claim: . 1. In a slngle sideband selective filter system of the character described: a local oscillator; a mixer for combining a signal under examination and the signal from 20 said oscillator; said single sideband selective filter including means generatin-a said carrier signal and being connect,-d to receive the output of said mixer; means to derive a pair of signals from said single sideband filter having a frequency related to the difference between the 25 'frequency of the signal at the output of said mixer and the freqljency of said carrier signal,, one of the signals lead;ng the other si.-nal for the frequency of the signal at the output of said mixer being greater than the freqliency of said carrier -signal, said other s-ignal leading 30 s@lid one signal for the frequency of the signal at the output of said mixer being less than the frequency of said carrier si,-nal; a trigger circuit adapted to operate only in either one of two operating states, said trigger circuit being connected to receive said signals and being 35 adapted to produce pulses having a repetition rate related to the f.-equeiicy of said signals and having a duty cycle related to each of the phase relationships between said signals; means prod-ucing a first steady voltage related to the duty cycle of said Pulses: means producing 40 a second steady voltage related to the repetition rate of said pulses; a gain inverting circuit providing in-phase ampli'.ication of applied signal between a selected level and one operatirig liniit and providing out-of-phase amplification of applied si,-nals betwe.-n the selected level @i5 aid another operating limit, said gain inverting circuit being connected to receive the combination of the first and second steady volta,@es; and means for. so applying the signal at the output of said gain inverting circuit -to said local osc-luator to vary- the frequency thereof as to 50 lock the s'@gnal under examination within the passband of said sin,-le sideband selective filter. .2. In a single sideband selective Mter system having a pair of channels, one receivin.- a carrier signal, the other receiving said carrier signal shifted in phase: a variable 55 ..gr@-quency local oscillator; a mixer for combining a signal under examination and the signal from said oscillator; said single sideband selective filter including m6ans generating said carrier si-,nal and being connected to receive the oulput of said mixer; meaus to derive a signal 60 from each ofsaid channels havng a frequency equal to the difference between the Lrequency of the signal at the output of said mixer and the freque@ncy of said carrier signal, the signal from one channel leading t@e signal from the other channel for the frequency of the signal 65 at the output of said mixer being greater than the frequency of said carrier signal, the signal from said other channel leading the signal from said one channel for the freouency of the signal at the output of said mixer being less than the frequency ofsaid carrier signal; means 70 to derive half cycles of said signals of or)posite polarity; a trigger circuit adapted to operate oniy in either one of t@vo operating states; said trigger circuit being connected to receive the combination of said half-cycle sigrials and being adapted to initiate an output pulse when 75 the combined signals pass in one direction through a

