart, as shown in Figure 3. Youuse this chart to find the number of sidebandpairs and then compute the bandwidth. Firstyou calculate the modulation index :

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where P "" modulation index.D "" peak devi­ation, and m "" modulating frequency.

Then you ex amine the chart 10 see howmany sidebands there are on each side of thecarrier. If the curve comes off the baseline aline-width or more, I include that sideband.The simple bandwidth formulas you find intextbooks are all different and can be con­sidered unreliable. Use the chart . The worsecase example is an FM signal that has beendeviated 5 kHz with a modulating frequencyof 3 kHz. The modulation index is 1.67, giv­ing us four sideband pairs, or eight side­bands of 3 kHz, requiring an es timated 24kHz bandwidth to contai n it. This is quiteacceptable for voice when it occurs on lybriefly. Packet uses a high tone of 2.2 kHz,

ject is to get a downward deflection.Some radios produce a noticeable differ­

ence in the two AFSK packet tones. Here,you may want to do the alignment using themore critical high tone . As you might ex­pect, any frequency error throws everythingoff, so make sure both the meter's receiverand the transmitter arc tuned to the same fre­quency.

packet stations. The really poor ones drivethe meter against the 15 reading (over devia­tion) , wh ile no audio of cou rse, drops itdown to O. I chose the 213-scale 10 readingas the best calibration setting.

Without a signal prese nt, the discrimina­tor outputs a noise waveform, so the calibra­tion pot on the meter is al igned to centerabout this 10 reading. Calibration needs tobe performed each time the frequency ischanged. I usually monitor the frequency fora minute to make sure there is no interfer­ence, and then recheck ca librati on . Anyanomaly causes me to change freque ncy andrecalibrarc.

When a packet is received, the meter willdeflect downward for the good guys and up­wards for the bad guys. It's important thatyou only measure signals that are full-quiet­ing , as noise will throw the reading off. Ifind it best to kee p the circuit portable andtake it to the transmitter for alignment . RF isbad news for consistent readings, however.You can avoid this by both removing thescanner antenna and placing a dummy loadon the transmit ter. If yo ur TNC does nothave a variable deviation adjustment (a de­sign defect), the common method is to wirea 10k ohm potentiometer into the audio lineto the transmitter. Don't depend on high orlow jumpers to operate correctly-these aresucker settings.

After these initial steps, I usu ally com­mand the TNC into the "calibrate" mode andsend the high tone. Another good method isto command the TNC to the "co nve rse"mode and hold down the "return" key. I thenquickly adjust the aud io pot to my calculated3 kHz deviation reading (about 8.0 on mymeter) . Unless you calibrate your meter to aknown sou rce you are only guessing aboutwhat the readings mean, as each discrimi na­tor is different. 1f you can' t find a calibrationsource, you can listen to signals on the air ortune your station by car to get an initial esti­mate . After a couple of days you will quick­ly come to know what is good and what isbad by monitoring the performance of bothyour own and other packet stations. The ob-

24 73 Amateur Radio Today. August, 1993

1.0 Parts List

(Oplional)

All ressrces 1/8 watt

AS1t270-627AS1t27o-1754RSIt278-105RSIt276-1711(Optional)

Case PlasticM1 1 mA meterJ 1 BNC jackte-1 LJA-32401 2N222201 .0 0 1N914Al .R2 10kR3 330kR4 1 MegRS 1kR6 2.2kR7 9.1kAS 4.7kR9 8.2kRl0 12kR11 22kR12 l OOkR1 3 10k PotentiometerR14 1511 Comes with AS meterC1 0.01 ).IF Ceramic discC2 1 uF ElectrolyticDrilled and etched PC boards are available lrom FAR Circuits, 18N640 Fieldct. Dundee IL 60118 1()( $4 plus $1 .50 S&H.

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Figure 3. Bessel f unction chan..

critical high tone. and 12 kHz for the lowtone. Th is reduced bandwidth is much lessaffected by the freque ncy drift between sta­tions. and is not distorted by the transmitterdeviatio n limiting ci rcuits. By listening tothe audio produced by 5 kHz deviation youwill notice that it sounds raspy and terrible.while the 3 U lz sounds very pure.

Conclusion

The recommended setting for packet is 3kHz deviation. With thi s meter you canquickly adjust your station. and others. 10ensure thai the transmitter hasn' t gone intolimiting. and that the bandwidth is optimizedfor the typical receiver. By spending a littletime tuning up. you will benefi t the entire

non was able 10 connect with distant nodesthat I thought were unreachable due 10 mypower level or antenna height. Loan the me­ter OUI. and make s ure everyo ne gets achance to use it. Thanks go 10 Will iamCrowl N6M WS for an excellent article and arepeatable circuit design. and to Joe BuswellK5JB who helped me firs t 10 cal ibrate themete r and then to understand FM modu la­lion. III

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26 73 Amateur Radio Today. August. 1993