US2463073A - Oscillator - Google Patents

Description

R. c. WEBB March l, 1949.

OSCILLATOR Filed Jan. 26, 1945 W, N W 5 W, d 5 5 .o0 yfl. 3 l.

kann.. Y1.- 9 .J w. www# 0K ZI 1 ....:w N1 i 'vvvvvvvv .PHASE SHIFT/NG NETWORK P/MJE sli/Fr A/Eng/oek 2'5 E CL/PPER OUTPUT INVENToR. ,QM/M20 6.' /f

BY f? A rro/vfy Patented Mar. 1, 1949 Richard, C.. Webb West Lafayette, 1nd assigner @Radio Qcrncraticn cf America" Ncw Ycrk, of Delaware This invention is related to electronic oscilla.l

tors.` In its more particular formthe. inventiony is related: to a variableff qllcllly electr-011119 USCH- lator of the general by@ termined. b y a combination,` pacity elements only.

The invention, herein to be set" forthl and described` is furtherparllticularly characterized throughrthe usem it or a form of phasel s ift circ cuit wherein a. soecalled ladder type network is utilized to form a feedback path between the out put terminals and the. input terminals of a vacuum tube amplier. In its particularI form, the inc-` vention. provides an. improved form of ladder type network which offers many economical,Y theoretic-q cal and practical advantages over heretofore known forms of resistance and capacity networks when used as a feedback path in a phase shift oscillator.

In the past various circuits have been utilized to` provide the effect `of phase shift and essen-v, tially, these circuits utilized were either series resistance and shunt capacity` elements forming ,a ladder type network or series capacity and shunt resistance elementsv also forming a ladder type network, with the former circuit constituting -a low pass filter arrangement andv the latter con-` stituting a high pass filter arrangement. In all such uses of circuits of this character, it became evident that while `various forms of `resistance and capacity elements would t the requirements necessary to provide the controlling elementsfor a phase shift'oscillator, they did not always funcf tion in a satisfactory manner because of the loads ing of the circuits, as will hereinafterbe set forth, and because of the fact that in most instances, the limit frequencies which could be realized from circuits of this character were not such as to en: able these circuits to nd wide application.

In all circuits of the prior art utilizing the phase shift principleand a resistance and capacity network, it was necessary to meet satisfactorily two basic requirements of which one was that'the product of the gain of the amplifier and the transmission response of the selected phase shift network should be made at least equal to unity and preferably greater than unity'; and, second, that the total phase shift around `the complete circuit must be 360 electrical degrees (2r rathaus) or an integral multiple thereof. It is well known, of course', that in such circuits a 180 phase is alwaysprcyidcd by the vacuum tube of.v resistance and, ca-

frecucncyl which; is developed in an oscillator of this character is deamplifier between s inputv grid and its output plate circuit and, accordingly,' thrremavining ISO phase. shift requiredl to meet. vtheforscillating con i Jclcns must be ioyided bythe phase shfil.` r Work.

These. oscillatorsV of the prior. art had certain! outstanding advantagesV and cameV about sult of utilizing` the resistance andfcapacity elements over circuits utilizing the beat frequency: oscillators. Among these dvautagesl were the facts that the hase shirt escalator provided great degreek of freedom from frequency drift and made it unnecessary to continually] check llle'calibrallcn Diller cdvenicscs cams ab' t because of the tact that it was possible totV o.`- vide a uniformly good wave formldutpht and yet to provide a considerably more compact, 'lighter and cheaper oscillator' thanfwcouldwbe"possible with the beat f 'r'equency oscillator. However,

prior` arf` arrangements have been of such vari yl that either all `of the capacity elements or all'of they resistance elements of4 ladder'netvrk ar usually in series and formed as the' lic "ult of? cnil bielas s rlllrllltycl, llsllcllyfthrcc ci Incr filter Sections ci eitherthc 'lllsll D'ss cr "thc "16W pls type c Y. c