3,150,322 5 ref-@rer@ce level and beinl- adap,-,d 'lo ter-rnLnat@- saidrlilse when the combined signals pass in the opposite d@rectioia throu.-h a reference lev--I; =@,-ars prodici-P..- a --nrst steady voltage related to the duty cycle o'L said PU"@Ses; means produc;n,@ a second steady voltage related to the 5 repetition rate of said pullses; an inverting circuit providin.a in-pl-lase amplil-ication ol. applied signal bc@tween a selected It--vel and one oi)eratl:--- limit and provid-.'--g out-of-phase amplification for appl,'ed s;.gnals betvicen the selected level and another operafing limit; said in- 1( vertin- eirellit being conneced to receive the comb-,-Iation of the first and second steady voltages; and means for applying the output of said invertin.- circuit lo said local oscillator to vary the freqlency tnereof; whereby the signal under examina"ion is locked at selected posi- 5 tions within the passband of said selectiv@ filter. 3. Apparat -as for maintainid- a s;gnal at sel@ected positi-Ons wilhin the pass band of a superheterodyne systeni, the apparatus comprisin.-: a vari-able freq,,ie--lcy local oscillator; means for mixing a signal apdlied to sa:id appa- 20 ratus and the si,-nal from sa-id osciila'Lor to produce a modulated s-ignal; signal gencratin.- means producing a pair of signals having a s-.Iected phase differe,2ce; means producin.- a first signal as the difference bet,;veen sa-id modulated signal and on.- of said pair of signals; means 25 producin.- a second si,-nal as the difference between said modulated si,-nal and the other of said i)air of signals; a tri,-ger circuit having an initial condition of stabi,ity and an actlated conditio-@i of stab-.'Iity and being co-nnected to receive the combinalion of the first and second sigrals; 30 said trig,@er circuit produc;n,- outt)ut pulses when tri g,-red from one condilon of stability to the other in response to said combination o'L signals appeed to the inpil@ ther,-of; said output pulses havin,@ a repeti@Lion rate equal to the frequency of said signals aiad ha,,,ing a 'irst dilty 35 cycle for the first signal leadin.- the second signal and havin- a second duty cycle for the second si.-nal leadin.- th, first si,-nal; means producing a -,9xst steady voltage proportional to 'the repetition rate of said pulses, means responsive to the duty cycl@e of said pulses to rroduce a 40 second steady voltage of one vali,-e related to said firq duty cycle and of another value related to said second duty cycle; an inverti-,ig eircilit providing phase invers;on only of signal@s applied thereto havin.- amplitudes w;thin one op-.ratin.- limit and a reference level; means to apply 45 the combination of the first and second st,-ady voltages to said inverting circuit; and means to apply the outpat of said inverting circuit to said local oscillator to v,)ry the frequency thereof, @,vhereby the si,-nal applied to said apparatus is locked at selected locations v,,ithin thf, pass 9A band of sa;d a-pi)aratLs. 4. ApparatLs for maintainln.- a si,-nal at selected positions within the pass ba--d of a sup,-rheterodyne system, the apparatus comprising: a volta.-e con' rolled local osci'llator; means for mixLTg a si,-nal applied to said apparatus 55 and the signal from said oscillator to produce a r-riodulated signal; signal gen.-retiTi.- m,-ans producing a oa-ir of si,-nals hav-ing a selected phase di-LTerence; m-.ans producin.- a first signal as the dfference bet-,veen sa;d modulated signal and one ol' said pair of signals; means i)roduc;ng a C-0 second signal as the differenc.- between said - morli-ilated sigr-,al and the o,her of said pa-'@r of s;,-nals; a trig,@er circ,,iit havir@g an inilial condition of stability and an actu-,ted condition of stability; ar--Ctifier circuit conilected lo apply @o the @@'npLit of said trigger circuit tac combinatio-Ti of hal.f c,,cles of one polarily of said flrst signal aiid half cycles o, opposite polarity of said secord -si,@nali; said trigger circuit ;nitiat@'ng an oil-tD@@t puise when triggered from o-ie condition of stability to the o@h,- r in response to tcombir-ation si6-nal ap-,oli,-d to the ir.,out thereof passing in one d;rection throi-igh a reference level and terininat,ng t@ie 0,,itrL,.t pul,@-- when triggered from said other condition ol. stal@iiity to the first in response to the combinafion si.@nal applied at the input thereof passing in the opposite direction through a refererce level; means producin.- a first steady voltage proportio-nal to the repeti'-,ion rate of 's said pui -@s; means responsive to the duty cycl.