US2225897A - Oscillator circuit - Google Patents

Description

Dec. 24, 1940. D. A. BELL 2,225,897

OSCILLATOR CIRCUIT Filed Feb. 18, 1958 2 A l A l n- 7 a vvvvv vvv K7 /5 y i 1 19.3 DISTRIBUTED 2 7 CAPAC/TA/VCE I1-; 0F RESISTOR 16; i J

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ATTORN EY Patented Dec. 24, 1940 PATENT OFFICE OSCILLATOR CIRCUIT David Arthur Bell, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application February 18, 1938, Serial No. 191,156 In Great Britain March 6, 1937 3 Claims.

This invention relates to electron discharge device oscillation generators and more articularly to such generators of the kind in which regeneration is obtained by coupling between a tuned output circuit and an input circuit. The invention seeks to provide improved oscillation generators of the kind referred to which, although employing ordinary tuned circuits with coils of normal Q value, shall be of high stability, which shall not be liable to change materially in frequency as a result of small changes in operating potentials or as a result of ageing of the electron discharge device, and whose frequency shall be very little affected bythe harmonic components generated in the electron discharge device.

According to this invention, an electron discharge device oscillation generator comprises in combination an electron discharge device, a circuit resonant to the desired frequency to be generated and energized from an output electrode of said device, means closely coupling said circuit to an oscillation input electrode of said device whereby a high degree of positive or regenerative feed back for oscillation generation is obtained ative or degenerative feed back to said device.

Preferably the resonant circuit is a parallel tuned circuit connected at one end to the output electrode of the electron discharge device and coupled at the other to the oscillation input electrode thereof, an intermediate point thereon being connected, through a circuit including a resistive feed back impedance to a point which is at cathode potential as regards the oscillation fre- 35 quency, e. g., to earth or to the cathode itself.

By the expression resistive as herein employed in connection with the means providing degenerative feed back, is meant that the current through the impedance by means of which said ,feed back is obtained, is substantially in .phase with the voltage across said impedance, and that the ratio of said current to said voltage is independent or approximately independent of frequency.

The invention is illustrated in the accompanying diagrammatic drawing in which,

Figure 1 shows a circuit diagram of a preferred embodiment of the invention;

Fig. 2 shows a modification comprising a tuned circuit made up of an inductance paralleled by two capacitors in series;

Fig. 3 shows another modification using in one part of the circuit an inductance in series with a resistor having an inherent distributed capacitance; and

and resistive means for applying a degree of neg- Fi 4 shows still another modification inwhich a series combination of inductance, resistance, and distributed capacitance serve as a substantially pure resistance at the operational frequencies. ,5,

Referring to Fig. 1, which shows one way of carrying out the invention, a triode or other thermionic valve 1 has its anode 2 connected to its control grid 3 through a parallel tuned circuit in series with a coupling condenser 4 the said cou- 10 were connected merely through the usual anode potential source M (shunted by the usual condenser 15) to the cathode 5, the generator would be an ordinary well known generator having the well known defects of instability as regards fre- 25 quency. In accordance with this invention, however, a resistance I6 is included in the lead between the intermediate tap l3 and the cathode 5. This resistance l6 provides resistive degenerative ornegative feed back and its provision enables 30 the control grid 3 to be closely coupled to the tuned circuits 1, 8, 9as is desirable for stability of freq'uency--and also greatly reduces the generation of harmonic voltages of such phase as would tend to disturb the frequency of oscillation. The circles V represent voltmeters.

In a practical example of generator as just described operating at a frequency of 80 kilocycles per second, the valve I was of the type now known under the trade designation Marconi 40 MH4; the tuned circuit consisted of an inductance 'l' of 200 microhenries shunted by a total capacity (including stray capacity) of about 0.02 micro-farad; the coupling condenser 4 was of 0.15 micro-farad; the grid leak 6 was of one quarter of a megohm; and the resistance l6 providing degenerative feed back was of 1,000 ohms.

