US2300996A

US2300996A - Electric oscillation generating circuits

Info

Publication number: US2300996A
Application number: US378530A
Authority: US
Inventors: Vanderlyn Philip Barnet; White Eric Lawrence Casling
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1940-02-15
Filing date: 1941-02-12
Publication date: 1942-11-03
Anticipated expiration: 1959-11-03
Also published as: GB540941A

Description

Nov. 3, 1942. p, VANDERLYN AL 2,300,996

ELECTRIC OSCILLATION GENERATING CIRCUITS Filed Feb. 12, 1941 IN'VENTORS )3.

ATTORNEY Patented Nov. 3, 1942 1 ELECTRIC OSCILLATION GENERATING CIRCUITS Philip Barnet Vanderlyn, Finchley, and Eric Lawrence Casling White, Hillingdon, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application February 12, 1941, Serial No. 378,530 In Great Britain February 15,1940

10 Claims. (01. 250-36) This invention relates to electric oscillation generating circuits employing thermionic valves.

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 thefrequencystability because the combination of non-linearity of the valve characteristic with the leakage reactance of the 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 bias 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 coupling condenser. In this case the grid drawscurrent more rapidly than it can be dissipated through a grid leak resistor of high ohmic value.

Thirdly,sing1evalve oscillator circuits involve a certain amountof mechanical inconvenience caused by the necessity for having two coupled coils or one coilwith a tapping point, particularly 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 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 phasereverser. Even in the case of such proposals, however, it is still difficult to fix a limit to the oscillation amplitude without involving the'fiow of grid current withfits attendant disadvantages.

It is the object of the present invention to provide a two-valve oscillator circuit in which the disadvantages mentioned above are eliminated 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 tensions supply, said impedance being large compared with 1/9, where g equals the mutual conductance of said valves within the operating frequency range. The arrangement also includes a resonant circuit which forms the coupling impedance between the anode of one of said valves and a control electrode of the second valve.

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 ofthe 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 isconnected betwen 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 such a manner that an increase in oscillation amplitude lowers the grid bias potentials thus tending to reduce the magnitude of anysuch changes in amplitude. J

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 oi the accompanying drawing.

In the drawing,

Fig. 1 shows an embodiment of the invention in one of its simpler forms,

Fig. 2 shows a modification oi the invention wherein pentode tubes are used as well as other circuit elements contributing to an advantageous performance of the oscillator, and

Fig. 3 shows still another modificationof the invention a feature of which is the use of a choke impedance in the common cathode circuit for the tubes of the oscillator. V

Referring to Figure l of the drawing, a triode valve I is shown having a tuned circuit LC connected in its anode lead, the anode being coupled to the grid of a second triode valve 2 through a condenser 3 and leak resistance 4. The cathodes of the valves I and 2 are connected to the negative terminal of the source of high tension supply S through a resistance RI and a suitable mean bias potential for both valves is derived from a potentiometer comprising resistances R2, R3 across the source of supply S. The bias may, of course, be derived from a suitable tapping point on the source S, and it will be understood that although triode valves have been shown, it'

may in some cases be preferable to employ tetrode or pentode valves.

Consideration of the circuit will show that a certain minimum swing of grid potential of the valve 2 will suffice to produce the maximum possible current through the valve I and any increase only serves to make the current Waveform more rectangular,the chief effect of which is to add to the generation of odd harmonics. The voltage amplitude across the tuned circuit LC then depends upon the amplitude of the current swing which is dependent chiefly upon the value of the resistance RI, the positive bias potential applied to the grids and on the impedance at resonance of the circuit LC.

If the resistance RI is large and suitable valves are used, it may be arranged that quite a large amplitude exists across the tuned circuit LC without either of the valves I or 2 passing grid current. The generated oscillations may be taken from the circuit LC direct or from the anode circuit of the valve 2, as variations of load will in the latter case have practically no effect on the frequency generated, particularly if the valve 2 is a tetrode or a pentode. In most cases it will be desirable to take the output from a further resonant circuit connected in the anode circuit of the valve 2 if an approximation to a pure sine wave is required. Since considerable harmonics are present in the anode current of the valve 2, particularly odd harmonics, the resonant circuit may be tuned to one of these harmonics thus providing an output at a multiple of the generated frequency.

The circuit depends for its correct operation upon the impedance connected between the common cathodes'and the negative terminal of the source of supply being several times as great as 1/0 of the valves. At high frequencies the stray capacities of both cathodes set a limit to the value of this impedance with the result that with modern receiving type valves, the useful upper frequency limit of the circuit is about 10 megacycles per second. 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 generated and the third harmonic will make it possible to operate up to about 30 megacycles per second. An arrangement with such a cathode circuit is shown in Figure 2 of the drawing in which the resistance RI is shown shunted by a condenser CI, this parallel combination being connected in series with a coil CR, a further coil CC in series with a condenser C2 being connected in parallel with the combination CR, RI and CL The distributed capacity of the circuit connections between the point of interconnection of the cathodes in tubes I and 2 and the negative line condenser which appears just to the left of the inductance CC. Figure 2 differs from Figure 1 in that pentode valves are employed and the output is taken from a tuned circuit LI, CI connected in the anode lead of the valve 2.