The frequency of oscillation in a system cof this. tyre ls varied by providing' Vsllllalll means" trcl the, Yalllc ci cites f' l resistance clclncritc.- Wllcrclcccursc is tc n to, ins cf c lcw pass llltcr ic fcrln 'clic cli sc shifting network, itis cil-lacet that bcccllsc cfj the increased aitcncclllcn this type ci nctvtcrk clcs lc harmcnics ci lllc csclllctlns ilccilcncyq there is ccnslccrcb c tendency fcr nonlinear sister# ory b th the capacity and ticns generated Wlill'nillc feedback a nlc'r. .lc be greatly rcclcccd. in magnitude citer nass sc tnlrcuen the nltcr nctwcrls with, tllc result 'that thc wai/c forni nnally rcsllliins ci the culcuttel: minals is ccnsirlcrably irnprcycd- Lllscwis lnce the ,circuit ccnstants Q1 parameters ci tlflc. phase shifting netwerk ,must lc varied in crdc tc change the resulting frequency, inc netwerk ccn:

stents are automatically adjusted to syc thc same loss characteristics t0 these harmonics at any frequency to which the oscillator may he ad:

justes. lThis becomes a particularly significant feature when consideration is given to the design of adjustable filters to follow a standard variable frequency oscillator.

With these forms of oscillators being known, the present invention incorporates as additional features and as elements serving to stabiliZethe oscillator and to improve its range `and hper-v ating characteristics thcuse of a numberof filter afieaovs sections, usually at least three, with cathode follower tubes connected between each section of the filter network and at its input and its output ends. A cathode follower tube used in such connection between each section of the phase shifting network improves the operation to the extent that the phase shifting elements in the network can be considered as being driven from a substantially zero source impedance and operating into an effectively infinite load resistance, which is in contrast to the already described arrangements of the prior art wherein each -section is operated out of the eifective'impedance of the preceding section or sections and is loaded by the impedance of each of the succeeding section or sections. As a result, the frequency range to which the oscillator herein to be described may be tuned is increased substantially over that which was heretofore obtainable.

With `the foregoing characteristic features having been generally described, it becomes an object of the present invention to provide alresistance and capacity phase shift oscillator circuit of improved operating stability and wave form characteristics which shall be effective over a wide frequency range.

A further object of this invention is the utilization of circuit constants and tube elements in such a way as to improve the frequency characteristics of a phase shift network.

Still a further object of the invention is the provision of an oscillator circuit from which separate output voltages are readily obtainable atthe correct phase shift relations to obtain polyphase output signals from the system.

A further object of the invention is to provide cathode follower tubes to interconnect different sections of either low pass or high pass filter combinations with the cathode follower tubes being connected in such a way as to improve the operation and stability of the oscillator.

,Other objects and advantages of the invention are those of providing an oscillator circuit which is relatively simple in its arrangement and construction; the provision of an oscillator circuit which is positive in its operation and which is effective over a wide frequency range; the provision of an oscillator circuit which overcomes one or more known defects of the prior art and which, at the same time, is highly eilicient in its use and operation and which is relatively inexpensive to install, light in weight and compact from the space requirements.

Otherobjects and advantages of the invention will, of course, become apparent and those suggest themselves to those skilled in the art to which the invention is directed when the following specication and claims are considered in the light of the appended drawings wherein:

Fig. 1 is a schematic representation of a phase shift oscillator; Fig. 2 is a diagrammatic representation of one form of suitable phase shifting network; Fig. 3 represents one form of circuit for practicing the invention in accordance withthe present disclosure; Fig. 4 is a vectorial representation of the phase shift effects obtained inthe network of Fig. 3; and Fig. 5 is a schematic representation of the general form shown by Fig. 1, with the addition of a clipper circuit to provide signal limiting.

Referring now to Fig. 1 for a further understanding of the invention, the phase shift oscillator tube II is arranged to receive impressed signal voltages on its input grid or control electrode I3 from an appropriate form of the phase shifting network I5. One form of the phase shifting network, as shown by Fig. 2 particularly, comprises an arrangement of series resistance elements Il, I8, I9 and so forth, bridged by parallel condenser elements 2l, 22, 23, etc., all located intermediate the input terminals 24, 24' and the output terminals '25, 25'. The circuit arrangement of Fig. 2 will thus be seen to comprise essentially the form of a low pass filter network and to provide for frequency variances when used in the oscillator of Fig. 1, for instance, when the resistance and capacity combinations are appropriately changed in value. The'gvarious condenser elements 2|, 22, 23, etc. are connected to be driven together so that the capacities increase and decrease simultaneously under the influence of a single control element which has been conventionally indicated by the uni-control connection provided by way of the linkage Z'I. With the phase shifting network of this general type, input signals (as in Fig. 1, for example) are supplied to the terminal 24 by way of the conductor 29 connected to the plate or anode 3| of the tube II and across the output resistor 33.4 Plate voltage is supplied to the tube II by way of a' source (not shown) connected intermediate the terminal points 35, with the sourcebeing poled as indicated. The cathode 31 of the tubev II connects to ground 39, as indicated, and output signals are derived at output terminal 4I,'Which is connected across the output terminals of the phase shifting net- Work.

