"Smart" Squelch for SSB

Editor's Note: W9MKV and W9YAN's "Smart Squelch" overwhelmed the competition to win the first 73 Magazine Hom.Brew Contest. Theauthors receiYed II $250 prize In addition to the normal article payment. You can build this trend-sett lrlg project; W9MKVoflers 8 PC boardlor $7.00 and a complete parts kit is availab le from Radloklt, Box 411 , Greenville NH 03048, lor $49.95. Congratulations to W9MKV andW9YAN lor a Job well done.

•

Photo A. Squelch unit is attached to the right side o f the HF SSB transceiver. RectangularLEOs above the control knob indicate circuit status. (Pho to by KA9FJSj44 73 Magazine· August, 1982

frank S, Reid W9MKVPO Box 528J8loomin~lOn IN 47401

David A Link W9VAN21J Westt'l"n DriveBloom;ngton IN 47401

This c ircu it detects thehuman voice but ig­

nores noise, steady tones,and the Russi an wood peck­er HF radar pulses . It re­quires no receiver mod ifica­tion and wo rks even whenvo ice signals are below thenoise leve l.

A sque lch turns off re­ceiver audio to eli minateannoying background noisew hen there is no signa l.Squelch circuits in AM andFM receivers are carrier-op­erated, On single sideband,which has no c a rr ie r,squelching is more d ifficult.Most SSB rigs with sq uelch,e.g.. the popu la r 2-metermult imode t ran sc e ivers,use age (5-meter) vo ltage toopen squelch in SSB mode.Age-operated squelch is ad­equate for st rong signa ls onrelatively quiet channels.Agc and VOX-type squelch-

es open for any noise or het­e rodyne that exceeds a pre­se t level . Weak signa ls of­ten are missed because theth res hold mu st be setabove the noise level.

White noise sometimescan ma ke you imagine t inyvoices in the noise, bu t itwon 't fo o l the Sm a rtSq uelc h. Detect ing unread­ably-weak signa ls is worth­while if a change of anten­na d ire ct ion or re ce ivercontrol settings will makethem usable.

The audio-ope ratedsquelch circuit described

here is simila r in principleto Motorola 's "Consta nt Si­ned" squelch, a discrete­component circuit with 22transistors.

Discriminating theHuman Voice

People normally speakabout three syll ables persecond. The sq ue lch work sby detecting vo ice-band en­ergy (500-3000 H z) which isvaryi ng in frequency at arate of 0.5 to 3.25 H z.

The circuit is a type ofFM detector. It is insensi­tive to amplitude va riationsthroughout the range wherethe input stage is not drivento sa t u ra tio n but bac k­gro u nd noi se is st ro ngenough to sa turate the lim­iter. The sq ue lc h wor kspro perly with most speaker­level signa ls. You can con­nect it di rect ly to the re­ceiver's detector output.ad just ing ga in of input buff­e r a mpli fe r U1A as neces­sa ry.

Performance

A rec e iver t un e d toWW V pro vide s a gooddemonstration of the cir­cuit's capabi lities . Squelchopens fo r voice announce­ments and ignores the restof the transmissions.

The sq uelc h can turn onwell within the first spokensyllable. Speed of responsedepends mostly upon therise-time of ac tive low-passfilte r U3A. The receiver ismu ted one second af te r thelast voice detection. Thebeginni ng of a steady tone

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opens the sq uelch only mo­mentarily. l t opens intermi t­tently on music . Responseto CW depends on codespeed and tone.

A single squelch c ircu itcan contro l mult ip le receiv­ers, unsq uelching them a llwhen any rece iver detects avo ice signal. (We like tomonitor HF aircraft and ma­r ine frequencies plus 144.2M Hz-the 2-mete r ssecalling freque ncv.I

The sq ue lch is usefu lwhen rf radiation fromcomp uter systems over­whelms the normal sq ue lchin a VHF FM receiver. It' sa lso good fo r moni toringVHF/U HF mo b ile-te le ­phone c hanne ls in systemswhere a co nstant idle toneis transmitted while no callis in progress. The c ircu itha s other app lications as a

" smart" VOX (voice-operat­ed switc h) for transmitters,recorders, intercoms, secu­rity systems, remote-basesystems, and repeater equip­ment .

Circuit Description

U1A is a unity-gain sum­ming ampl ifier, input buff­e r. and low-pass filter with3-kHz cutoff. U1A d rivesU1 8, a th ird-order high-passact ive filter with 3-d B cut­off at 500 Hz. We chosehigh-perform ance FET-in­put operational amplifiersso that act ive filters co ulduse high resista nces andsma ll capacito rs. The n 084quad o p-amp chip is eq uiv­a lent to the Nationa l lF357.

