US2243540A - Oscillation generator - Google Patents

Description

S. W. SEELYEY OSGILLATION GENERATOR Filed NOV. 3, 1938 May27, 1941,

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Patented May 27, 1941 UNHTED STATES PATENT OFFEQE 2,243,540 oscnLA'rIoN GENERATOR Stuart W. Seeley, Bayside, Long Island, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 3, 1938, Serial No. 238,500

7 Claims. (Cl. 250-36) of the oscillator tube. More specifically, the pri- 5 mary object of the invention is to compensate for a drift in the oscillator frequency caused by a variation in that part of the input capacity of the grid circuit of the oscillator electrontube which is due to the finite electron transit time between the cathode and first positive element in thetube.

Another object is to provide compensating means in the form of a space discharge path for automatically compensating for the effect upon the frequency of the oscillator caused by variations in the anode voltage supply. In the preferred embodiment of the invention this space discharge path takes the form of another electron tube disposed in parallel relation to the oscillator tube.

A further object is to provide such an oscillator as above described, wherein the compensating space path contributes to the oscillator output in order to provide a more eficient generator of high frequency oscillations.

The theory underlying the present invention will now be given: It appears that the largest single factor afiecting the frequency of the oscillator as its operating potentials are varied is the change in input capacity with anode current. The total apparent input capacity ofan electron tube is usually composed of three factors. First,

the static capacity between the metallic electrode elements. This type of capacity is due to the physical geometry of the tube and is not affected by operating potentials. Secondly, the capacity due to the Miller effect. This second type. of capacity is due to reactive currents flowing between the grid and a positive electrode element whose potential is varying at the same frequency but 180 out of phase with that of the grid. Third, the so-called transit time input capacity which is due to the instantaneous unbalance between approaching and departing electrons in the vicinity of the grid. The present invention is believed to be concerned mainly with this last type of input capacity, i. e., that which is due to finite electron transit time, although it is to be distinctlyunderstood that the invention is not to be limited by any theory advanced for the results achieved.

If the input capacity of a typical tube is measure-d between grid and all other elements bypassed together, it will be found that the input capacity depends to some extent upon the amount and the electron velocity of the space current passing through the grid. As an illustration, the input capacity of 605 vacuum tube with 200 volts on its anode will increase as the grid potential is decreased from cut-off toward zero value. This increase is very nearly proportional to the anode current up to 10 m. a. (8.5 volts bias) at which point it, amounts to about 1.0 ,u f.

As the grid bias is decreased further, the anode current continues to rise, but the capacity curve levels off, showing a capacity saturation characteristic. It also appears that as the grid is made positive, the input capacity may actually decrease, but this is extremely difficult to measure in the presence of grid conductance.

Ordinarily, it would seem that an increas in plate voltage of an oscillator would tend to raise the input capacity and thus decrease the frequency. However, I have found the reverse to be true due to the saturation characteristic of the capacity curve. A tube with grid leak bias and normal plate potentials will operate over the flat portion of the capacity curve for the major portion of its cycle unless the developed direct current bias is greater than the value at the knee of the curve (8.5 volts in the above case). If the anode voltage is increased, the maximum value of capacity remains the same but the tube operates with a greater portion of its cycle in region of less capacity. It will be understood, of course, that it is the integrated value over the cycle which determines the effect on frequency.

According to the present invention, any tendency on the part of the oscillator tube to have its input capacity change in one sense with change in anode voltage is off-set or compensated by another electron tube whose input capacity tends to change in an opposite sense with the same change in anode voltage. This compensation is accomplished by paralleling the input and output circuits of the oscillator, electron tube whose frequency increases due to a decrease in input capacity because of an increase in anode voltage, with the grid and anode circuits, respectively, of another vacuum tube having an opposite effect on input capacity with change in anode voltage. In this way the compensating or stabilizing tube contributes to the oscillator output to provide a more eflicient oscillation generator circuit, while the compensating tube nevertheless has an initial bias characteristic different from that of the oscillator.

The following is a detailed description of the invention in combination with a drawing, wherein Figs. 1 and 2 show, by way of illustration only, two oscillator circuits embodying the invention.

