US2424905A - Automatic amplitude control for variable frequency oscillators - Google Patents

Description

Patented July 29, 1947 AUTOMATIC AMPLITUDE CONTROL FOR VARIABLE FREQUENCY OSCILLATORS .Ranald D. Scheldorf, Haddon Heights, N. 5., assignor to Radio Gorporation of America, a corporation oil Delaware Application August ill, 1943, Serial No. 498,124

. 3 Claims. (Cl. 250-36) -means for connecting thereto different inductive and capacitive elements, depending upon the operating frequency band, have been employed extensively in multiband radio receivers and transmitters. One disadvantage of circuits of this type is that the amplitude of oscillation varies considerably over the several frequency bands and over difierent portions of each band. Since it is often desirable to maintain the oscillation amplitude at a substantially constant value over the full range'of operating frequencies, it has heretofore often been necessary to employ separate individually balanced oscillation circuits.

The instant invention contemplates a simple and efilcient circuit for controlling automatically the amplitude of oscillation provided by a multigrid oscillator tube. Briefly, the invention comprises a first triode interposed as a variable resister in the screen voltage supply circuit connecting the anode and screen electrodes of a pentode oscillator tube. A diode rectifier is connected to a suitable point in the oscillation circuit to derive therefrom a positive control voltage characteristic of the amplitude of oscillations. This control voltage is applied to the control electrode of a second triode which is directly coupled to the control electrode of the first triode which functions as the screen electrode resistor. The circuit effectively comprises a two-stage amplifier for the control voltage which regulates the operating voltage applied to the screen electrode oi the pentode oscillator to maintain constant the regeneration, and hence the amplitude of oscillations generated thereby at a substantially constant value notwithstanding changes in the values of the oscillation circuit components.

Among the objects of the invention are to provide an improved thermionic tube oscillation circuit having automatic control of the amplitude of oscillations generated therein. Another object of the invention is to provide an improved automatic amplitude control circuit for a multiband thermionic tube oscillation circuit. An additional object of the invention is to provide an improved automatic amplitude control circuit for a multiband pentode oscillator. A further object of the invention is to provide an improved automatic amplitude control for a multigrid oscillatortube wherein the operating voltage applied to the oscillator tube screen electrode is controlled by a thermionic tube circuit responsive to the amplitude of oscillations generated by the oscillator tube.

The invention will be further described by reference to the accompanying drawing of which the single figure is a schematic circuit diagram of a preferred embodiment thereof.

Referring to the drawing, the oscillation circuit comprises a pentode oscillator tube I havin its control electrode connected through a conventional parallelwise connected grid capacitor 2 and grid resistor 3 to the movable element of a first wave-change switch 4. Similarly, the cathode of the pentode oscillator tube I is connected to the movable element of a second wave-change switch 5. The movable elements of the first and second wave-change switches 4, 5, respectively, may be ganged as indicated by the dash line 6. The fixed contacts of the wave-change switches t and 5 are connected'to tapped points on an inductor i, one end of which is connected to ground. Avariable tuning capacitor 8 is connected between the movable element of the first wave-change switch t and the grounded end of the inductor l. The suppressor electrode of the pentode oscillator tube l is grounded. The pentode anode electrode is connected to a source of positive potential, and also is connected through a capacitor 9 to ground.

The anode of the pentode oscillator I is connected to the anode of a first triode I Ii of a. double triode tube I i. The cathode of the first triode I0 is connected to the screen electrode of the pentode oscillator tube I, and also is connected through a fixed capacitor I2 to ground.- The anode of the second triode I3 of the double triode ll is connected to the control electrode of the first triode l0, and also is connected through a grid coupling resistor I4 to the cathode of the first triode II). The cathode of the second triode I3 is connected through a cathode resistor IE to ground, and also through a bleeder resistor I 6 to the screen electrode of the pentode oscillator tube l.