Since the current amplitude depends upon the positive bias applied to both valves, and since no grid current flows with the result that this bias may be provided by a high impedance source such as a rectifier, it is possible to provide a simple automatic amplitude stabilisation circuit as shown in Figure 3. In this figure an amplifier valve 5 is shown connected between the output from the oscillator and a rectifier diode 5, the amplifier being provided in order to improve the sensitivity of the automatic control and also to remove the load of the rectifier from the circuit L2, C2 which would spoil the frequency stability and waveform.

According to Fig. 3 the output from the oscillator is taken from the anode circuit of the valve I which includes the condenser C4, the resistance R4 and tuned circuit L2, C2 the junction between the resistance R4 and the tuned circuit L2, 02 being connected to the grid of the valve 2. If the output should be taken from a circuit connected in the anode lead of the valve 2, the input to the valve 5 should be taken from the anode circuit of the valve 2. As in Figure 2, pentode valves are employed in the arrangement shown in Figure 3.

Figure 3 differs both from Figure 1 and Figure 2 also in that a choke coil RI is employed in place of the resistance RI, the provision of this choke serving to increase the sensitivity of the control. The choke provides a high impedance at the operating frequency but a low resistance so that small changes in the bias potential cause comparatively large changes in the total feed current and hence in the amplitude of the current swing.

A bias potential derived from a variable resistance R5 is applied to the rectifier 6 so that the output has to reach an amplitude, after amplification by the valve 5, equal to the bias applied to the diode 6 before the automatic control commences to function. Variation of this biasing 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 terminal of the source S is indicated by a dotted with the invention, is such that a grid swing is applied to the valve 2 several times as great as is required for the production of maximum current amplitude and the current waveform is in consequence deformed so as to be substantially rectangular. This method of operation is by no means essential and if a particularly pure sinusoidal output waveform is required, it may be an advantage to reduce the grid swing by taking a fraction of the voltage across the tuned circuit. As in the case of any oscillator operating on an almost linear portion of the valve characteristic, the positive feedback adjustment to secure a given amplitude will then be very critical and the amplitude stabilising circuit arrangement shown in Figure 3 will be particularly useful. The operation can in this case be regarded as changing the amount of the positive feedback by varying the slope of the valves I and 2 in consequence of varying the mean currents in dependence upon the output of the rectifier 6. The resistance R4 may be made adjustable in order to provide a suitable amount of feedback initially.

The method of altering the feed currents to the valves in the arrangement described is 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 3 for example the rectifier 6 may serve to apply a negative bias potential to a pentode valve connected in place of the choke coil RI. It will be understood that further modifications in the circuit arrangements described may be made.

We claim:

1. An oscillation generating circuit arrangement comprising in combination two valves, said valves having each a cathode, a control electrode and an anode, a source of high tension supply and an impedance common to said cathodes and connected between said cathodes and the negative terminal of said source of supply, circuit connections from the anodes of said valves to the positive terminal of said source of supply, and a resonant circuit constituting a coupling impedance effective between the anode of one of said valves and the control electrode of the other of said valves, said common impedance being high compared with the space path impedance of said valves within the operating range of frequencies.

2. An oscillation generating circuit according to claim 1 comprising a source of bias potential connected to said control electrodes and means for varying said bias potential whereby the average feed current of said valves is controlled.

An, oscillation generating circuit according to claim 1 wherein means are provided for varying the ohmic resistance of said common impedance, whereby the average feed current of said valves is controlled.

4. An oscillation generating circuit according to claim 1 wherein said resonant circuit is parallel tuned to the frequency or the oscillations to be generated and said resonant circuit is connected between the anode or one of said valves and the positive terminal of said source of high tension supply and wherein a condenser is connected between said anode and the control electrode of said valve.

5. An oscillation generating circuit according to claim 1 comprising a source of bias potential connected to said control electrodes, and rectification means whereby the mean bias potential applied to the grids of both valves is made dependent upon" the oscillationamplitude across the resonant circuit, said means being coupled to said circuit in such manner that an increase in oscillation amplitude lowers the grid biasv potential, thus tending to reduce the magnitude of any such changes in amplitude.

6. An oscillation generating circuit according to claim 1 wherein said common impedance is constituted by a simple resistance.

'7. An oscillation generating circuit according to claim 1 wherein said common impedance is constituted by a complex network comprising an inductance in series with a condenser, said series connected inductance and condenser being shunted by an inductance in series with a parallel combination comprising a resistance and a further condenser.

8. An oscillation generating circuit according to claim 1 wherein said common impedance is constituted by a choke coil, whereby the sensitivity of control is increased.

9. An oscillation generator comprising two cooperating electron discharge tubes, each tube having a cathode, an anode and at least one grid, a common cathode impedance of high ohmic value relative to the space path impedance of said tubes, a source of direct current operating potential having its negative terminal connected to said cathodes through said cathode impedance, and having its positive terminalconnected through other impedances to said anodes, at least one of said impedances being resonant to the operating frequency, and regenerative coupling means suitably connecting control grids in both of said tubes to one of said anodesand to said resonant circuit for causing oscillations to be sustained at asubstantially constant amplitude at said operating frequency.

10. A generator according to claim 9 and having means including other grid electrodes in said tubes for limiting the amplitude of the oscillations.

PHILIP BARNET VANDERLYN. ERIC LAWRENCE CASLING WHITE.