US2268366A - Electronic oscillator control device - Google Patents

Description

Dec. 30, 1941. l, WOLF-F ELECTRONIC OSCILLATOR'CONTROL DEVICE Original Filed Jan. 29, l191553 2 Sheets-Sheel l Patented Dec. 30, 1941 ELECTRONIC OSCILLATOR CONTROL DEVICE Irving Wolff, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Original application January Z9, 1938, Serial No. 187,596, now Patent No. 2,210,518, dated August 6, 1940. Divided and this application January 19, 1940, Serial No. 314,648

(Cl. Z50-36) 6 Claims.

This application is a division of my copending application Serial No. 187,596, filed January 29, 1938, for Electronic oscillator control device Patent No. 2,210,518 granted August 6, 1940.

My invention relates to the regulation of the frequency and amplitude of an electron discharge oscillator, and more particularly to a constant current device for maintaining th'e frequency and/or amplitude of an electron discharge oscillator constant by controlling the magnetic field, the filament current, or the electrode potentials.

I am aware of the various methods of obtaining a constant frequency in which the voltage developed across a circuit which is resonant near the desired frequency is used to change the frequency generator in a manner which will tend to stabilize the frequency. Electron discharge devices such as magnetrons and Barkhausen- Kurz oscillators present new problems in frequency and amplitude control because of th'e ultra high frequencies involved and because of their inherent erratic operation. For these reasons, the usual methods of control would be relatively ineffective; in addition, changes in the resonant frequency of the usual control circuit, due largely to changes of temperature, have a greater effect at ultra high frequencies than at lower frequencies.

It is an object of my invention, therefore, to provide means for stabilizing the frequency output of an electronic oscillator.

Another object of Vmy invention is to provide a biasing .potential for the control of the operating characteristics of an electronic discharge device.

A further object of my invention is to provide a magnetron'oscillator having stable frequency clfiaracteristics.

It is also an object of my invention to provide means for compensating for the inherent instability of operation of an electron discharge oscillator resulting from the regenerative effect of electron bombardment on the hot cathode.

A still further object of my invention is to provide means for automatically controlling the nagnetic eld intensity of a magnetron oscilla- My invention will be better understood from the following description when considered in connection with the accompanying drawings. Its scope is indicated by the appended claims.

.Referring to the drawings,

Figure 1 is a schematic diagram of one embodiment of my invention;

Figure 2 is a schematic diagram of a modication which controls frequency independently of changes in amplitude;

Figures 3 and 4 are diagrams showing operating characteristics of a magnetron oscillator; and

Figure 5 is a schematic diagram representing a modification of my invention which may be employed to maintain the amplitude of an oscillator at a constant value.

Referring particularly to Figure 1, an enclosed quarter Wave antenna is shown at l. This device consists of a conductive rod 3 approximately a quarter wave long, mounted within and attached to the grounded base of a hollow cylindrical metal case 5. An opening 'l is provided through which radiant energy from the oscillator to be controlled may induce a voltage on the quarter wave antenna 3. While I prefer to obtain th'e control voltage by radiation from the oscillator, obviously it may be obtained by direct, capacitive, or inductive coupling.

The combined inductance of the rod 3 and the capacitance of the shield 5 are designed so as to resonate near the frequency which is to be maintained. It has been discovered that a resonant circuit of this nature has an exceptionally high Q, i. e., high ratio of reactance to resistance, and consequently when its resonant frequency bears such a relation to the oscillator frequency that said oscillator frequency falls on the slope of the resonance curve, a small change in the oscillator frequency results in a large change in voltage von the resonant circuit. In addition, it has been found that it is possible to design a resonant circuit of this nature which is inherent- -ly stable with respect to temperature variations. A coupling coil 21 is placed within the outer case 5 in order to loosely couple the antenna to a thermionic tube ll through' a coupling capacitor I3. Tube Il consists of a heat-er 2l, a unipotental cathode I9, and an anode electrode l1. The cathode i9 is connected to one terminal of capacitor I3, to the ungrounded end of a diode load resistor 23, and to the control grid 28 of a second thermionic tube 25. Tube 25 is preferably a pentode similar to RCA type 57 which contains a control grid, a screen grid, a suppressor grid, an anode, a unipotential cathode, and a heater.

