US2251629A

US2251629A - Parasitic oscillation elimination

Info

Publication number: US2251629A
Application number: US345768A
Authority: US
Inventors: Laurance M Leeds
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1940-07-16
Filing date: 1940-07-16
Publication date: 1941-08-05
Anticipated expiration: 1958-08-05
Also published as: BE448814A

Description

Aug. 5, 1941. u M. LEEDS 2,251,629

PARASITIC OSCILLATION ELIMINATION Filed July 16. 1940 I I LI L INDUCTIVE Inventor: Laurence M. Leeds,

His Attorney.

Patented Aug. 5, 1941 PARAsrrIc OSCILLATION ELIMINATION Laurance M. Leeds, Rotterdam Junction, N. Y.,

assignor to General Electric Company, a. corporation of New York elimination July 16, 1940, Serial N6. 345,768

(c1. ris -171) Claims.

My invention relates to high frequency circuits and more particularly to means for damping undesired oscillations in such high frequency circuits.

In circuits carrying high frequency current it frequently arises that certain portions of the circuit must be made of such length as to have appreciable inductance, such as .to resonate, with circuit capacities and produce undesired oscillations. It has been proposed to shunt such a high frequency circuit by a resistance to damp out the undesired oscillation. In certain cases, however, the resistance itself placed in shunt to the conductors, may have an amount of inductance comparable to the inductance of the conductor itself. It is accordingly an object of my invention to provide an improved and simplified high frequency circuit in which undesired oscillations are minimized.

, It is a further object of my invention to provide a high frequency circuit having improved and simplified means for damping undesired oscillations therein.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of'operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1

of the drawing illustrates a preferred embodiment In Fig. 1, a pair of electron discharge devices lb and H are connected to operate in balanced or push-pull relation to amplify a wave from a source, not shown, which wave appears across an input inductance l2. The amplified wave is impressed across an output inductance I3 and is eventually utilized in a load circuit, not shown.

In the input circuit for the devices I0 and .H the ends of the inductance I2 are connected respectively to the control electrodes [4 and of the devices 10 and II. The cathodes It and [1 are connected to ground.

The central point of the inductance I2 is connected through a choke coil I8 and a source [9 terelectrode and circuit capacities, resonate with the inductance 12 at the frequency of the wave impressed thereon.

In the output circuit of the devices l0 and I 1, the anodes 23 and 24 thereof are connected to opposite ends of the inductance 13, respectively through

conductors

38 and 39; A condenser 25 is connected fromthe anode 23 to ground and a second condenser 26 is connected from the anode 24 to ground. These two condensers in series, together with unavoidable interelectrode and circuitcapacities, resonate with the inductance 13 at the frequency of the wave impressed on the inductance l2; The central point of the inductance 131's connected through a choke coil 21 and a source 28 of anode potential, by-passed by a condenser 29,to ground.

In high power amplifiers, in which my present invention is primarily useful, triode electron discharge devices are commonly used which require neutralization. Conventional neutralizing means including a pair of condensers 38 and 3| are provided to neutralize the devices I 0 and I I. The

condenser 30 is connectedfrom the anode 23 througha conductor 32 to the control electrode l5 of the discharge device I I. Similarly the condenser 3| is connected from the anode 24 through a conductor 33to the control electrode M of the device It). I I

In the operation of such an amplifier at high frequencies it has been found that the

conductors

32 and 33,; which often must be of considerable length, may have an inductance resonant with the interelectrode capacities in the devices ID and Hand withinductance and capacity in other portions of the circuit at a frequency or frequencies, near or in the band of frequencies within which it is desired to operate the amplifier. Parasitic oscillation may, of course, be produced at any frequency determined by circuit conditions. Such undesired or parasitic oscillations may completely destroy the usefulness of the amplifier if the oscillations are strong, or may result in spurious radiations and reduced useful output if the oscillations are weak. For satisfactory operation, parasitic oscillations must be completely eliminated. It has been proposed to insert damping resistances in the

conductors

32 and 33, but such resistances absorb substantial amounts ofpower of the frequency which it is desired to amplify. It has also been proposed to connect a resistance in shunt to some portion of ,each; circuit in which undesired oscillations persist, sothat the circuit is critically damped and undesired oscillations cannot build up. It has been found, however, that the shunt resistance 34 and the condenser 35 may therefore be said to be series resonant. Likewise the resistance 36 and the condenser 3'! may be called series resonant. The

resistors

34 and 36, being in series resonant circuits, therefore act as pure resistances and effectively damp oscillations which tend to occur through the

conductors

32 and 33.

In the usual case encountered in practice, parasitic oscillations occurring in the neutralizing circuit are at frequencies substantially higher than the operating frequency. With

resistors

34 and 36 and their associated

condensers

35 and 31 series resonant at the parasitic frequency and connected in shunt to

conductors

32 and 33, there is substantially no damping at the operating or power frequency, yet the circuit may be critically damped at the frequency of undesired oscillations.