- of said t ,e pulses to pro-di-ice a second steady vo' a@ of one value I )-Or a duty cycle greafer than one-half and of another valle for a duly cycle less than onehalf; an invertin.@ circuit providin@- phase ilnv-.rsion only of signals applied therello having aniplitudes within one ope-raling Li-nit and a reicrence means to appi-y ,he combination o'l the first and second steady vol.,ages to said inverting circuit; -Tid means to a-rply the o@,itput of said iqverting circl-lit to said local oscillator. 5. A c@,,cu;t responsive to tiie lead-lag phase reletionsh;p bel,7Nveen a pair of signa,s, said circuit comprising: rectlfier means producing half cycles of one polarity from one of said signals applied thereto and producing half cyr-'Ies of oppos:lte polaricy from the other of said signals -,tpplied the-reto; a trigger circuit adapted lo operate only in eilhe-r one of tivo operating stat,-s; ipeans to apply the combination of said half-cyc'le si.-:@@s to the ilip-Lit of said trigger circuit; said trig,-er circuit producing pulses when tri,-,-ered from one operating state to another in response to sional applied to the in-Dut thereof, said pulses haiiing a repetition rate related to the frequency of said pair of signals and having a duty cycle related to each of said phase relationships between said pair of signals; means to produce a first steady voltage related to the repetition rate of said nuises; means to produce a secon-d steady volta,-e related to the duty e-ycle of said puls--s; an invertin- ci.-cuit providing out-of-phase amp iucation of si.-nals @appeed 'Lhereto havin.@ an amp]-itude between a selected valil-c and on.- operating limil and prov.,din- inphase ampli--@icaLion o'L s-'@gnals applied thereto hav'm@g a@i .inip'iitude between said setected va@'.ue and atiother oioeratin.- limit; and means includi-iig said inverlin- c-' cuit . ir and producing an otitput signal -'Lrom the combination of the flrst and second steady voltages. References Citzd in the fil.@ o'L this Patent UIN!TED ST,@ITES PATENTS 2,836,712 Crosby ---------------- May 27, 1958 2,-088,527 Folliensbee et al - -------- May 26, 1959 2,916,618 Adams et al - ------------ Dec. 8, 1959 2,924,706 Sassler ---------------- Feb. 9, 1960 2,930,891 Lai--atos ---------------- Mar. 29, 1960 31060,326 VVatson ---------------- Oc'L. 23, 1962 3,076,141 Baumel ---------------- Jan. 29, 1963

Ujlited States Patent Office p -a 37150@322 3,150,3@22 .kRECE'x',TE,l S!GR-JAL 7 OCKE',,@T4- -A-PPA:&@',ATUS USE@TG =GGET.' CZ,7L'@CU-z@. !N' CC@iN-iP,.CL LOG:? or @o W. V.-m Sa-an, CaHf., gssi Hev@,l@-t-t-Packa-d Com-@ai-y, Palo Alto, CaEit'., a co@-po- 5 r@-tion of California F-,Ied A-L@g. 20, 196.2, Ser. iNo. 217,374 5 C (C]. 325-420) Th.s iniontion relates to 'the method and means -,'or 10 automati-@ally locl,;ng a signal at selec'Led positions -viithin the passband of a freciuen--y-s-,Iective system. Frequencies about a s--Iectcd sideband in a modllation system inay readily be extracted by using a tLL-ied filter or other narrwv passoand device. Wnere exlreme- 15 ly high selectivity is reqa@red about a h-@'- frec. -,:icy -,h -U sideband, it is kno@vi to provide a lo@v pass filter to pass the di,i'erence freq@aency of an ap-,olied freqi-lency and a carrer reqL,-,ncy and to mix this diffe.-etice fi-equ-,ney a.-ain viith the carr.;er freqlieicy. This has the ei@ect of 20 shifting the actlal and image frequency characlerist:cs of the low pass filter in irequency by the va',ue 04: th-- carrier L-equency. Undesirable sidebands created by mixing the difference frequency agah-i with the carriar frequency may be cancelled using anotner s-imilar chati- 25 nel opoi-ating on the same appl;ed and carrier frequenc:ies, one of w'@iich is shifted in Dhase a@-icrle by 90'. FreqL,@-ncy selective devices of this type are describ-ld in the liter-,,'Ltire (see "A Third lvfethod of Generatio@i and Detection of Sin.