The phase shifting network shown particularly by Fig. 2 and schematically represented by Fig. 1 at I5 is such that a phase shift of substantially is applied to the signals between the input terminals 24' and 24' Vand the output terminals 25 and 25' at some established frequency. A like phase shift of 180 is produced within the tube IIl between the grid I3 and the plate or anode 3| for all frequencies within the audio or supersonic range. Thus, where the product of the gain in the amplifier tube I I and the transmission response of the phase shifting network I5 is equal to or greaterthan unity and the total phase shift around the complete circuit is exactly 2f radians (360 electrical degrees or an integral multiple thereof), it is apparent that oscillations will be developed.

It is evident that the frequency vof these oscillations will be dependent upon the values given to the resistance elements and the capacity elements, as indicated in Fig. 2, and the oscillation frequency can readily be determinedmathematically by well-known formulae. In arrangements of this character, it is apparent that a phase shift of 180", for instance, occurs between the input terminals and theoutput terminals, as in Fig. 2, but, also, it is apparent that each section of the lter, that'is, for instance, the section comprising the Vresistance II and the condenser 2 I, is loaded by all other sections following it. Accordingly, the phase shift in each section of the filter or phase shift network of Fig. 2 is non-uniform. The circuitA arrangement of Fig. 3, accordingly, provides, in addition to the elements shown to accomplish phase shift as in Figs. 1 and 2, several ,vacuum tubes used to divide the lter sections in appropriate manner.

Referring now to the phase shift or filter network andthe phase shift oscillator of Fig. 3, the separate filter sections-of Fig. 2, which are utilized lin Fig. 3,'are identified by like reference numerals with a prime attached. Also, the secused resistance sections all being connected to ground. Also, it will be appreciated that within the scope of this invention, the phase shift effect may be brought about by virtue of high pass filter sections replacing the low pass filter sections in.- dicated in which capacity elements will be 1ocated in positions in the phase shifting network corresponding to the locations of the resistances shown, and resistance elements Will be substituted in the locations shown for the capacity elements. In either instance, the phase shift between the input terminals and the output terminals is still provided as the 180 electrical degrees necessary provide for sustained oscillations.

The foregoing oscillator thus is highly stable and readily controlled as to frequency by a mere adjustment of all the condenser elements 2|',

. 2,2 and 23 simultaneously by virtue of the unicontrol arrangement of 21. Further, the oscillator herein shown is such, as above explained, that a uniform phase shift occurs in each `'section of the filter. Accordingly, with the three-section lter being provided and a 180 electrical degrees phase shift being introduced between the input terminals and the output terminals, it may be appreciated that a 60 phase shift occurs in each section, provided the value of all the resistances l1', |8 and I9' is kept equal and the value of all the capacity elements 2|', 22 an-d 23' is similarly maintained equal. With this in mind, reference may be made to the diagrammatic showing of Fig. 4 where the input voltage available in the input terminals 24 and 24 may be assumed` to be represented by the vector O-Ez4, 24'. `The voltage applied to the input of the tube 5| after passing through the phase shifting network l1 and 2| and being retarded with respect tothe input voltage may be assumed to be that voltage represented at the point 19 and, accordingly, represented by the vector O-E'm. Similarly, the voltage available at the input of the tube 59, after the input signal has been passed through the phase shifting network I8 and 22 will be that voltage which appears at the point 8| and which is applied to the grid 51 and, hence, is represented by the vector O-Eai. Lastly, the voltage available after passing through the third phase shifting network I9 and 23 is applied to the input or control grid B3 of the tube 65 in accordance with the voltage which appears at the point 83, and, accordingly, is 180 out of phase with regard to the input voltage at the input terminal 24 and applied to the grid 45 of the tube 41. Since no phase shift occurs between the input grid 63 and the cathode 13 of the voltage available at the grid I3 of the tube is that voltage which appears at point 25 and is represented by the vector O-E25. Because the cathode follower connected tubes are herein utilized and because the amplication provided by each of these tubes is less than unity, the voltages are all reduced in magnitude in excess of the amount that naturally occurs in passage through each phase shifting element and the vectors represented by the diagram of Fig. 4

the tube 55,

are intended to indicate schematically both the i phase displacement and the assumed relative magnitude of the various voltages.