Circuits as just described have been experimentally tested for working frequencies of 80 k. 0. per second, 320 k. 0. per second, and 560 k. c. and have been found remarkably free from frequency deviation as a result of applied voltage variations. For example, in the k. 0. case, an increase in anode voltage from to 237 volts produced a frequency increase of only 2 cycles per 55 If now an intermediate tap I3 on the inductance 1 of the tuned circuit 2Q million, while a variation of filament voltage from 2.5 to 4.42 (the normal voltage being 4) resulted in a frequency increase of 16 cycles per million. In the 320 k. 0. case variations of anode voltage by 5 111% produced frequency changes, respectively, of -0.8 cycles per million and +0.45 cycles per million, variations of filament voltage of producing frequency changes, respectively, of +0.6 cyclesper million and 0.6 cycles per mil- 10 lion. In this connection it may be noted that frequency changes due to variations (in the same direction) of anode and filament voltages are opposite in direction so that, in cases Where these voltages are derived from a common source, a

T5 very high degree of constancy should be obtained. For the 560 k. 0. case variations of :12% in anode voltage produced variations of about :5 cycles per million and variations of i 10% in filament voltage produced variations of about :4

30 cycles per million. Reduction of the harmonic content is consequent upon the use of resistive negative feed back which, in particular, tends to suppress harmonic voltages of such phase relationship to the anode current of the valve as would cause disturbance of the frequency of oscillation. Frequency variations due to grid current are minimized by the close coupling between tuned circuit and grid and for the same reason frequency variations due to variations in interelectrode capacity of the valve can be greatly reduced. These are very important advantages.

In the modification shown in Fig. 2 the tuned circuit capacity is made up of two capacities l1,

IS, in series, and instead of taking the tap to a :35: point l3 on the inductance I of said circuit 1, l1, N3, the tap is taken to a point (here marked l3) between the two condensers l1, 18. In Fig. 2 the anode feed is shown applied via a choke I9 direct to the anode.

. In some cases, especially where high frequencies are in question, distributed capacity across the degenerative feed back resistance may result in said feed back being insufficiently resistive in nature. In such a case either of the embodiments 5 already described may be modified by connecting an inductance in series with the resistance. In practice the reactance of this inductance is small compared to its ohmic resistance so that the required value of inductance is approximately CR 50 where C is the distributed capacity measured in farads across the resistance and R the resistance in ohms and the inductance is measured in henrys. Fig. 3 shows the circuit of Fig. 1 modified in this way, 20 being the series inductance, and the brok- 55 en line condenser 2| represents the distributed capacitance across the resistance l6. Such a series combination of inductance 20, resistance 16, and (distributed) shunt capacity 2| will act as a substantially pure resistance for all values of 60 frequency such that p C R. is negligible compared to unity,

( is the frequency) Similarly, where the resistance element presents an undesirable amount of inductance (this difliculty may arise at low frequencies) a network consisting of a series branch of inductance and resistance shunted by a series branch of capacity 5 and resistance may be employed (in place of a simple resistance l6) as a complex network of resistive nature for providing the degenerative feed back. Fig. 4 shows the circuit of Fig. 1 modified in this .manner, the network comprising a 10 branch consisting of resistance 22 and series inductance 23 shunted by a branch consisting of resistance 24 and series capacity 25.

It will be seen that in the illustrated arrangements close coupling to the grid 3 is effected by choosing the value of the coupling condenser 4 to be of low impedance at the oscillation frequency while the coupling of the anode and grid to the tuned circuit may be so chosen that the efi'ects of space charge variations in the valve upon the frequency generated substantially cancel out, leaving substantial immunity from frequency variations from this cause.

I claim:

1. A device for canceling out the effects of space 25 charge variations in the discharge tube of an oscillation generator, said tube comprising a cathode, an anode, and a grid; said device including a resonant circuit interconnecting the cathode and anode, a regenerative feed-back circuit to said grid from a suitable point on said resonant circuit, and means for degeneratively coupling said anode and grid comprising impedance means connecting said cathode to an intermediate point in the capacitive leg of said resonant circuit.

2. An oscillation generator comprising an electron discharge tube having at least a cathode, an anode and acontrol grid, a parallel resonant circuit connected at one end to the anode and having close coupling means at the other end for supplying regenerative feed-back energy to said control grid, and means to stabilize the frequency of the oscillations generated, said means being constituted by a substantially pure resistance in series with a coupling capacitor and in circuit between the cathode and an intermediate point in 4 the capacitive leg of said parallel resonant circuit.

3. An oscillation generator comprising an electron discharge tube having at least a cathode, an anode and a control grid, a parallel resonant circuit formed by an inductance and two serially connected capacitors, a conductive connection from the anode to one end of said resonant circuit, a capacitor for closely coupling the other end of said resonant circuit to the control grid and for supplying the same with regenerative feedback energy, and a degenerative feed-back circuit constituted by a substantially pure resistance connected to the junction between said two serially connected capacitors and coupled to the cathode.

DAVID ARTHUR BELL.

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