If desired, such a series resonant combination of resistance and capacity may be connected in shunt to some other portion of a circuit resonant at an undesired frequency, such other portion being, for example, an

anode conductor

38 or 39, or a portion of the choke coil I8 or 21, or a surface or portion of a neutralizing condenser 33 or 3|.

In Fig. 2 there is illustrated apparatus similar to that of Fig. l in which such a series combination of resistance and capacity is connected in shunt to a portion of the anode circuit. In this figure many of the elements are like those illustrated in Fig. 1 and are given like reference characters. The devices Hi and II, however, respectively have screen electrodes 40 and 4!, in addition to anodes 23 and 24, cathodes l6 and I7, and control electrodes I4 and 15. These screen electrodes minimize capacitive reaction between the anodes 23 and 24 and the control electrodes hi and I5. gether and through a source 42 of potential to ground and are respectively by-passed to ground through

respective condensers

43 and 44. Such screen electrodes 43 and 4| are frequently utilized to avoid the necessity of neutralization.

Very high frequency parasitic oscillations are frequently observed in such an electron ,diS- charge amplifier. Such oscillations may occur in a path through the conductor 33, the space between anode 23 and electrode 40, condenser 43, and thus through ground and condenser 25 back to the conductor 38. A similar circuit exists through the conductor 39, the space between the anode 24 and the electrode 4!, the condenser 44 to ground, and thus through the condenser 26 back to the conductor 39. Such circuits usually tend to oscillate at a very high frequency. The inductance of the

conductors

38 and 39 plays a large part in these oscillating circuits.

In order to eliminate such undesired parasitic oscillations, a series combination of a capacitor 45 and a resistor 43 is connected in shunt to the conductor 38, and a similar series combination of a capacitor 41 and a resistor 48 is connected in shunt to the conductor 39. The capacitor 45 They are connected tois adjusted to resonate at the undesired parasitic frequency with the inductance of the resistor 46, which exists due to its finite length. The series combination of capacity and resistance, being series resonant, presents a pure resistance at the undesired frequency in the parasitic oscillation circuit. This resistance 4'6 may be adjusted to produce critical damping of the undesired parasitic oscillation, so that such oscillation is effectively eliminated. The series combination of capacitor 41 and resistor 43 is similarly made series resonant, so that it likewise presents a pure resistance at the undesired parasitic frequency in shunt to the conductor 39. The resistance 43 also may be adjusted to produce critical damping in the circuit for the parasitic oscillation.

It may be noted that conventional neutralizing means 30, 3i are shown in addition to the screen electrodes 4!) and 4!. When such screen electrodes are used at very high frequencies, it is difficult or impossible to ground them for the high frequency currents and consequently some feedback is experienced to the control electrodes I4, 15. The neutralizing condensers 33, 3| are provided to improve the operation at such high frequencies.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed, and I aim by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. Radio apparatus comprising a circuit in which oscillations of an undesired frequency tend to exist, said circuit including a conductor of finite length, which thereby exhibits inductance, across which such oscillations appear, and a resistance and a capacitor serially connected between spaced points on said conductor, said resistance, due to its finite length, being inductive and being resonant at said undesired frequency with said capacitor, whereby a pure resistance is presented between said spaced points at said undesired frequency effectively to damp said oscillation.

2. In a high frequency power amplifier, an electron discharge device having an anode, a cathode, and a control electrode, a circuit connected between said anode and control electrode and including a conductor of substantial length, said circuit having a tendency to break into oscillation at an undesired frequency, said conductor due to its length having substantial inductance, and a series combination of resistance and capacity connected in shunt to said conductor, said resistance being inductive due to its finite length and being resonant at said undesired frequency with said capacity effectively to damp the oscillation in said circuit.

3. An electron discharge amplifier including a circuit which tends to oscillate at an undesired frequency, a portion of said circuit being a conductor of finite length whereby it is inductive, and a series combination of resistance and capacity connected in shunt to said conductor, said resistance also being inductive due to its finite length and being series resonant at said undesired frequency with said capacity effectively to damp said oscillation.

4. In a high frequency power amplifier having a triode electron discharge device and neutralizing means therefor, said neutralizing means including a conductor of substantial length, the inductance of which tends to resonate with other portions of said amplifier at an undesired frequency, and means in shunt to said conductor for damping oscillations of said undesired frequency, said means comprising a series combination of resistance and capacity, said resistance being of finite length and exhibiting inductance, and said capacity being of such value with respect to said inductance as to resonate therewith at said undesired frequency effectively to damp said neutralizing means.

5. In combination, an electron discharge amplifier device having an anode and a pair of control electrodes, said anode and one of said control electrodes being adjacent each other, a circuit through said anode and said adjacent control electrode including a conductor of finite length which thereby exhibits inductance, said circuit tending to oscillate at an undesired frequency, and a series combination of a resistor and a condenser in shunt to said conductor, said resistor having finite length and exhibiting inductance and said condenser being resonant therewith at said undesired frequency effectively to damp said circuit.

' LAURANCE M. LEEDS.