A NEW TYPE OF SELECTIVE CIRCUIT AND SOME APPLICATXOJXS

BY

H. H. SCOTT

PRINTED IN THE U.S.A.

A NEW TYPE OF SELECTIVE CIRCUIT AND SOME APPLICATIONS*

Surnmnry-This paper describes the use aj the a'nverss feetkbnck principle la obiain slror71ly sel~clive circuits. fmporlant adaunlagss o j such circuils are ( I ) in- ductances orc not necessary, (2 ) "1?~ning" ma?/ bc ~ h a n g ~ d b$, merely v a r p a g re- rdstancoa, (3) rcadg adaptnhilily jm use at w r y Entu jrequencjes, (4) 'tuning" mny he aariad over wide ranges of frequency while ~naintaining a aelectivily curve which is a conslant pacmfage junclion of f l ie tuned frequency, I.5) the possdbilil!/ of using a aingle SF! offrequency-determining elmnsnts for ''tuningn severul ampEifying 8iages, and ( 6 ) the general simplicit?/ of con.rlrt~rlion and operation whea c o m p a ~ e d udlh many other Eypes of equipment designed lo produce eqtaivnleat re.eulls.

Man?] uses for a circuit of this type immediately suggest thern8elue8. Tu-o im- pnrlnnt rappla'colioas are dsscrihed, including a noael type of nnalgrzer and an oscil- In fox having extremel!) gnnrl wave form.

1. INTRODUCTTOW

S ELECTITfE circuits in common use today consist gene~nlly of cnmbinntiorls of inductance and cnpncitnnce or their mechanical equivalents. Illustrat,ions are the many t u n ~ d circuits usrrl i n

radio receivers, transmitters, and other 1,ypes of communicntion equip- ment. Among the selective mechnnicnl or electromechanical deviccs may be included crystals ('qurtrtx, Rochclle salt, etc.) and various t,yprs of rnngnetostrictinn and m~chan iea l filters.

In the i n t ~ r ~ s t s of simplifivd dmign nrld low construction cost it ~ R P become gcnernl pract.ice t o adjust the tuning of electrical circuits, so far RS possible, by varying thc ronrf duct- nnccs to remain fixed. T h i ~ is g~nvmlly quen- cies, but at Eower frequ~ncics the conde large so that conventional types of v a r i ~ ble nond~nsers ca~lriot be used. Anot,hcr s~rious objcctian to rircnits of thir t y p ~ is the difficulty of ob- t,aining satisfuctory inductances for use at lower frequencies.

The electmmeeha,nicsl t y p ~ s of selective circuits, such as crystals, mngnetostriction devices, and rncchnnical filters, are even more re- stricted so far RS chanj=jng the r e s o ~ ~ a ~ t frequency is concerned, since in such devices the frequenry is de t~rmincd mainly by the rnc~hanicsl dimensions and other characteristics which are not. variable. Also, several important factors such as size and expense limit the use of such devices to rclativcly high frequencies.

Irl aliowi Lory at h' :d are sui

ng the in igher fret 'Frciantly

* Decimal classification: R141.2. Original manuscript received by the Institute, 0ctrll)ar ti, 1!)37.

221i

Some circuits h n v ~ bccn developed in which thr: chnngr of tuning can he nccomplished by ac l jus t i~~~f variable resistnnces, b u t lixvc not xttxined wide accept,ance due, perhnps, to the cornplicatetl ns t~ l rc of the circuits involved or t8he gener~l unnvaiIabilit y of satisfactory news- sitry circuit elements such as, for instance, dynrttrons. The "resistmanee tuning" of Cabotqs an illust.mtioa of t,his genera1 type of rircnit. An obvious advantage of being able t o adjust the tuning of a circuit by varying re.rist:inceu orily is the f:~cL IllttL, at Iow frequencies, suitable vm-inble inductitnces or cap~citances are not avnilsble, with the result that in most con~entional types of circuits low-frequency tuning is adjustable only in steps.

One of the most important uses for tuned circuits at low frequencies is in the analysis of cIcctrical wave fonns, and, in particular, in rccent years the nndgsis of noise wave forms as obtainrd from n microphone ~ n d amplifier. In order to meet this demand for analyzers, various in- struments have been developed. Types involving merely tuned cir- cuits consisting of inductsnces and capaeitnnccx have generally berm zhthxnrlon~d due to the elriborate switching arrangements necessary for cnrnpkrte cot- rag^. of the low-frtqucncy ranges, the difficulties in crb- tnining ~ntisfactory ~lclcetivity s t the lower f r ~ q l l p ~ ~ i ~ ~ , ~ n d to the susceptibility of this type of circuit to magnetic pickup and interfer- ence, pxrticrdarly from power lines. Anal yz~ r s of thr: het prodyne t.ypc? have, acrnrdingly, gained wide accept ancr.2 Ln these devices the wave form t o be analyxecl i s heterodyned against a pmctically sinusoidnl wsre form nbtnincd from a local oscillator, and the resrrlting heat note is passed through a sharply s~lectivc filter, generttlly of some electro- mechanical type consisting of crystnls, magnetostrirtion rods, or ~ i m i - lsr clevices. An outstanding characteristic aE an anaIyzcr of this type is that the band width in cyeIes is practically constant, regardl~ss af thc frequency t o which the analyzer is tuned. Changes in band width can only be accomplished by changes in the electromechnnicnl rcaon:tat circuits or hy the more common method of switching otllcr resonant circuits into the system.

For many types ol work such arrangements are quite sittisfnctory, hut far noise anslysis t h y are, in general, a compromise. It is generally sgreed Rmong users of sorrnd-measuring equipment tha t the most srtiisfnctory nnalyzer for nnise analysis ~1110uld have s. band width or

' Fen-nil Czbot, ''Rc.sistsnce tuning,* PROC. I.R.E., vnl. 22, pp. 709-731; ,Ju~ie, (1034).

1;or infcrrm~tiun on conventional types nf analyzers see L. B. Ar~uimbnu, "Wave analysis, Gen. /?ad. E x p . , vol. R, pp. 12-14; Jane.Tuly, (1!):1:<); M. S. Mend and T. 33. Berry, "A portable Frequency anal~~zer," Gen. Elec. Rev., vol. 37, pp. 378-383;; A n ~ u s t , ~ (1!>34); and 11. H. Scott,, The annlysis of complex sounds of eonstant p ~ t c h , Cen. Rad. Exp . , vol. 9, pp. 5-8; May, (1935).

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RELATIVE RESPONSE ln OEC18ELS

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