A rectified D. C. voltage is developed across resistor 23 which is positive with respect to ground and which is applied to the control grid. It is necessary to provide a somewhat greater positive potential for the cathode 29 in order to operate the tube at the proper point on its characteristic. I prefer to use a constant voltage gaseous regulator tube such as RCA type 8'14 for this purpose. The degenerative effect of a series resistance in the cathode circuit is thereby eliminated. I have accomplished this by connecting a limiting resistor 33 in series with regulator tube 35 between ground and the positive termina-l of a high voltage supply 24. The cathode 29 is then connected to the movable arm 25 of a low resistance potentiometer 31 sh'unted across said regulator tube. The effective grid bias on tube 25 is the difference between the voltage appearing from grid to ground and that appearing from cathode to ground. This bias may be adjusted by means of the variable contact 26. Resistor 33 and potentiometer 31 are of such value that the steady current through them from lsource 24 is large with respect to the cathode current of tube 25. rIhe cathode potential then remains substantially constant when th'e cathode current changes.

The anode 39 is connected to the positive terminal of the high voltage supply. through a plate load resistor 4I and directly to the control grid 3U of tube 43, which is also a type 57 tube. This direct connection places grid 30 at a positive potential with respect to ground, and consequently it is necessary to operate its cathode at a slightly higher positive potential, This is accomplished by means of a tap at a point of suitable potential on a bleeder resistor 41 connected between the positive potential source and ground. The same bleeder resistor supplies potential for the screen grid 49 and also for the screen grid 5| of tube 25.

The anode 32 of tube 43 is connected directly to grid 53 of a fourth thermionic tube 55, which is preferably a low plate impedance triode, such as RCA type 2A3. The anode 51 of this triode is connected directly to the positive terminal of the potential source. The voltage drop from anode 51 to cathode 59, due to the anode current in tube 55, provides the necessary positive anode potential for this tube. Resistor 6 I, which is connected between grid 53 and cathode 59, provides a connection to the positive potential source for the anode 32 of tube 43, and in addition the voltage drop across 6I, due to the anode current of 43, places grid 53 at a suitable negative potential with respect to cathode 59. A connection is made from cathode 59 to the oscillator which is to be controlled.

The constant current characteristic of this device may be utilized in different ways, depending upon the requirements. If a controlled potential is required, a resistor may be connected between cathode 59 and ground, and the operating potential for the oscillator taken from the drop across this resistor. A preferred method is illustrated in Fig. 1, in which the grid 63 of a Barkhausen-Kurz oscillator 64 is connected to cathode 59. This type of oscillator is so well known that it is not necessary to show its connections in detail. An antenna 66 provides the radiation through which coupling is effected to antenna I.

It is well known that the frequency. of electron discharge oscillators is critically dependent upon various electrode potentials. In a Barkhausen- Kurz oscillator, for example, the frequency increases as a function of the positive grid potential. It is evident, therefore, that the frequency of such an oscillator may be controlled by the device illustrated.

Assume that the system is operating in a stable condition on the high frequency slope of the frequency-amplitude characteristic of resonant circuit I. Energy from the oscillator 64 is radiated from antenna 65, and picked up by antenna 3. If the frequency becomes slightly higher due to a change in supply voltage, or any other cause, the operations which will tend to return the frequency to the original value can be traced in the following manner: The R. F. voltage on the resonant circuit I decreases; the D. C. potential on grid 28 becomes less positive; the anode current in tube 25 decreases; the potential on anode 39 and grid 30 becomes more positive; the anode current in tube 43 increases; the potential on anode 32 and grid 53 becomes less positive; the anode-cathode impedance of tube 55 increases; and the positive grid voltage on grid 63 decreases, which lowers the frequency. The system will, therefore, stabilize itself at some point dependent upon the circuit constants, and will tend to maintain the oscillation frequency constant regardless of the Variation of other factors entering into its operation. In brief, this embodiment of my invention may be described as a degenerative constant current regulating device operated by the frequency of the generated oscillations.

It is, of' course, possible to operate the oscillator on the low frequency slope of the frequency-output characteristic of resonant circuit I, but it is then necessary to compensate for the consequent phase reversal by adding or deducting one stage of amplification.

In applying my invention to a. magnetron oscillator, reference is made to Fig. 3, which graphically illustrates the frequency and amplitude of a magnetron oscillator as functions of the magnetic field intensity. It may be seen that the amplitude passes through a maximum and the frequency increases as the field intensity is increased. When a magnetron is operated on the peak of the amplitude-field intensity curve, the frequency can be controlled by small changes in the magnetic field strength without affecting the amplitude. However, where it is desirable to control the oscillation amplitude manually, by varying the cathode current, for example, or

where it is not desirable to operate the magnetron on the peak of the amplitude-field strength curve, it is necessary to provide means for controlling the frequency independently of changes of amplitude. In Fig. 2 I have shown a modification of my invention which accomplishes this. While shown in connection with a magnetron oscillator, it is obvious that the principle may also be applied to any electron discharge oscillator.

