US2373437A - Twin-discharge tube oscillator - Google Patents

Description

April 10, 1945- P. B. VANDERLYN ET AL ,4

TWIN-DISCHARGE TUBE OSCILLATOR Filed Aug. 18, 1942 PHILIP B. VANDERLYN & ERIC .c. WHITE A'ITORNEY Patented Apr. 10, 1945 TWIN-DISCHARGE TUBE OSCILLATOR Philip Barnet Vanderlyn, Fincliley N.3, and Eric Lawrence Casling White, Chiswick WA, London, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, a company of Great Britain England,

Application August 18, 1942, Serial No. 455,180

In Great Britain February 15,1940

7 Claims.

This invention relates to electric oscillation generating circuits employing thermionic valvesb This application is a continuation in part of 1 my application Serial No. 378,530 which was filed February 12, 1941, and which matured into Patent No. 2,300,996 on November 3, 1942.

It is well known to generate electric oscillations by means of a thermionic valve having regenerative coupling between the anode and grid circuits, but any single valve oscillator circuit,

whatever the nature of the circuit details, is subject to certain disadvantages.

Firstly, the oscillation generated builds up to a limiting amplitude which is always such that grid current flows during a portion of the cycle. This has two deleterious effects, namely, (a) it spoils the frequency stability because the combination of non-linearity of the valve characteristie with the leakage reactance ofthe grid coupling coil causes the grid current flow to be asymmetrical with respect to an oscillatory cycle in the main tuned circuit, and (b) it accentuates the production of harmonics, particularly the second harmonic.

Secondly, limitation of amplitude by automatic 1 operating frequency range. The-arrangement albias produced either by grid current in a grid leak resistance or by cathode current in a cathode resistance is liable to cause the generation of oscillations in bursts, due to temporary blocking of the grid by charges accumulated on the grid condenser. In this case the grid draws current more rapidly than it can be dissipated through a grid leak resistor of high ohmic value.

Thirdly, single valve oscillator circuits involve a certain amount of mechanical inconvenience caused by the necessity for having two coupled coils or one coil with a tapping point, particuarly when a large frequency range has to be covered by coil changing. Again, in many oscillator circuits, particularly some forms in which one tapped coil is used, neither side of the main tuning condenser is at earth potential and this is I inconvenient.

It has previously been proposed to avoid the mechanical troubles mentioned, by the use of two valve oscillator circuits in which the second valve is used as a phase reverser. Even in the case of such proposals, however, it is still difiicult to fix a limit to the oscillation amplitude without involving the flow of grid current with its attendant disadvantages.

It is the object of the present .invention to provide a two-valve oscillator circuit in which the disadvantages mentioned above areeliminated or reduced and a further object of the" invention is to provide a circuit for maintaining the output of an oscillator constant as the generated frequency is varied.

According to the present invention, a circuit arrangement for generating electric oscillations includes two valves, the cathodes of which are associated with a common impedance set up between said cathodes and the negative terminal of a source of high tension supply, said impedance being large compared with 1/g,-where g equals the mutual conductance of said valves within the so includes a resonant circuit which forms the coupling impedance between the anode of one of said valves and a control electrode of the sec ondvalve.

In order to regulate the amplitude of the generated oscillations, the average current feed of the valves is controlled .by alteration of the grid bias potential applied to the valves or by alteration of the D. C. resistance of said common impedance. The values of the components and operating potentials are selected so that no grid current flows in either valve,

In particular circuit arrangements according to the invention, a parallel tuned circuit resonant at the desired oscillation frequency is connected between the anode of one of the valves and the positive terminal of the source of H. T. supply, and this anode is also coupled through a condenser to the control grid of the second valve. The mean bias potential applied to the grids of both valves may be made dependent upon the oscillation amplitude across the resonant circuit by means of a rectifier coupled, through an amplifier if necessary, to said circuit, in suchmanner that an increase in oscillation amplitude lowers the grid bias potentials thus tending to reduce the magnitude of any such changes in amplitude.

