US2516812A - Electron discharge tube circuits - Google Patents

Description

July 25, 1950 J. R. TILLMAN 2,516,312

ELECTRON DISCHARGE TUBE CIRCUITS Filed Nov. 12, 1946 2 Sheets-Sheet 1 J. R. TILLMAN ELECTRON DISCHARGE TUBE CIRCUITS July 25, 1950 2 Sheets-Sheet 2 Filed Nov. 12, 1946 JOHN fP/C/V/VAfi TMAM/M; ATTORNEY Patented July 25, 1950 UNITED STATES PATENT OFFICE ELECTRQN DISCHARGE TUBE CIRCUITS John Richard Tillman, North Harrow, England Application November 12, 1946, Serial No. 709,339 In Great Britain November 14, 1945 2 Claims.

This invention relates to variable impedance electron discharge tube circuit arrangements that is to say to discharge tube circuit arrangements in which one or more discharge tubes is or are employed to act as variable impedance means presenting variable impedance (usually though not necessarily variable reactance) of a value which is controlled in dependence upon a control signal.

. The use of a discharge tube asa variable reactance is well known and a number. of circuits in which a tube is so employed has been proposed.

There are numerous practical applications for a variable impedance discharge tube circuit, one

of the most importantv being that in Which the circuit is utilised to effect frequency modulation. In such an application the tube which is to present variable impedance is usually arranged to provide part of the reactance in a frequency determining circuit of an oscillator and the react- I ance so provided is caused to vary in dependence upon the instantaneous amplitude of a modulating wave so that the frequency of the oscillator is modulated in accordance with the said modulating wave. r

In one well known form of variable reactance electron discharge tube circuit arrangement the variable reactance is obtained between the anode and cathode of the tube and, where the tube is to be employed to control the frequency of an oscillator, these electrodes are connected across the normally provided parallel tuned circuit of said oscillator so that the effective reactance presented between them is included therein. A

upon the tube and circuit parameters) the relationship between susceptance and modulating voltage amplitude is substantially lineara common requirement.

In another known type of variable reactance electron discharge tube circuit arrangement a multi-electrode tube, e. g. a pentode or hexode, is employed and the quadrature and modulating or controlling voltages are applied to different grids instead of to the same grid, the quadrature voltage being applied to one control grid and the modulating voltage being applied to another grid 2 I 7 which isnearer the anode so as to serve to control the proportion of the total emitted cathode current reaching the anode.

These and other known variable reactance electron discharge tube'circuit arrangements have the important defect of lacking stability or constancy of performance, the reactance presented for a given amplitude of modulating or control voltage being by no means always the same in a given circuit. Even in the absence of any control or modulating voltage the change of anode current with respect to change of voltage on the grid receiving the quadrature voltage is dependent, inter alia, on the H. T. voltage, the L. T. voltage and ageing of the tube and the susceptance manifested is therefore similarly dependent on these factors. Although the adverse results of this can be reduced by stabilising the H. T.,and L. T. voltages, stabilisation of L. T. voltage is often difficult and there still remains inconstancy due to tube ageing. It has been proposed to meet the difiiculty by providing additional circuits for stabilising the center frequency but such proposals obviously involve additional circuit complexity and, moreover, are not suitable for application in the not uncommon case in which the modulating or control voltage includes a unidirectional component. I

The present invention seeks to avoid these defects and to provide relatively simple variable impedance discharge tube circuits of high stability. I

According to this invention a variable impedance discharge tube circuit arrangement comprises a tube having at least a cathode, two control electrodes and an output electrode; means for subjecting one of said control electrodes and said output electrode to voltages which are in predetermined phase relation (in quadrature where pure reactance is required) means for applying a controlling voltage to another of said control electrodes; and means-for aprent with respectto the potential on said one control electrode approximately independent of its value in the absence of degeneration, said other control electrode havinglittle elfect on the total cathode current but serving to control the proportion of said current reaching the output the latter voltage is preferably applied to the first control grid and the controlling voltage to the second control grid. In either case the screen grid (between the two input grids) is held at some anchored potential in the ordinary way.

