US2423505A - Bridge stabilized oscillator - Google Patents

Description

July 8, 1947.

` v. D. LANDON BRIDGE STABILIZED OSILLATOR Filqd Aug. 9, 1943 INVENroR. Yer-non .D L andan AToe/VEK Patented July 8, 1947 BRIDGE STABILIZED OSCILLATOR Vernon D. Landon, Princeton, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application. August 9, 1943,' Serial No.` 497,926

(Cl. Z50-36) 2 Claims. l

This invention relates to bridge-*stabilized oscillators and has for its principal object to provide frequency stability in such an oscillator when the same is also arranged to be tunable over a wide range of frequencies.

It is another object of my invention to provide an oscillator in which frequency stability is obtained by means of a thermally controlled resistance element which affects the balance of a bridged-T network in accordance with the amplitude of oscillation.

It isv still another object of my invention to provide a two-stage oscillator of great stability wherein the feedback from the secondstage to the` rst is obtained through a bridged-T network. Constant amplitude as well as constant frequency isA obtained by the use of a thermally responsive resistanceV element operating to control the phase of the feedback potentials through the bridge network so asV to` maintain oscillations at the lowest possible amplitude consistent with the quantity of useful power which is to be derived from the oscillator.

In the. achievement of the foregoing objects, I provide a circuit arrangement which isA operable in the same manner as Class A ampliersr so as to derive output energy which is quite free from harmonics and other unwanted frequencies. Hence, it is a further object of my invention to provide a system which will operate to produce a substantially pure sine wave, and which will, as far as possible, be independent of temperature changes and yother adversely variable conditions such as variations in the driving D. C. voltage, humidity, jarring of the apparatus, and age of the electronic tubes.

My invention will now be described in more detail, reference being made to the accompanying' drawing, the sole figure of which represents diagrammatically a preferred circuit arrangement.

Referring to the drawing, I show therein two electron discharge tubes I and 2, these being preferably of the pento'de type. In each of these tubes is a cathode 3, a control grid 4, a screen grid 5, a suppressor grid 6, and an anode 1. The input circuit for tube I comprises an inductance 8 and an adjustable capacitor 9 which constitutes a tank circuit tunable to the desired frequency of operation. The capacitor 9 is directly connected to ground and to cathode resistor I il. Resistor I I preferably has a very low impedance, say of the order of 10 ohms. It is connected between the lower end of the inductance 8 and ground. The

purpose of thisl resistor I I is to prevent oscillation at unwanted frequencies.

The control grid 4 in tube 2- is coupled to the anode circuit of tube I by means of capacitor 25. 'Tube 2 has an input circuit whichincludes grid leak resistor 26. The outputV circuit for tube 2 is tuned by means of a parallel-arranged inductance 28 and capacitance 29'. A core 21 of powdered, iron. or rnagnetitev is made adjustable for tuning purposes.

The inductance coils 8. and 28 are interconnected by means ofv a T-bridgek circuit which is used for feedback purposes. This circuit comprises two inductances I2 and I3 shunted by a resistor I4. The junction between inductance-s I2 and I3 is connected through a variable resistanee ele-ment I5 and through a capacitor IS to ground.

Anode potential is fed to the anode 'I of tube I from any suitable source (indicated as +B) through an inductive impedance I'I, the latter being preferablyshunted by a resistor I8. Inductance I1 together with a capacitor 23 provide series tuning of the output circuit for tube I. Anode potential for the anode 'I in tube 2 is derived from the sam-e D. C. source through a resistor I9 and through!inductance 28. A, capacitor 20- isolates the D. C-. anode potential from the input circuit for tube Ii. Bypass condensers 2 Iy provide Aa-n A. C. path from each of the screen grids 5 to ground. Bypass condensers 22- serve the same purpose with respect to the A. C. potentials on the cathodes 3f. The proper value of screen grid potentials is obtained by the use of three resist-ors 24, one being in a common lead and the other two being individual to theY screen grids.

Any voltage variation which appears in the output circuit of tube 2 affects the temperature of the variable resistance element I5. This element is preferably so constructed that it provides the necessaryr compensation so as to maintain constant the phase angleof the feedback potentials froml the output circuit of tube 2 across the bridged-T network to they input circuit of tube I. Preferably-therefore, the' resistance' element I5 iscomp'osedof a very fine platin-um wire. In an embodiment of the invention which was actually constructed, this wire had a diameter of 0.8 mil and its length was 1A; inch. The Value of this resistance element when cold was approximately 2 ohms. The increase in the resistance of this element caused by the action of the radio frequency current was in a direction to produce a balance in the bridged-T network. The resistance I4 bridging the two coils I2 and I3 requires a value 3 of 4(R-|R) for a balance, R and R' being the coil and variable resistance values respectively.