In order that the nature of the invention may be more clearly understood, alternative forms of circuit arrangement embodying the invention will now be described in greater detail by way of example with reference to the three figures of the I accompanying drawing.

In the drawing, I Fig. 1 shows an embodiment of the invention in one of its simpler forms;

Fig. 2 shows a modification wherein pentode tubes are used and wherein a choke impedance is substituted for a pure resistance in the common cathode circuit; and

Fig. 3 shows still another modification of the invention.

supply S through a resistance RI and a suitable mean bias potential for both valves is derived, from a potentiometer comprising resistances R-Z;

' R3 across the source of SUDPIXS. ?1 e..bias iIiiill,

of course, be derived froma suitabletappingmoint on the source S, and it will be understood thatalthough triode valves have been shown, it may in some cases be preferable to employ tetrode or pentode valves. i

Consideration of the circuit will show that a certain minimum swing of grid potential of thevalve 2 will sufilce to produce the maximum pos;

sible current through the valve l and any increase only; serves to make the current: waveform more rectangular,- the; chief; efiect. of; which is; to; add

to the generation of odd harmonics. I'hevoltage amplitude across. the: tuned cironitIsQ then de p nds upon; the amplitude of; the-current. swin whi h s: d p ndent chiefly: upon. the-value of the resistance; RI, the;. positivebifi-S; potential applied;

to the grids. and; on: the, impedance at. resonance the resistance 13]: is, large and; suitable. valves aroused; it ay be arranged that, quite. a. large a udes. exists. across. the tuned circuit.

the circuit LC direct or from the anode circuit of the: valre 2; as variations. 01: load will: in! the latter case have practically no. efiect'on the T ene ted oscillations may be taken from frequency. generated-particularly it the valve 2;

is a: e rode; or; pentode. n most: cases-it will; be. desirable to take the; output. from. a iurther resonant. cir uit. connected? in. the anode circuit.

fr he alve zf i an approximati n tola pu Sine wave is required. Since considerable harmonics: arerpresent the. anode current, of; the valve 2; particularlyoddharmonics; the: resonant circuit may be tuned; to, one ct these; harmonics, thus.

providing an output at a multiple or the gen.-

eratedfrequeneyi v e The circuit: depends for its; correct operation. upon-the impedance connected between thecommen a des a d; the-negative terminal of thesource of supplybeing" several times as. great as 1/9 Qf t e: alves; At-high-frequencies the stray capacitiesofi both; cathodes set a. limit-to, the Value of this impedance: with the resultthat with modern; receivingtype, valves, the useful: upper frequency limit of the circuitis about 1,0;megacycles persecond? The; impedance'in question should be high not only at the fundamental frequency but. also; at all the odd harmonics; and; the. provision of a complex cathode circuit giving shunt. resonance at both the fundamental: frequency. nerated: and the third harmonic will. make it possibleto operate up to about30zmegaoyclesper second.

Referring now to Fig, 2, we show: a modification wherein the two pentodetubes. l; and: 2* possess a common cathode impedanfieconsistin f a chok coil T. This choke ooil-servestoincrease ea'ee e iv y o he control. It. pr vides a hi h the current swing. The circuit arrangement is further described as follows:

The anode potential is supplied to the tubes I and 2 from direct current source 5 fed through a common resistor 8 andindividualtr'esistors 9 and I0. Thescreen grids of these tubes are also supplied from the same source through a resistor H.

"The suppressor grids are directly connected to the cathefieaas is. usual. Certain high frequency components are by-passed to ground through capacitor (-13,.