It was pointed out above that a phase shiftlng network of the character generally described might readily be utilized for the development of polyphase output signals at any frequency to which the oscillator is to be tuned. If reference is now made further to the showing of Fig. 3 with the vector diagram of Fig. 4 in min-d, it will be appreciated that the voltage represented by the vector O-Evg is out ofvphase with regard to the voltage represented by the vector O-Ezs so that, accordingly, if itv be assumed that the voltage represented by vector O-Ezs constitutes one of the three phases, a second phase may readily be obtained from the voltage represented by the vector O-Efia It also will be observed that the voltage represented by the vector O-Esi is out ofphase with each of the voltages O-Ezs and O--Ep by 60, lagging O-Ew by 60 and leading O-E25 by 60 so that if now a voltage be obtained which is out of phase with regard to the voltage O-Eai, the obtained voltage will be 120 out of phase with regard to each of the voltages O-Ezs and O-Evs. out that the voltage O-Esi at the input grid 51 of the tube 59 and since it is a well known fact that a 180 phase shift occurs within a vacuum tube between the grid and plate electrodes, it will be appreciated that the voltage appearing across the load resistor 81 connected to the plate 89 of the tube 59 is 180 out of phase with respect to the input voltages supplied to this tube. Accordingly, with the vector diagram of Fig. 4 in mind, the conductor 9| may be connected across the cathode resistor 61 of the tube 65 to supply to the terminal points 93 and 93 voltages in phase with the voltage represented by vector O-Ez5 and representing one phase (marked qsII) of a three-phase supply.

Likewise, from what has been stated above, there will be supplied to a conductor 95 connected across the cathode resistor 55 of the tube 5| to supply the terminal points 91 and S1 a voltage wave which is in phase with the voltage represented as the vector O-Ew. Thus, at the terminals S1 and 91 a second phase (marked I) of a three-phase supply is made available. Likewise, there is supplied to the conductor 99 connected to the terminal points |0| and |0| a third phase (marked III) which is derived ln accordance with the voltage appearing across the load resistor 81 connected to the tube 59.

\To utilize the developed voltages represented as I, II and QSIII, it is only necessary to equalize these voltages by means of simple potentiometers (not shown for simplicity of illustration) and adjustment of the voltage amplitudes can readily be accomplished in well known manner without in any way affecting the phase relationship of the developed voltages at any frequency within the range of the oscillator.

While the invention above described, particularly in connection with Fig. 3, shows three-phase operation assumed, it should be understood that any number of phases may be developed if desired. This is readily accomplished by changing the number of sections in the network. The development of polyphase voltages is not possible with the arrangement of Fig. 2 because there the loading per section is not uniform due to the loading of one section upon another section. The arrangement of Fig. 3, when thus providing the three separate phases, makes it possible to utilize the oscillator together with suitable power amplifiers to control the speed of rotation of synchronous or induction motors and makes readily possible all of the advantages of three or more phase operations of such devices, as well as utilization of the circuit hereinabove described in connection with the operation of Selsyn or Autosyn indicators, timing and control circuits, circular sweeps for cathode ray tubes, and the like. IThezforegolng descriptionhas set forth par- It was above pointed is that which appears' connection that the low pass type of phase shift-v ing network has certain advantages notA present inthe high pass type ofnetwork which, for ex.- ample, would be particularly emphasized bythe. increased reduction of harmonic content present in the output signal developed across the resistor 33y as, the output from the tube Il' which istobe fed back` by way of*y conductor 29- to the phase sl'liftingnetwork.` The use of the cathode follower tubes 41, 51,1519 and65, for instance, also tends to.improvetheoperation from the standpoint offf both the 10W pass lter and the high ras'sltaf type of phase shifting network.,` Where the lowy Pass type 0f. filter. Phase ShifiineolnetWQrR-las harain disclosed, is utilized,` the harmonicsA presentY` inr the developed signal; are reduced by passage through theresistor elements lbefore again applying the signall to the grid of the feedback tn be I I; whereas if` the high passtype of network is utilized, the series capacity elements often tendfto effect an accentuation of the harmonics so as thereby to` tend to increase theA total distortion present in the oscillating signal.`

Amplitudelimiting of the developed signals to control the maximum amplitude of the oscillations present in the circuit is desirable under maar operating conditions Actordiraiil` iii., a Circuit ofthe type showaby FiaA 3, ier iniaiie, ii. becomes Convenient at times, i9, talig, afiilaeiaga of the improved filteringactionrr of the low pass mier-type 0f phasashiiiiag network in @eating tion. with a Suitablev form af: gaat limitar er clipper circuit immediately preceding the phase shitting network, A Circuit af,- this general form has been schematically-4 and, diagrammatically represented by Fig. 5l whe-re the, feedback conaccuenta-H9111 tbatieedbak tube. lir iS. shew as connecting into a clipper circuit. ldd, which` has been diagrammatically shown,

The clipper circuit |05 receives the feedback signal and passes the. output to thephase shiftgma network: i-1 The remaining. portion. of, tha

circuit operai/estas shown and described by Eig.