-le Sideband Signals," Donald K. -NVea- 30 ver, Proceedings of the IPE, vol 44, No. 12, Dece@nb.-r 1956, pa,@es 1703-1705). NVhere this schem-. is used 'lor sel--ctivity in appl,.eat-@ons requirin.- a, capacitor to block the thc=al drift vol.tages produced by the @-nodu"ato-rs, a nulli ap,,Pears in 35 the freq@uency cilaractei-istic, of the syster@i at t-l-le carrier frequency. An auto-x-,atic frequency control circuit operatin@ o-n this type of passband characteri-St;c may lock onto the nall and thereby cause a docrease -Ii s:l-mal transmission. It is thus d-@sirable in a freqllCncy 40 selective system of thl:s t,,Tpe to pro-,@ide an automat:c frequency control circuit vihich locks the desired si,-11-,.@l withl,-l the passbar@d at selected nos,.tions on e,.tner side of the null in frequency characteristics. Accordingly, it is a t)rincinal objcct oil the t)res,-nt in45 v---iition to pro,,ide a sigiial loc-kin@ circuit Vnich is capable of lockin.@ a si-nal -,,i-'@thin the passband at ei,h,-r side of the null in 'Lhe f@-eauency characte,-istic of a frequepcy seleelive system. In accordance with a preferred embodiment of the 5( Dr-.sent @.nvention two sigiials are produced wh;ch have a freq-@ioncy --qual to &-e dl:iTer--nce betnve--n the frequency of an applied signal and a carrier frequency. These signals @re derived from the frequency select-t,,Ie eircu,'t and are separeled by a r.-lative phase argle of 55 90'. If the fi-cqtie-.icy of the applied s:,-nal is larger than the carrier 'reque-@icy, one of 'Lhe Si,-nals jeads the other sianal in quadrature. If the frequ----cN, ol. the applied s;giial is less than tl,-. carrier frequency, the one si,anal lags the other si.-nal in cluadrature. These si-- 60 nals are used to control the re-pet;tion rale and d-Litl,7 cycle of puises produced by a trigg,-r circuit. The pulses P.re app]7ed to a circuit v,,h;ch T)ro.,_iuces a -teady volta@e propo.-tional to 'Lhe rei)ctitio--,i rate of th.- ruises atid to a circuit which prodi,-ces a steady volta---e proportional to the dlty cycle of the pu,is,-s. TI-c com't)inat-ion of th-,se steady volta,-es is tpplied to an i-qv@-rtin.- clrcuit ivhii@ii provid.-s in-;)hase ainpliflcalion of signals havin.- an amolitud-. above a critic,,ll value an-d vi-@'lich p,-Ovides out-of-P@-ase amPL-iication of s;-.nals having an arilioli10 tude below the critical value. T-be oii'@put 7;olta.-e o4-@ the in-verfl,n c;rcu-it is the-@i used 'La co-iitrol 'Lre@e-.ited Sep'. 22, '6-96A@ que.-ey of a si.-iia'l which is mixed with an i-.iput signal to prcduce the sig-;ial applied to the -,-'rea-uency selective circuit. -r and inciden,al ob,@-ets ol' the present invention Oth@ i - ap@ it.Lrom a readir@- o4 this spe.-i-'qcation and xi ll b ia -CL , iaq insicet-on of the accoi-apanyi @ d@-awin- vihich shovis n. a sc-he--nal c diagra-m oj' the s-ignal locicing apparatus of .a prese.-@t iq-,,ention. P-,cferz'r,..- to the drawing, s,'@,--@al appearin.- at inp,,it -Lerm,'@-,ial 9 and the s-,@n,-.1 from local oscillator 11 are r@-I-.Xed togeti-ey in mixer 'e,3. The outpul o-'L mixer 13 is a- . - li e _L),-,I:ed to the se'@ec+ ve -i'@lter desigiiated - nerally as 15 wh@@ch coristitul-,s a selecti-ve netv,,ork- that tyoically may p@-oiide a passba-@id of 200 cycles ba.-iidwidth about a c@,rr;cr freouc-ncy of 3 n-iegacycles. A fus'L chanpel of tie sele@,ti@ve filter 15 coinectin@ t,ie ou' ut of mixer @p '-,3 and oLi'LDut @'erm;nal 1,7 inc@iudes s,-rially conv@ected mixer 19, low pass ilter 2-i and balanced inixer 23. A -.-cond cha-iiti@-I of th@3 sel@ective 4!lt@-r includes serially co-inected m:lxer 25, low pass filter 27 and balaticed mixer 29. T!2e cai-rier osgi'l a,or 31 is connected to the --.rixers --@9 and 23 o-'L the first chaunp-I and is connected throu,-h phase netv;ork 33 to tLe -@-ixers 25 and 29 of the seco-ad c-h,-.iinel. A first recti.Lying - @ietvork iTiclud;pg resist-,r 35 a-@id diode 37 is cornected to receiva t-@e output of low pass fl!ter '21. A second rectii'yiiicircliit in-cluding resistor 39 and diode 41 is conrec@Led to receive the output of low pass filter 27. The outputs of 'Lhe rectiiy;n- circu;ts ai-e corf,L)'ned usi g res@stors n 43 anj :,,s a-tid are a-.