The control; grids for the two tubes l and 2 are 1. interconnected. through an inductive impedance 1;;2 shunted by a capacitor C2.

in shunt with resistor R3, both forming an imimpedance at the operating frequency'butalowresistance such that small changesin bias poten tial produce comparatively; large change in the total feed current and hence in the amplitude of Capacitor C1 is pedancepath; between the control grid of tube i and ground. A feed-back circuit is shown connected between the anode of tube l and the control. grid of tube 2 and comprising capacitor C4 and resistor R4. The control grid of tube 2 is coupled across capacitor'Cfi to the control grid of. anamplifier tube 5... This: last mentioned; con.-

.trol= grid; is. also connected to an; input circuit which includes. resistors l2, and. 23. both connected to ground, whereas: resistor la, is a cathode resistor. Tube 5' is supplied. with anode potential through resistors l4 and; l- Its s r en. rid is also suppliedfrom; the same source S through resistor Hi. v p Since the current amplitude. depends upon the positive biasapplied to tubes,- l nd, and s n e; the grid current is inappreciable; the grid bias may be provided. by. hiehimpedanee o ce $11. as a. diode. rectifier 6;. An output comp nent from tube. 5 is rectified. in the; tube. 6, thus, providina a simple oma amp itud abilisat on circuit. This. arrangement, is such; as. to improve; thesensitivity of' h au oma ic; con ro d a s orem ret e lo dz he iee i er rom he c u t a L 62.; which the wisewould; spoil the. frequencystabi ity -a dnweve; i rmt will be seen. f r he more. that; the output from the. oscillator is taken front theianede circuit; oiualve i whiohincll des. capacitor C4, the resistance R4, andgtuned circuit t e; junction between the. resistance R4 and the tunedcircuit L2; C2;- being connected tot-he grid of; th velvet. If the output should;

potential, can be used to effect adjustment of the amplitude of the output oscillations.

It has. been stated above that the normal adjustment of an oscillator operated in accordance; with theinventiomis; such. that a grid swing is applied: to the valve 2:se .Q. a1 times as; great as is required for the production of maximum current. amplitude and. the, cur-rent waveform. is in consequence deformed so as to be substar-itiallyv rectan ular. This methodof operation is by no means essential and; it a particularly pure sinu soidal; output waveform; is; required, it may be an advantage-to reduce the rid: swin -1hr tak afr iQlSiQD. of the voltage across the tunedcircuits. As in the case of any oscinatoroperating on an: almost linearportion of the valve. characteristic,

the positive feedbacloadjustment-to secure a given. amplitude will then be very. critical. and the am plitude stabilizing; circuit arran ement shown in 1 2,373,437 Figure 2 will be particularly useful." The "opera;

tion 'can in this case be regarded a changing the amount of the positive feedback by varying the slope of the valves l and 2 in consequence of varying the mean currents in dependence upon the output of the rectifier v'5. The resistance R4 may be made adjustable in order to provide'a suitable amount of feedback initially. v

The method of alteringthe feed currents to the valves in the arrangement des'cribedis by alteration of grid bias voltage, but it will be understood that the same result may be achieved by varying the ohmic resistance of the impedance common to the cathodes of the two valves. In the arrangement shown in Figure 2 for example the rectifier 6 may serve to apply a negative bias potential to a pentode valve connected in place of the choke'coil I.

In the third modification of our invention shown in Figure 3, a discharge tube P is inserted between the cathodes of the tubes I and 2 and ground. This tube P replaces the choke coil I in Fig. 2, and has a control grid 9 connected to a tap on a potentiometer IB which is in circuit with a resistor 20. Gain control potentials are thus obtained through the action of the'rectifier tube 6. The potentiometer I8 is preferably shunted by capacitor IS.

The screen grid potential for tube P is derived from a tap on the potentiometer I! which interconnects the two terminals of the source S.

The control grid in tube l is connected to a tap on potentiometer 2| which is in series with resistor R5, both potentiometer and resistor being connected across the terminals of source S.

The adjustment of bias potential applied to the control grid in tube P provide a convenient means for adjusting the feed currents of the valves l and 2. However, the control potentials of tubes l and 2 do not need to be negatively 40 biased, since the control grids of these tubes are connected to a tapping point on the potentiometer 2!. In other respects the circuit arrangement of Fig. 3 is similar to that of Fig. 2 and the operation thereof may be well understood in view of the foregoing description.

Other modifications of our invention and other circuit arrangements may suggest themselves to those skilled in the art. The invention is, therefore, limited only in accordance with the claims.