3. The lirrer.-` draait conventionally shown at |05 is ofV any well known form of clipper and limiter. It serves as a signal gate to clip symmetrically both the positive and negative half cycles of the signal wave fed back by way of conductor 2&1` as soon. as that wave reaches a predetermined threshold value. A symmetrically clipped sine wave of the` character developed in the circuit of Fig. 3, for instance, will then be found to.4 contain the fundamental frequency component plus5 harmonicsv4 which are odd multiples of the iuridarnentaly wave, with the odd harmonics progressively decreasing in amplitude relative tothe iundarnentalv with very considerable decreaseof the higher harmonies.' Furthermore, the amplitude, of; the fundamental increases but very little above a predetermined threshold limit even in spite of the removal,l of substantial top,` and bottom sections of the wave by clippingaction. The harmonic components.irltrodnood by the clipping action are reduced to negligible size due-to the filtering action of the low-passl networkw The clipper circuit included Winn-nutrie, unit its may compriseany. form, Q fab-d. tubo in strumentality so constituted as to provide both grid and plate limiting, for instance, so that with the clipping effected, an extremely rapid action form of amplitude control is obtainable. In this K' than thefhigh pass type above briefly mentioned. It willbeappreciated in this` voltage. with circuit and the. signals connection, it should 15e; pointed out, however,

that the. effectivenessl of the, alrlplitudeV CQntrol produced by the clippingcircuit 105, as` shown inFig. 5, is;` of maximum effect only when the oscillator is .used as a singlephaSYSOurceof signal K theoutput signals, derived at the output:v terminals 15.an d 16 because, under these circumstances, the ltering, effect` or action, of: the phaseshiftingfnetwork,isutilized toits fullest extent only when.,

terminals 24; and 2,4 and'. 252, and; 25.".

the signals ,Shealdt be. raf Fia 3,-, and,` an arririiiudaA control; circuit utilizing. the usual diode rectifier on AVC system, isto be preferred, Afcircuitot thisA typelwould` then oier the adi/ antageV of; per-rI mitting no` distortion to occur at any` point onthe from` each section ofl the will be @duallyA free 0i of; three phasevoltages,

phase shifting network harmonic. content.

Inthelight off the foregoing description, it-will be apparent that various I rlodiflcation'smay be made' in the circuit herein shownV without in any way: departing from either the spirit or. scope of` what has, herein been set forth, and, accordingly, it. is. believed that suchv modifications as fall fairly within they spirit and scopeofe what is-v hereafter claimed may readilybe made, u

Having now described the invention, I claim;

l'. Anfelectronic oscillaton circuit including, in combination, a. feedback tube having, an input circuit and; an` output circuit, a; phase" shifting network comprising a iilterA including a` plurality. or: separate sections of resistancei and? capacity elements, said filter having its input circuit( connecztedl` to, receive the, output signals from the 40 feedback tube and its output circuit connected to supply signals tothe input circuit of the feedback. tube, anda thermionic tube connected be l tween each separate section ofthe resistance and the capacity filter element, said individual lter sections each being connected with individual tubes in cathode follower manner` to provide a low.A impedance source to drive each section of thev phase shaft network, the complete phase shift network and tube combination being so arranged that the; signal voltages passed therethrough ar. rive atvthe, output of the filter in opposite polarity to the signal 'voltages` at the output of the feedf back tube.

2 An electronic, escalatesaircaitiacimiiaa1a combination, a thermionic `feedback amplifier tube` having an input circw't andan output circuit, a phase shifting network comprising a ijllter including a pluralityr o f separate sections of` resistance and capacity` elements, said filter having its: input circuitv connected to receive the output signals from the` feedback tube and its output circuit connected to supply signals tothe input feedback tube, and a thermionic tube connected between each substantially like section offthe resistance, and the capacity elements oiV the phase, shifting network,- said individual network sections, each being connected to `said tubes toreceive input signal voltages from thefimme'diately preceding tube` in cathode followermanner to provide a low. impedance source lsection of the phase shift network, and'to deliveroutput signal voltages to the input circuit of the next suceeding tube, so that a phase shift is introduced between the input and the z the. Signal; is,L permitted to` pass through` the, complete network betweenw its input its output terminals Y 11 output of the complete phase shifting network and tubes to supply the signal voltages passing therethrough to the input of the feedback tube in opposite polarity from the signal voltages available at the output of the feedback tube.