Referring to Fig. 1, there is shown an electron tube oscillator I for the generation of high frequency oscillations. The grid and anode electrodes of this oscillator tube are coupled to opposite terminals of a tuned oscillatory circuit comprising an inductor -2 and a parallel connected variable capacitor 3, while the cathode of tube I is connected to an intermediate point on the inductor 2. A suitable bias resistor 4 supplies bias to the grid of tube I, while the source of anode potential +B supplies a positive polarizing potential to the anode of tube I through a radio frequency choke coil 5.

In order to eliminate or reduce the effect of variations in the anode supply +B upon the frequency of oscillations, there is provided a compensating electron tube 6 whose grid-cathode or input capacity is disposed in parallel relation to the grid-cathode or input capacity of the tube I, and whose anode circuit is in parallel relation to the output circuit of the oscillator tube I. More specifically, the grid of electron or vacuum tube 6 is disposed in parallel relation to the grid or input circuit of tube I, while the anode of tube 6 is disposed in parallel relation to the anode or output circuit of tube I. The anode of electron tube 6 thus obtains its positive polarizing potential from the same source of anode supply +B through choke coil 5. The negative bias of compensating tube 6 is obtained through grid resistor I and potentiometer 8, the latter supplying a high negative potential to the grid of 6, such that it never draws grid current. The adjustment of the negative bias on the grid of the compensating tube 6 by means of tap 9 enables the change of input capacity of the oscillator tube I with change in anode voltage to be counterbalanced by an opposite change in input capacity of tube 6. By changing the degree of negative bias on the grid of tube 6, we can thus overcornpensate or undercompensate or equalize over the operating cycle the change in input capacity of the oscillator tube, provided that the total variation of input capacity of the compensating tube can be made greater or less respectively than the input capacity of the oscillator tube. V

In View of the fact that the anodes of both tubes I and '5 are operating in parallel relation,

' both serve to supply energy to the output circuit of the oscillator, although it will be appreciated that tube would not oscillate by itself. It should be understood that compensating tube 6 operates over a different portion of the grid characteristic than does oscillator tube I. Because of the fact that tube 6 contributes to the oscillator output, there is obtained a more efficient circuit than an arrangement wherein the anode circuits are not connected in parallel relation from a radio frequency standpoint.

. Fig. 2 is an arrangement very similar to that of Fig. 1, except that tubes I and 6 of Fig. 1 are combined in a single envelope known as a twin triode oscillator. A tube of this type'is sold under the trade designation 6A6,.b'y the RCA Manufacturing Co., Inc. The circuit of Fig. 2 is substantially the same as that of Fig. 1, except that the input circuit of the oscillator portion of the tube comprises a single turn inductor 2' in parallel relation to a capacitor 3|, while the output circuit of the oscillator portion of the tube includes an inductive feed-back coil IO which is coupled to the inductor 2' to augment the gain on low frequencies, in order that the oscillator may cover a band of frequencies. The bias on the grid of the oscillator space path is obtained by virtue of the grid leak condenser combination II, while the bias on the grid of the compensating or stabilizing space path is obtained through resistor I and tap 9, the latter of which is variable. over a suitable battery I2. Except for the foregoing differences, the circuit of Fig. 2 functions in substantially the same manner as does the circuit of Fig. 1, in order to provide a stabilized ultra high frequency oscillator.

What is claimed is:

1'. In. an oscillation generator comprising two electrode structures each having a grid, a cathode, and an anode, said structures being connected in parallel for high frequency, a common source of anode potential for both structures, the'method of minimizing changes. in the frequency of oscillation arising from changes in input capacity due to changes in the potential applied to said anodes, which comprises varying the internal capacity of one of said structures oppositely relative to the other in response solely to said same changes in potential applied to said anodes, and adding together the alternating current output energy of said structures.