U1C and U1D are limiteramplifiers with a combinedgai n of 85 d B. U1 D's outputis voice-band aud io turnedin to co nsta n t-a m pl it u d e

sq ua re waves. The sq ua rewaves trigge r CMOS mono­stab le m ultivib ra to r U2 .Output of U2 is a train of.33-m ill iseco nd pu lses. onefo r each aud io cycle. Theaverage voltage of U2's o ut­pu t is proportional to the in­put frequency. U2 a nd thefo llowing low-pass filte rform a frequency-to-volt­age converter, i.e ., FM de­tecto r, somewhat sim ila r toa n auto mobile tachometerci rc ui t.

Active low-pass f ilt erU3A cuts off a t 3.25 Hz, thebest compromise betwee nnolse-fals ing and the rate atwhi ch people speak svlla­bles.' Note tha t U3A has nobias network even thoughthe amplifier use s a single­pola rity power supp ly. U2'save raged pulses keep theoutput o f U3A at 5 to &

vo lts with normal noi se in­put from the receiver. R1 7,which sets U2's period. canbe va ried to keep U3A's qui­escent output voltage nea rthe cente r of its range.

On very quiet channel sthe re may not be e noughpulse s from U2 to keep U3Apro perl y biased. Fa lse de­tec ts may occur as U3A'soutpu t goes in and out of itslinear range. You can injectextra noi se or low-leveltone into the squelc h ci r­c u it's a ux ilia ry input toachieve the desired resultsfor your particul ar applica­tion .

U3A's output is ac-co u­pled to U3B. which a mpli­fies with a ga in of 2, andthence to U3C. a unity-ga ininverte r. U3B and U3C to­gether fo rm a phase splitte rw ith a gain of 2. The phase

73 Magazine. August ,1982 45

splitter provides posmve­going outputs for positiveand negative frequency de­via tions of the rece ive raudio.

Comparators U4A andU4 B d etec t t he ra te-o f­cha nge-of-f req ue ncy sig­nals fro m the phase-spl itteroutputs . If the vo ltage atthe inverting (-) input ofU4A or U48 exceeds the ref­erence voltage set bysquelch-threshold controlR30, then the low-going lev­e l at the comparators' par­alleled open-collector out­puts disc ha rges C20th rough R34 and D1 . Thedisc ha rge time constant is10 m illiseco nds. C20. R35.and comparator U4C forma time-delay circuit whichholds squelch open duringits one-second period. Eachdetector output longer than

10 milliseconds resets thetimer for another one sec­ond. R35 controls length ofdelay .

U4C's outp ut is t h esquelc h-open signal (activehigh). U4C tu rns on hystere­s is-switc h transistor Q1(whic h lights LED2) and ac­tivates output-driver U4D.As shown. U4D's outputgoes high to unsquelch. Weused normally-closed relaycontacts so that the speakeris enabled when the relay istu rned off or if power is re­moved from the squelc hcircuit. To reverse the se nseof the output, excha nge t he(+) and (-) inputs of U4D.(J umpers are provided onthe PC board.) U4D's open­col lector output can drive arelay in the speaker lead, asshown. or a gated amplifier,ana log gate, optoisolator.

or TT L o r CMOS logic ci r­cuit. The comparator out­put can si nk 50 rnA max­rmum.

The squelch is more sen­sitive a fter open ing than be­fo re. The se nsitivity changeis ca lled hysteresis . With nohysteresis, the sq uelch mayd rop out while someone is

talki ng. If there is too muchhysteresis , squelch thresh­old becomes hard to adjustproperly. Detector compar­ators U4A and U4B havetwo leve ls of hysteresis.Po siti ve-feed back res istorR32 prevents comparato rosc illation and lowers theth reshold slightly during a

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48 73Magazine . August , 1982

Fig. J. Component layout

Photo B. Circuit board and chassis detail. The only external connections required are receiver audio, speaker, and12 volts dc. (Photo b y KA9F/5)

detec t. Q1 conduc ts whil esq uelch is o pen, further re­ducing the threshold volt­age via R42 and 02. R42 de­term ines the amount o f hys­teresi s. The 100k valueshown for R42 provide ssmooth squelch operation.