We claim:

1. An oscillation generator comprising two cooperating electron discharge tubes, one of which functions as a phase inverter with respect to the other, said tubes having each a cathode, an anode, and at least one grid, a common cathode impedance connected from the cathodes of said tubes to a source of operating potential, said source having its positive terminal in circuit with the anodes of said tubes, an amplifier stage having an input circuit subject tocontrol by energy from said generator, an automatic gain control circuit connected between the output side of said amplifier stage and the common cathode impedance of said tubes, said gain control circuit being arranged to regulate the amplitude of the oscillations generated, and means includin a resonant circuit interposed between said grids for causing the oscillatory swings in the two said tubes to be contraphasal.

2. An oscillation generator in accordance with claim 1 in which said common cathode impedance includes the space path of a dischargetube.

3. An oscillation generator comprising two cooperating electron discharge devices, each device the two cathodes being interconnected, an electron discharge tubethe space path of which constitutes a common cathode impedance which connects said cathodes to thenegative terminal of a direct current source, impedances interconnecting the positive terminal of said source and said anodes, a resonant circuit interconnecting the control grids offsaid tubes, a feed-back circuit coupling the anode of one discharge device to the control grid of the other discharge device, a bias potential source for said grids, and means for so controlling the bias potentials applied to said rids that variations in the oscillation amplitude are minimized.

4. An oscillation generator according to claim 3 in which the last said means includes an amplifier coupled to said generator, and a rectifying circuit in shunt with a bias resistor for dissipat- 29 ing certain output components from said amplifier.

5. A circuit arrangement comprising an oscillation generator having two contraphasally operable discharge. devices each device having at least a cathode, an anode, and a control grid, the

cathodes being interconnected, a direct current source having positive terminal connections through resistive impedances to the anodes of said tubes, an impedance connected between the negative terminal of said source and said cathodes, means including a resonant circuit having suitable connections to said control grids for maintaining contraphasal control thereof, a feedback coupling means connected between the anode of one device and the control grid of the other device, an amplifier stage having an input circuit under control of output energy derived from said generator, a gain control circuit operable by output energy from said amplifier stage and effective to stabilize the amplitude of the os cillations generated, the bias potential applied to said control grids being subject to the action of said gain control circuit, and means including a rectifier in said gain control circuit for pro- 4 5 ducing a threshold action therein.

6. A circuit arrangement comprising an oscillation generator. having two contraphasally operable discharge devices, each device having oathode, anode, and control electrodes, the cathodes 0 being interconnected, a direct current source having positive terminal connections through resistive impedances to the anodes of said tubes, an inductive impedance connected between the negative terminal of said source and said cathodes, 55 means including a resonant circuit having suitable connections to said control electrodes for maintaining contraphasal control thereof, a feedback coupling means connected between the anode of one device and the control electrode of the other device, an amplifier stage having an input circuit under control of output energy derived from said generator, a gain control circuit operable by output energy from said amplifier stage and efiective to vary the bias potential on said control electrodes, which potential is primarily derived from the voltage drop across the cathode circuit impedance, and means including a rectiher in said gain control circuit for producing a threshold action therein.

'7. A circuit arrangement comprising an oscillation generator having two contraphasally operable discharge devices, a third discharge device the space path. of which constitutes a variable impedance in a cathode circuit common to the 75 two discharge devices of said generator, a source having an anode, a-control grid and a cathode,

f; direcfi curnept havins: msitive; terminal; eon:-

nectiqns tw anpdes m. magenamtor. clevicas. and,

vipe; in said. geneztatqr an amplifier sta ehavini 1 841 input circuit: under: ennhml; Qf. Qu nut nemy denlved tram; said; generatar; 11.3mm. contmlf k fler flgevand. eflieqtivem vaty th yimgedanca ot aid hird dischar a device, and meansdn ludins a. rectifier. in; saidg in; c ntrol circuit for prqduw in a threshold. action therein,

VANDERLXR

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