The movable element of the first wave-change switch 4 is connected to the anode of a diode rectifier tube I7. The cathode of the diode rectifier tube I1 is connected to the control electrode of the second triode I 3 of the double triode tube II, and is also connected through a conventional cathode resistance-capacitance network It to ground.

In operation, a screen voltage to the screen electrode of the pentode oscillator tube I is supplied through the variable resistance comprising \the anode-cathode circuit of the first triode it of b i I I r 9,494,908

the double triode ll. Anode voltage for the second triode llof the double triode tube ll is obin, and applied to the control electrode of thefirst triode Ill to vary the cathodeanode resistance of the first triode. Variations in the cathode-anode resistance of the first triode l provide variable screen voltage on the Dentode oscillator l, which automatically controls the amplitude of oscillations generated thereby. The triode portions l0, l3 respectively, 01' the double triode control tube ll eflectively comprise a twostage amplifier responsive to the control voltage derived from the diode rectifier I! for varying the screen voltage on the pentode-oscillator in response to the amplitude of oscillations generated thereby.

It will be seen that an increase in the amplitude of oscillations generated ,by the pentode oscillator will provide increased positive bias on the control electrode of the second triode ll of the double triode control tube'l I, causing the second triode to draw more plate current and thereby biasing the first triode ill toward anode cur,-

rent cutofl. The effect of increasing the bias of the first triode l0 toward anode current cutoff condition efiectively raises the anode-cathode resistance oi. the tube and lowers the screen voltage applied to the .pentode oscillato i, which in turn, decreases the amplitude of oscillations generated in the oscillator tube circuit.

Furthermore, lowering oi! the voltage applied to the screen electrode of the pentode oscillator tube l lowers the fixed bias applied to the second triode cathode electrode, thereby increasing the apparent gain of the amplifier system.

Thus the invention described comprises a novel and efiicient means for controlling automatically the amplitude of oscillations of a multigrid thermionic tube multiband oscillator circuit whereby control voltages derived from the oscillation circuit provide efiective control of the screen voltage of the oscillator tube.

I claim as my invention:

1. An automatic amplitude control circuit or a variable frequency oscillator employing a thermionic discharge tube having at least a cathode. an anode, a screen electrode and a control grid comprising means including a second thermionic discharge tube providing a variable impedance path from said anode to said screen electrode, rectifier means coupled to said oscillator tube control grid and responsive to the amplitude of oscillations generated by said oscillator for derivinl control voltages, and means torapplying said control voltages to vary the impedance of said variable impedance path to control regeneration in said oscillator by varying the voltage on said screen electrode.

2. An'automatic amplitude control circuit for a variable frequency oscillator employing a thermionic discharge tube having at least a cathode, an anode, a screen electrode and a control grid 4 comprising meansincluding a second thermionic discharge tube providing a variable impedance path fromv said anode to said screen electrode, rectifier means coupled to said oscillator tube control grid and responsive to the amplitude of oscillations generated by said oscillator for deriving control voltages. thermionic tube means for amplifying said control voltages and means for applying said amplified control voltages to said second thermionic tube to vary the impedance of said variable impedance path to control regeneration in said oscillator by varying the voltageon said screen electrode.

3. An automatic amplitude control circuit for a variable frequency oscillator employing a thermionic discharge tube having at least a cathode, an anode, a screen electrode and a control electrode comprising a source of operating voltage for said anode, means including a second thermionic discharge tube having at least at cathode, an anode and a control electrode providing a variable impedance path for said anode voltage to said screen electrode, diode rectifier means coupled to said oscillator tube control electrode and responsive to the amplitude of oscillations generated by said oscillator for deriving control voltages, means including a third thermionic tube for amplifying said control voltages, and a connection from said third tube to the control electrode of said second tube for applying said amplified control voltages to vary the impedance of said variable impedance path to vary the voltage applied to said screen electrode 01' said oscillator tube.

RANALD D. SCHELDORF.

REFERENCES CITED The following references are of record in the file or this patent:

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