Thus when the resistance element is cold, there is less attenuation in the bridge, which causes an increase in the voltage applied to the amplifier input and the amplitude of oscillation increases. With this increase in amplifier output the bridge is brought more nearly into balance. Furthermore, the attenuation is increased, thereby decreasing the input voltage. Thus, it is evident that by such a means the amplitude of oscillations is automatically stabilized.

It will be understood by those skilled in the art that the degree of unbalance of the bridge circuit as caused by the amplifier output current results in a voltage variation of input potentials applied to the grid of tube l. However, the gain of the system is so adjusted that substantially no grid current is permitted to flow.

It will be apparent that many advantages are to be derived from the construction of my oscillation generator in the manner heretofore set forth. The system operates with no tube overloading and produces a very pure sinusoidal output. Furthermore, the system can be made to operate substantially independent of anode voltage variations, which fact has been demonstrated by actual tests on apparatus constructed in accordance with the invention. For example, the following data was obtained during such tests:

Frequency deviation from 107 cil/cles` with, given change in supply voltages for various oscillator types Per cent Per cent Decrease in Decrease in Felgcy Filament Piate vou- C 015s' Voltage age y 10 70 0 2O 260 O 600 0 1, 450 10 10 60 zo Y 2o 1.100

The effectiveness of the circuit in holding constant frequency is brought about by the use of the bridged-T network, operated near the balance point. The eiectiveness is a function of the rate of change of phase with a change in frequency. This factor is highest when the balance point. Operation is automatically maintained at a point as close to the balance point as is possible while maintaining suicient feedback to sustain oscillation.

My invention is, of course, capable of modification in various ways, as will be clear to those skilled in the art.

I claim:

1. An oscillation generator comprising two cascaded stages of electron discharge tubes, each tube having an input circuit and an output circuit, tuning means in the input circuit of the first stage and in the output circuit of the second stage, a series resonant circuit in the outbridged-T network is operated Very close to the f put circuit of the first stage, a circuit for coupling said series resonant circuit to the input circuit of the second stage, a resistor of relatively loW ohmic value interconnecting two points on the input circuit tuning means of the first stage, one of said points being grounded, means including a feedback circuit connected between the output circuit of thesecond stage and the input circuit of the first stage for causing oscillations of relatively low amplitude to be generated and for stabilizing the frequency of said oscillations, said feedback circuit being constituted as a T-bridge network composed of two series-inductive elements shunted by a single resistive impedance, a variable resistance element being connected at one end to the junction between said inductive elernents and having its other end capacitively coupled to ground, said variable resistance element possessing a positive temperature coefficient of such value that phase compensation results from variations in the amplitude of the feedback energy.

2. An oscillation generator comprising a pair of electron discharge tubes, each tube having a pair of input electrodes and a pair of output electrodes, a tuned input circuit connected between the input electrodes of the rst tube, a resonant output circuit connected between the output electrodes of the second tube, a series resonant circuit connected between the output electrodes of the rst tube, a circuit for coupling the input electrodes of the second tube to said series resonant circuit, means for grounding said tuned input circuit and said resonant output circuit, and a feedback circuit coupling said resonant output circuit to said tuned input circuit, said feedback circuit being constituted as a T-bridge network composed of an inductive link shunted by a resistor and a variable resistance connected at one end to a point intermediate the ends of said inductive link and having its other end capacitively coupled to ground, said variable resistance possessing a, positive temperature coeiiicient, said T-network being so adjusted as to maintain sufficient feedback to sustain oscillations.

VERNON D. LANDON.

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

UNITED STATES PATENTS Number Name Date 1,897,639 Kreer Feb. 14, 1933 2,173,427 Scott Sept. 19, 1939 1,568,065 Gunn Jan. 5, 1926 2,210,303 Polydoro Aug. 6, 1940 2,163,403 Meacham June 20, 1939 2,319,965 Wise May 25, 1943 FOREIGN PATENTS Number Country Date 682,205 France Feb. 11, 1930

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