'2. A stabilized oscillation generationcircuit comprising a first electrode structure constituting an oscillator and including a cathode, an anode and an intermediate grid electrode, means including a source of potential for maintaining said anode positive relative to said cathode, an oscillatory circuit coupled to said anode, grid and cathode, means for applying a. biasing potential to said grid relative to said cathode, the frequency of said oscillator tending to change with change in anode potential, and av second electrode structure located outside the electron path of said first structure and also including an anode, a cathode, and a grid electrode, con-- nections between the corresponding electrodes of said two electrode structures for paralleling the same from a radio frequency standpoint, said source of potential being also connected between the anode and the cathode of said second structure, whereby said second electrode structure contributesto the output of said. oscillator,. and means for applying a negative potential to the grid of said second structure of such value that said second structure operates over a different portion of thegrid characteristic than said first structure and the frequency change tendency of saidoscillator caused by avariation of said source of anode potential is substantially ofiset by an opposite frequencychange tendency caused by said same variation in anode potential. r

3. The combination with an electron discharge device oscillator having grid, anode and cathode electrodes so connected and having such potentials applied thereto as toproduce high frequency oscillations, theirequency of said oscillator tending to change with change in anode potential; of another electron discharge device having its grid} anode andcathode: coupled in parallel relation from an alternating current standpoint to the grid, anode and cathode of said oscillator, whereby the outputs of both said devices are paralleled, a common source of anode potential for both of said devices, and means applying a controllable negative potential to the grid of said second device of such value that said second device operates over a different portion of the grid characteristic than said electron discharge device oscillator and the grid of said second de vice never draws current, and the frequency change tendency of said oscillator caused by a variation of said source of anode potential is substantially offset by an opposite frequency change tendency caused by said same variation of said source.

4. In a high frequency system, in combination, first and second electrode structures within a single evacuated envelope, each of said structures including an anode, a grid and a source of electrons, means including a source of potential for the anode of said first structure and a resonant circuit coupled to the anode, grid and source of electrons of said first structure for causing the same to produce oscillations the frequency of which changes with change in anode potental, connections of low impedance to energy of the operating frequency between corresponding grid and electron source electrodes of said structures, and a direct connection of low impedance to energy of the operating frequency between the anodes of both structures whereby said source of potential is common to both anodes and the eifective input capacity of said second structure is connected across a portion of said resonant circuit, and means applying a negative potential to the grid of said second structure of such value that said second structure operates over a difierent portion of the grid characteristic than said first structure and the input capacity of said second structure produces a change in the efiective capacity of said resonant circuit with change in anode potential which is opposite to the change produced by said first structure with the same change in anode potential and independent of the frequency produced by said first structure.

5. A high frequency oscillation generator comprising two electron discharge device electrode structures each having an anode, a grid and electron emitting means, and each structure characterized by an input capacity which depends upon anode potential and a rate of change of input capacity which depends upon grid potential, a common source of anode potential for said structures, means for biasing said grids sufiiciently differently so that said rates of change are opposite in sign, whereby the variations in input capacity of one electrode structure due solely to fluctuations in the potential of said common anode supply tend to ofiset the variations in input capacity of the other device due to said same fluctuations, an oscillatory circuit,'and connections from said electrode structures to said oscillatory circuit for causing both of said structures to contribute to the output of said generator.

6. A high frequency oscillator comprising a pair of electron discharge devices each having an input capacity which varies with anode voltage, a common source of anode potential for said structures, means for biasing said devices suificiently difi'erently so that the input capacity of one device varies with anode potential oppositely in sense compared to the variation in input capacity of the other device due solely to the same change in anode potential, and a resonant circuit, and means for connecting said devices in parallel relation, from a radio frequency standpoint, to said resonant circuit, whereby both devices contribute to the output of said oscillator.

7. In an oscillatory electron tube circuit including a first electrode structure containing a cath- I ode, a grid and an anode, a parallel resonant circuit coupled across said grid and anode, a connection from said cathode to an intermediate point on said resonant circuit, and a second electrode structure also containing a cathode, a grid and an anode, connections between corresponding electrodes of said two structures for paralleling the same from an alternating current standpoint, a common potential source for said anodes, and means for biasing said grids differently and at such values that the input capacities of said structures vary in opposite sense with a change in the potential of said common potential source and independent of frequency.

STUART W. SEELEY.

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