-)LIied to 'Lhe input of tr'--er circuit 47. Pulses produced by t,ac tri-ger circulit 47 are ap1)lied to ti'7e freqil-ency-to-D.C. conie.-ter,49 and to the IoNv pass f;@"ter 51. n, e outp@its of t@@e conver'cr 49 and illter 51 are combined us-,n@ r-si-tors 5' and 55 and -ar-- app,icd to the i:,verLi--Lig circuit 57. -lbe invertirig circu,-, @'j7 in-ciii-des trans:stor 59 haiing i s base e' el -t le trode conr@ec@Led fo receive applied signal. Scrially conrected d@ode 61 ar@d Ze@ier diode 63 connect 'che collector a-iid em'tter electrodes of the trans;stor. The out-rut of tLe invert-@Ti,- c,.rcuit is @ieri-.7ed from the corr.nion connectiori oj' t'@le diod, es. -Tn operation, a si,-Pal titid--r examinatio-@l app@-aring at ir@,Dut term-;nal 9 is mixed witr. the frecricacy of local oscillator @l I in m. ixer 13. Th-- olitput of rp.@er 13 is applied to ',I-ie frequc-.ney-se'lective single-sidp-band fl]ter 15 which t)rovides an extre@-iiely narroav passband a:,7oind t'--e freci@-lei,@ey of c,,--rier o@-cillator 31. Where the pass f-,I!ters 21 ar-d 27 of 'Lh-, -iqlter 15 are capacitively coupled to block D.C. from t.ae nixers 1-2 and 25, a n7Lill ki)pears in the ceiter of the pas.@band character'ist@C c@iL - iq',fp_r 15 abo-ct the frequenr-y of carri,-r oscillator 31. It is essential to lock a s-ignal within this passba-@id oa eitlier sicie of the nall by var3ir.,- the freqtieqcy of local oscillator 21. -lo this end the rectifyin- circuit i--Liel@,iding d;odes 37 and 41, trigger circuit 47, lwvv p-.iss f-@iter 51, the f.-,-,q-!i-.:icy-to-D.C. converter circuit 49, and the -1--iverfir@f.@, c-- cuit 5'1 are provided to produce a signal which loc@ks the si,-nal o@i eilher side c)f t@ie null ; th-. pass_n ba.,id of the single sdeba-iid filter 15. The signal aL t'ne c,,-lti)ut of ni-@er TLZ which has a freci ency of a_opro,-@iiu m,,itely 'Lhr@-e nie-acvcles for any freciuency applied to the ;nnut tetrqinal 9 @ii,L'I'in the indicated ran,@e, is applied to fne inruts cf mixers 19 and 25. The si---nal is mixed ixith ',he frct,:tency ff@ carrie-r csciiial@Or 33- in mixer 19 of tl-le first char@nel and is mixed wih the same frequency shifted 90 de-,Fees bv phase s!iift network 33 in mixer 25 of the second ch?@inel. Th-, resulting difference signals having the sa-,re frecraency are applicd to the lo-@,7,, pass f-@lters 21 and 27 in each of the channels. Thes-. fiiters are designed to cut o'f slarvl qt a loii aadio f-reou@ency, say 100 cycles, - y Pnd ar.- desig-icd to have zero t,-ansmiss@,On at D.C. T-h@- -p diff,-rence s;gnal appeari:,ig at tde ou@ u' o'l 1(,,Tl pass

3 filter 21 is r-etified by the retwork ii-icluding diode 37 and resistor @5. Tne difference si,- nal appea.-ing at the otitput of low pass filter 27 and shift-.d by 90 degrees relative to the signal appearin.a at the output of fflter 21 is rectified by the circuit includin.- diode 41 aid re-sistor 39. The positive half-cycle of the signal from the first channel and the negative half-cycle of signal from the second channel are added in tfie resistive network hicluding resistors 43 and 45 and are applic-d to the input of the trig.-er circuit 47. This trigger circuit,. whic-la may be of the type conimonly known as a Schrritt trigger, initieles an output pulse as an applied s;gnal passes in one direction throli.-h a given ret'erence '-,Cvel, say zero, and terminates the output pulse as the applied signal passes in the opposite direction throu.-h a -iven re@4erence level, say zero. ThLs, if the freqbency of tl,-. signal appearing at the output of the mixer 13 is greater than the frequency of carrier oscillator 31 the circuit operatesin the "forward" coidition producing tl-.e indicated waveforms. If the frequency at the output olt mixer 13 is less than the -'Lrequency of carrier oscillator 31 the circuit oi)erates i-.1 the "revers-," condition produ--in.