The ci rcu i t uses 25-30 rnAplu s relay current. Theeight-volt-regulator IC, US,

should be used for mobileoperation. Otherwise, theentire circu it can run from awell-regulated 12-volt sup­ply. (O mit US and add ajumper between input andou tput pins of US on the PCboard.)

Adjustment

LE01 lights w henever the

detector is active. listen toa voice signal and ad justthe threshold co nt ro l untilLE01 bl inks for every spo­ken syllab le, then make finead justments as necessaryfor noise conditions. Theenable switch allows you toadjust t he squelch beforeactivating the rel ay and al­lows you to unsquelch with-

ou t disturbing the thresholdsetting. Sc ale mark in gsaround the cont rol knobmake it easier to reset an of­ten-used level .

Construction

All-new parts cost about540, using referenced itemsfrom Radio Shack and ECG.Resourcefu l hams can build

2.7k 24.7k 46.8k 2Sk, linearpot 110k 418k 147k 156' 168' sl OOk 4130k 3150k 2160k (see text) 1

300' 1360' 21M 22.4M 1

42 @ $.08 ea. = $3.28

2 .103 1.291 2.991 1.491 .49

10.00$40.85

OtherFerrite beadsMiniature earphone jacksRelay, tz-vott SPOT (AS 275-000)Switch, miniatu re toggle SPSTControl knobHardware, PC board, chassis

PartsTotal

AS · , R6", R7, AS, A2lR25, R26,R39, R42A22, A27,R28A18-, nrs-A17A20R12, A15R3S, R37A32

.12

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Parts List

A11,A14A9, Rl 0, R23, A24R4",A3 lA30R13, R16, A34, R36A29A40A38

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1.60.99

2.382.00

$ 5.981.491.50.99.1'.20.10.49.49

Capacitors (All 20 V or more)C12 68pF 1ca. CS, C1 3 1000 pF 3C7, C8 4700pF,5 % 2C6 0.047uF, 5% 1C2 O.056uF 1C10, Cl1 0.1 uF 2C1S 0.22uF,S% 1C14 0.47 uF, 5% 1Cl , C4, C20,

e21 1 uF, electrolytic 4C16 4.7 uF, non,polarlzed(RS272-998) 1C18, C19 6 uF, electrolytic 2C9, C17 47uF,elactrolytic 2

SemiconductorsUl,U3 TL084CquadBIFET opamp 2U2 C04047CMQS multiylbrator {ECG 4047) 1U4 LM339quad comparator 1U5 7808 a-vert regu lator (opt ional- seetext) 101 2N2222 or equiv. emcc n NPN transistor 101,02 lN9l4 orequiY. silicondiode 2D3 lN4002 0r equiY. slllcond iode 1LEDl Red LED (rectangular) 1LED2 Green LED (rectangular) 1

Resistors (All '/4 Watt; • = 5"10)Rl,R2,R3,R33,A4l lk

48 73Magazine · August, 1982

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ElE(TRONICS

61 Lowell Road

Hudson, NH 03051

ca n be preset to precise fre­quencies.

This ci rcuit can be a start­ing point for many experi­ments. You co uld, for exam­ple, insert an ana log delaydevice between audio inputand output. If the delaywe re lo n ge r t ha n t hesquelch response time, thensq uelch would o pen beforethe fir st spoken sy lla b lereaches the loud speaker.

Digital techn iques couldperform the function of theana log ci rcu it describedhere. perhaps with improve­ments such as adaptivethreshold and prog ram-con­trolled time constants. Weare ex perimenting with am ic r 0 p ro c e s sor-b a sedvoice de tec tor which maybe the subject of a future 73art icle .•

References

'Don lancaster, Active FilterCookbook, Howard W. Sams &Co., Inc., 1975.' Motorola Micom HF SSB Trans­ceiver Service Manual, 1975.

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Conclusion

Although sque lch effec­t iveness may dimini sh o nvery c rowded amateurbands, a sensitive, d iscr imi­nating squelch is very use­ful for net ope rations andscheduled contacts, espe­cia lly with modern digital­ly-tuned rece ive rs which

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the ci rcuit fo r su bstantia llyless.

We built several proto­types on universal pri nted­ci rcuit ca rds. The only c riti­cal a rea is U1, where highlimiter-ampl ifier gain cancause feedback osci llationin some layouts . Keep com­ponent lead s as short aspossible . Use 5% toleran ceo r better for freque ncy-de­term ining components inact ive filters . The Rad ioShack rel ay's fra me mustbe insu lated from ground.Mounting the re lay on arubber pad quiets its c lick­ing and isolates it from vi­bration.

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50 73 Magazine. August, 1982