It will be seen that, with an arrangement in accordance with this invention, although the proportion of cathode current reaching the out put electrode and determined by the controlling voltage is dependent upon the geometry of the tube (which is a relatively stable factor) and on the operating potentials of the electrodes subsequent to the first input grid it is substantially independent of the magnitude of the cathode current. Thus, if IA is the output electrode (normally the anode) current; VG is the voltage on the control electrode (normally the first control electrode) receiving the phased voltage; IK is the total cathode current; and p the fraction of the total cathode current reaching the anode. (This varies substantially linearly with the voltage on the second input grid.)

in, L dV P (W and, in the presence of the applied degeneration will be substantially independent of In and of the value of (H dV in the absence of degeneration.

The invention is illustrated in the accompanye ing drawings which show, by way of example, threecircuit embodiments in accordance therewith, for providing variable reactance.

Fig. 1 discloses a pentode tube arranged in a circuit so as to act as a variable reactance tube, with degeneration applied to the control electrode to give high stability.

Fig. 2 is a modification of Fig. 1 in that two amplifier tubes are included in the degeneration and phase shifting circuits.

Fig. 3 shows lizing, the invention.

Referring to Fig. 1 a pentode V1 having a cathode K, three grids GI, G2, G3 and anode A is employed as a variable reactance tube to manifest controllable reactance between a terminal X connected to the anode and earth. Anode and screen grid potentials are applied from a stabilised source (not shown) connected between H. T.+ and H. T.. The cathode leg of the tube contains two series resistances RK and RNF of values represented by these references and the anode-earth circuit contains two series impedances Z122 dimensioned in accordance with well known principles. The control grid GI is connected to the junction point of Z1Z2 through the condenser C and also to the junction of the resistances RKRNF through the resistance R. In this way the grid GI receives quadrature voltage (i. e. voltage in quadrature with that on the anode A) and is also subjected to negative feed back or degeneration. A modulating or other controlling signal wave is applied at terminals IN between the suppressor grid G3 and the cathode, the lead to the latter electrode including a (where V61 is the voltage on the grid GI) in the practical electrical circuit utipresence of degeneration-i. e. with the circuit as shown; and gKO is the value when degeneration is removed-4. e. when RK+RNF is made efiectively equal to zero (e. g. by by-passing these resistances with a large capacity) then By making (RK+R1vr) giro large as compared to unity g1; may be made to approach closely to the value 1 RK+RNF with obviously greatly improved stability as compared with gKO.

A defect of the circuit shown in Fig. 1 is that the application of any considerable amount of degeneration results in a low value for gx and, for some purposes, this may be a serious disadvantage. In such cases the defect may be avoided by modifying the circuit by providing for voltage step-up between Z2 and GI, e. g. by replacing the condenser C by a step uptransformer or coupling Z2 to GI, through a stable amplifier. If the signal input circuit connected at IN is required to have one side earthy the resistances RKRNF should be shunted by an inductance of high inductive value and low ohmic resistance, and a low pass filter or some equivalent arrangement provided to pass the desired input-to the second input grid while excluding undesired high frequency input thereto.

In the modification shown in Fig. 2 two. additional tubes V2V3 are provided for applying degeneration and the quadrature voltage, taken as in Fig. 1 from across Z2 is applied to the first grid of tube V2 and thence via V3 to the grid GI of V1. As will be apparent the negative feed back loop includes the cathode circuit of V1 and the circuits of the tubes V2 and V3 to the grid G-I of tube V1. If, when the resistance R'NF is. reduced to zero, i. e. in the absence of degeneration, the voltage gain from the first control grid of V2 to GI of V1, is ,u; go represents dVGl (for tube V1) under these conditions; and

1 i GlZ is therate of change of cathode current of V1 with change of voltage on the first grid of V2; then, the value g of d1 k GiZ when the resistance R'NF is inserted is given by and is substantially independent of the cathode current In of tube V1 whence it follows that the required stabilisation is obtained.

Although the introduction of the additional tubes V2V3 introduces new variables, notably variation of [.6 with tube ageing, the stability of the susceptance produced is still very high because all the components likely to vary with time are in the feed back loop. As before the circuits of the grids G2 and G3 and of the anode A of tube V1 are not in the feed back loop. If desired negative feed back may be applied to either or both of the tubes VzVs, either separately or across them both, in addition to the overall feed back illustrated.