Referring to Fig. 2, it will be seen that resonant circuit I, rectifier II, amplifiers 25 andA 43 and control tube 55, with their associated resistors, are connected, and operate, in a manner identical with that shown in Fig. 1. The Barkhausen-Kurz oscillator has been replaced by a magnetron oscillator 80, whose connections are so well known that it is unnecessary to show them in detail. A magnetic field is supplied by an electromagnet 82. Two coils 84 of this electromagnet are serially connected between cathode 59 and ground. In addition, a resonant circuit 65 has been added which differs from resonant circuit I only in the fact that it is resonant on the opposite side of the desired frequency. Circuit I is resonant below, and therefore circuit 65 is resonantabove, the frequency which is to be maintained. A second rectifier 61, an additional pentode amplifier 69, and a second triode 1I have been added, and connected in the same manner as the first group shown in Fig. 1. Since the resonant circuits I and 65 are on opposite sides of the desired frequency, the voltage developed across diode load resistor 13 will increase when the voltage developed across resistor 23 is decreasing due to a shift of frequency, and vice versa. In order to obtain an in-phase control from both circuits, one amplifier tube has been omitted from this additional circuit. Tube 69 is connected directly to control tube TI. This results in one less phase reversal in the additional circuit than in the original circuit, and consequently the additional circuit is brought into phase with the original and their action in controlling the field current is additive. However, a variation in the amplitude of oscillation is applied in phase to the rectiiiers I I and 61, and consequently will appear out of phase in the control tubes 55 and 1|. The effect of an amplitude variation is therefore eliminated, and the device may be used to control frequency independently of unintentional or intentional changes of output. A further modification to be noted is the inclusion of a resistor 'I5 between the anode of tube 43 and the grid of tube 55. This is necessary in order to reduce the gain of the first system to that of the additional system so that uniform control will be exerted by each.

A third modication of my invention is shown in Fig. which is substantially a constant current control device which prevents changes in amplitude of a generated signal. Referring to Fig. 5, an aperiodic antenna is shown at Tl. The voltage developed in this antenna from the oscillator 91 which is to be controlled is impressed across the diode and cathode electrodes of a tube 19. The rectified D. C. potential which appears across a diode load resistor 8| is applied directly to the grid 83 of a thermionic amplifier pentode 85. Bias for the cathode 81 is obtained in a manner identical with that used in Figs. l and 2, by means of a regulator tube |08, a limiting resistor |02, and a potentiometer |53. The anode 89 is connected to the grid 9| of a low impedance triode 93 which is connected between the positive terminal of the potential source S5 and the element which is to be controlled in the oscillator, indicated by 97. Screen voltage is derived from a voltage divider |04. An increase in the amplitude of oscillation causes grid 83 to become more positive, which makes grid 9| less positive and reduces the voltage available for the oscillator 91, tending to reduce the oscillator amplitude.

My invention is not, of course, limited to the control of the operating potentials or the magnetic field intensity, but may, for example, be used to control lament current as shown in my copending application Serial No. 113,184, filed November 28, 1936. The embodiment shown in Fig. 5 is particularly applicable to the latter purpose.

It is further possible that a combination of the methods indicated in Figs. 5 and l may be desirable to control both the frequency and the amplitude of a magnetron oscillator. If this is done, it is necessary to consider the magnetron characteristic which is illustrated in Fig, 4. It will be seen from Fig. 4 that while the cathode current primarily regulates the output of a magnetron oscillator, it has also a secondary effect on the frequency. Itwill therefore be necessary to operate the frequency control device illustrated in Fig. l on the proper side of the required frequency and choose the proper number of intermediate amplifiers so that the undesired effect of a change of magnetic field intensity on the output, and the resultant secondary effect of a compensating change of cathode emission on the frequency do not interact so as to produce instability. The constants should be adjusted s0 that an increase in magnetic iield intensity causes an increase in output as well as increased frequency. Thus, if the magnetic field tends to increase in order t-o keep the frequency constant, the cathode emission, which therefore decreases in order to maintain the output constant, will also tend to increase the frequency. Since both tend to operate in the same direction on the frequency, stability will be reached that much sooner.

