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"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.
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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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Fig. 2. PC board (fo il side).
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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.60
.30
.30
.30
.40
.40
Parts List
A11,A14A9, Rl 0, R23, A24R4",A3 lA30R13, R16, A34, R36A29A40A38
s
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