With this arrangement, as the amount of degeneration is increased from zero the stability improves until a maximum is obtained after which additional degeneration produces no further improvement. This limiting value occurs by reason of the fact that the magnitude of In has some influence on p the influence being less smaller the value of Ix. As In is reduced to overcome this effect on also becomes smaller and accordingly larger gain is required from V2 and V3 (for a given degree of stability) to maintain a given amount of degeneration without the value of g becoming smaller. In practice it is unnecessary to reduce IK indefinitely to secure maximum stability for if the current is controlled in some way, e. g. by providing a control of the bias voltage of VGl in tube V1 and there is a moderate amount of feedback, e. g. -30 times it will be found that it is possible to adjust 1K to a value such that the residual dependence of g on cathode emission changes (due for example to ageing of the tube) is balanced by the dependence of p on IK. It is therefore preferred to provide for manual adjustment of the (steady) cathode current of In for by operating such adjustment it is possible, by trial and error, to achieve .a very high degree of stability, go and g falling (to an extent reduced by the degeneration) as the cathode emission falls (with ageing) but p increasing therewith.

Fig. 3 shows a practical circuit in which the invention is employed for frequency modulation. In view of the description of Fig. 2 it is thought that Fig. 3 will be largely self explanatory. Modulating potentials are applied at IN to the second input grid of tube V1 through a filter circuit FC the components of which are chosen to suit the frequency range of the modulating signals and exclude the high frequency. For the sake of giving maximum information suitable values of components are indicated in the figure but it is, of course, to be understood that these are by way of example only. It will be noted that control is provided of the bias voltage for V1. The inductance in the cathode lead of this tube should be of low ohmic resistance and the bias applied at GB should be sufficient to operate the grid Gs of V1 about the middle of the substantially linear range of control. The terminal X is connected to the drive circuit (not shown) of the oscillator to be frequency modulated the fixed elements of the frequency determining circuit of which are shown at FLC. A circuit as represented in Fig. 3 can be designed for any of a wide range of modulating signals, either alternating or D. C. (telegraphic) signals.

(susceptance) being obtained from its output circuit. Any of the illusmated embodiments can be modified in this way.

The invention is not limited to the particular circuits shown and many modifications may be made without departing from the scope thereof. Moreover, although the invention has been particularly described as applied to the provision of a variable reactance electron discharge tube circuit for frequency modulation purposes, it will be apparent that the said invention is not limited to this application but has numerous other applications, e. g. for automatic tuning purposes.

In the description of the drawings reference has been made throughout to variable reactance tube circuit arrangements and to the application of a quadrature input voltage. The invention is obviously not limited to arrangements in which a pure reactance is manifested and if desired a reactive impedance effect may be provided, 1. e. the reactance tube may be arranged to provide a resistive as well as a reactive component. In such a case the quadrature input will not be quadrature but will be at a phase angle removed from by an amount dependent on the resistive component required.

I claim:

1. In a frequency modulating system, an electronic discharge device connected to comprise a part of the impedance of the frequency determining circuit of an oscillator, an anode, a cathode and a plurality of grids in said device, a circuit connected between said cathode and anode including a pair of impedance devices and a pair of resistances, a connection including a third resistance from a point between said pair of resistances to the control grid of the device, a second connection including a condenser from a point between said impedances to said control grid, and a modulating circuit connected between the oathode and another grid to vary the gain of the device to in turn vary the frequency of the oscillator.

2. A frequency modulating system as claimed in claim 1 wherein said first connection includes additional discharge devices serving as amplifiers.

JOHN RICHARD TILLMAN.

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

UNITED STATES PATENTS Number Name Date 2,255,746 Brailsford Sept. 16, 1941 2,279,661 Crosby Apr. 14, 1942 2,382,436 Marble Aug. 14, 1945 2,445,508 Beleskas Jul 20, 1948 FOREIGN PATENTS Number Country Date 450,136 Great Britain July 6, 1936

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