- the indicated waveforms. It can be seen that when the circuit operates -in the "reverse" condition, tr-'@gger circu,.t 47 prolduces outt)ut nulses ha-,zing a greater duty cycl-@ than the pulses produced when the c;rcuit oderates iii the "fo-@-@vard" condition. Also, th-- repetition rate of the pulses produced by the trigger circuit 47 is eciual to the fr.-quency of the d-@fference sigr@als app,-aring at the ou',puts of filters- 21 and 27. These pulses when appeed to low pass filter 51 produce an output voltage whi:ch is related to the average value of the applied pulse@s, ivhic-,,i avera,@e value is large in tne "reverse" condition and small in th-. "forward" condition. At the same tirre, a steady ,,oltage -is prodticed by fie frequency-to-D.@'. converter 49 which is relate,d to the repetition -ate of the pulses. The combination of these tvio volta s provides 'ge an indication of the relatior.,ship between the Lre-qtiency of the signal appearin.@ at the output of mixer 13 and the carrier oscillator frequency. Thus for the ",LorNvard" conditio-@i, a low volta,@e which correspond-s to a lo,@v duty cycle and -,vbich increases with difference ireqi-lency is applied to th-- base electrode of transislor 59. For the ,, reverse" conditio-ii, a hi.-her -ioltage which correspo-.ids to a h:lgh duty cycle and @vhich increases with differenc-frequency is-adplied to the base electrodc@ of transistor 59. The ;nverting circuit 57 viiiieh includes transistor 59 provides outof-phase arr@plification of signals below the valtie of the largest volta,@e produe.-d in the "forward" conditior., and prov;des in-nhpse amplification of signals above ',he val@,,@e of the lowest voltage produc--d in the "reierse" condition. Transistor 59 is so biased that Zener diode 63 is rendered conductive in the reverse breakdown region and diode 61 is rendered nonconductive for lo,v volta,@es applied to the base electrode. The trans7s',Or thus operates as a common emitter staac and provides out-of-phase amplfflcation of applied signals. Zeiaer diode becories nonconductiv,.- inthe reverse voltage region and diode 61 becom--s cor.,ductive when tiie- volta.-e applied to the base electrode and amplified is Qufficiently large to reduce the collector voltage belo@w the Zener voltage of diode 63. Transistor 59 then op.-rat-.s as a conmonco'@lector sta.@e and -Drovides inphase amplification of a@2pEed sgnals. The control voltage applied to local oscillator !iL to vary its frequency thus increases from a low value as the frequency of the applied si,-nal L-icreases above the carrier fiequency and decreases from a higher value as the frequency of the applied signal decreases below the carrier frequency. If the local oscillator 1 1 is desi,-ned to sh,.ft frequency iTi respons-, to voltage va-riations about a reference potential, say ground, then 'Lh,- voltage applied thereto fron the inverting circuit 57 calises the frequency to shift above and below the frequency established by the reference.len,el on eacli side of the nuH in the frequency cnar3,150,322 acteristic of the selective fil@er 15. The transmission characteristic between i-.iput t'erminal 9 and the output terminal 17 i-@i one embodiment of the present invention is tp-us a passband 200 cycles wide about the frequency .5 of t:qe signal appearing at input terminal 9 and which has a nlill in the center of the passband that is approximately I cycle wide. The apparatus of the present invention thus provides the means for locking a signal on either side of the null 10 ir, the passband of a highly selective single sidebard filter. This permits selected frequency components of an applied complex waveform to be isolated within the passba.-d for independeit analysis and to be tracked autorjaatically over small ranges as the selected co-mponent 15 drifts in freq7aency with time and temperature. I