3. The circuit claimed in claim 2 comprising, in addition, means to derive output voltages each bearing a predetermined phaseal relationship to each other and each of like frequency.

4. An'V electronic oscillator circuit including, in combination, a thermionic feedback amplifier tube having an input circuit and an output circuit, a phase shifting network comprising a plurality of substantially like characteristic separated sections of resistance and capacity elements having the input circuit thereof connected to receive output signaly voltages from the feedback amplifier tube and the final output circuit thereof connected to supply phase shifted signal voltages to the input circuit of the feedback amplifier tube and also adapted to supply output signal voltages toa load circuit, a thermionic tube connected between each separated section of the resistance and the capacity phase shifting network, said tubes each having the individual section of the said phase shift network connected thereto in cathode follower fashion to provide a low impedance signal source for the next succeeding section of the phaseshift network so that a substantially equal phase shift is introduced in each network section and so that the signal voltages passed through the phase shifting network to the input of the feedback amplifier tube arrive at the said input in opposite polarity from the signals at the output of the feedback tube,

whereby oscillations are developed at frequencies dependent upon the magnitudes of the resistance and capacity elements forming the phase shifting network.

5. An electronic oscillator circuit including, in combination, a thermionic feedback tube having an input circuit and an output circuit, a phase shifting network comprising a plurality of isolated sections of resistance and capacity elements, said network having its` head end portion connected to receive output signal voltages from the feedback tube and its output end portion connected to supply signal voltages to the input circuit of the feedback tube, a thermionic tube sequence of which one tube is connected between each of the plurality of sections of the resistance and the capacity network, said individual network sections each being connected'toreceive the output signal voltages from the immediately preceding tube in cathode'follower manner and to deliver its output to the next thermionic tube of the sequence and the complete phase shifting network and tube combination being so arranged that the signal voltages passed therethrough ari rive vat the output of the network in opposite polarity to the signal voltages at the output of the feedback` tube.

6. An electronic feedback oscillator circuit including, in combination, a thermionic feedback tube having an input circuit and an output circuit, a multi-section phase shifting network of which each section includes at least one resistance and capacity element, said complete phase shifting networkhaving its input end connected to receive output signal voltages from the feedback tube and its output end connected to supply signal voltages to the input circuit of the feedback tube, a separate thermionic tube connected between eachindividual section of the resistance 'andthe capacity phase shifting network, said in;

dividual network sections each having the input thereto connected with a preceding and individually associated thermionic tube in cathode follower manner and the output of each network section being connected to supply signal voltagesto the input circuit of the following thermionic tube, the last network section being connected to= supply signal voltages to the feedback tube and'. the complete phase shift network and tube com-- bination being so arranged that signal voltages passed therethrough arrive at the output of the network and at the input of the feedback tube in opposite polarityrto the signal voltages at the output of the feedback tube.

'7. An electronic oscillator circuit including, in combination, a thermionic feedback amplifier tube having an input circuit and an output circuit, a phase shifting network comprising a plurality of separate substantially like sections of resistance and capacity elements having its input circuit connected to receive output signal voltages from the feedback amplifier tube and its output circuit connected to supply phase shifted signal voltages to theinput'circuit of the feedback amplifier tube and also adapted to supplyY output signal voltages to a load circuit, a thermionic tube connected betweeneach separate section of the resistance and the capacity phase shifting network, said tubes each having a phase shift network section connected thereto in cathode follower fashion to produce a low impedance source to drive each section of the phase shift network so that a substantially equal phase shift is introduced in each section of the phase shifting network yand so that the signal voltages passed through the phase shifting network to the input of the feedback amplifier tube arrive at the said input in opposite polarity from the signals at the output of the feedback tube, whereby oscillations are developed at frequencies dependent upon the magnitudes of the resistance and capacity elements forming the phase shifting network, and a plurality of output connections for obtaining separate phase shifted voltages all at like frequency. p

RICHARD C. WEBB.

REFERENCES CITED The following references are of record in the Afile of this patent:

OTHER REFERENCES Proc. of I. R. E.; vol. 33, No.' 8, August 1945,

pages 541-545; -Phase .Resistance- Capacitance Oscillators, by R. M Barrett, 250-36-27.

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