The magnetron characteristics shown in Figs. 3 and 4 were determined from actual operating conditions. It was noted, however, that the frequency characteristic may reverse itself under certain conditions. The explanation given here applies only to oscillators actually functioning as indicated in Figures 3 and 4. It will probably be necessary, therefore, to determine the frequency characteristic under each set of operating conditions and determine the control method accordingly.

Although I have shown several specific embodiments of my invention for purposes of illustration, other modifications will be apparent to those skilled in the art. For example, a battery may be substituted for the regulator tube shown in various amplifier cathode circuits. In Fig. 2, I have shown two control triodes and 1| connected in series, but these may be replaced by a single tube having two grids. Similarly, while I have shown a bleeder resistor 4l for obtaining various operating voltages, I am aware that proper potentials may be supplied by separate batteries. Nor is my invention limited to the particular types of tubes indicated. My invention, therefore, is not to be limited except as is necessitated by the prior art and the appended claims.

I claim as my invention:

l. In an oscillator including an electron discharge device in a magnetic iield in which the frequency of Aoscillation is dependent upon the intensity of said eld, the method of maintaining a constant frequency which comprises resonating said oscillatory currents at a frequency higher than the required frequency, simultaneously resonating said oscillatory currents at a frequency lower than the required frequency, rectifying said resonated currents, bringing said rectified currents into an aiding phase relationship and producing from the combined rectified currents a reaction on said magnetic field tending to oppose a change of frequency in said oscillator.

2. A frequency regulating device which includes a source of oscillations consisting of an oscillator in the field of an electromagnet, a concentric line resonator having an opening in the outer conductor thereof for receiving said oscillations, means coupled to said concentric line for obtaining a direct current potential whose amplitude is proportional to the frequency of said oscillations, a thermionic tube having anode, cathode and grid electrodes, means for impressing said direct current potential on said grid, a source of direct current for energizing said electromagnet, and means including the cathode-anode electron stream of said thermionic tube for passing said current through said electromagnet, whereby a change in oscillator frequency reacts on said electron stream to affect said oscillator in a manner which tends to compensate for said change in frequency.

3. The combination including a magnetron oscillator consisting of an electron discharge device, an electromagnet, and a direct current source for energizing said eiectromagnet, a first resonant means tuned to a frequency higher than the frequency of said magnetron, a first rectifier coupled to said rst resonant means, means including said first rectifier for obtaining a first direct current potential, a second resonant means tuned to a frequency lower than the frequency of said magnetron, a second rectifier coupled to said second resonant means, means including said second rectier for obtaining a second direct current potential, variable impedance means connecting said direct current energizing source to said electromagnet, said variable impedance means being controlled by said first and second direct current potentials so that said clectromagnet energizing current is controlled by a change in the frequency of said magnetron in a manner which tends to oppose said change of frequency, but is unaffected by a change in the amplitude of the oscillations of said magnetron.

4. A device of the character described in claim 3 in which one or more thermionic tubes constitute the variable impedance means connecting said direct current energizing source to said electromagnet.

5. A device for generating constant frequency oscillations which consists of a magnetron oscillator energized by an electroinagnet, a rst resonant circuit receptive to said oscillations, the resonant frequency of said. circuit bearing such a relation to the magnetron frequency that said magnetron frequency falls on the low frequency slope of the frequency-amplitude characteristic of said first resonant circuit, means including a first rectifier for obtaining a first direct current potential whose amplitude changes in proportion to changes in said magnetron frequency, a second similar resonant circuit tuned to such a frequency that the magnetron frequency falls on the high frequency slope of the frequency-amplitude characteristic of said second resonant circuit, means including a second rectifier for obtaining a second direct current potential Whose amplitude changes in inverse proportion to changes in said magnetron frequency, a first and second thermionic tube having anode, cathode, and grid electrodes, a source of energizing current for said electromagnet, means including the anode-cathode path of said first and second tubes for connecting said source to said electromagnet, means for applying said first direct current potential to the grid of said first tube so that the potential of said grid varies in inverse proportion to said rst direct current potential, and means applying said second direct current potential to the grid of said second tube so that the potential of said grid varies in proportion to said second direct current potential, whereby said first and second tubes similarly regulate the electromagnet energizing current of said magnetron in a manner which tends to oppose a change of frequency of said magnetron.

6. A device of the character described in claim 5 in Which an odd number of therinionic direct current amplifier stages constitute the means for applying said rst direct current potential to the grid of the first tube, and an even number of therinionic direct current amplifier stages constitute the means for applying said second direct current potential to the grid of said second tube.

IRVING WOLFF.

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