US2396224A - Oscillation generator - Google Patents

Description

March 12, 1946. M. ARTZT OSCILLATION GENERATOR led June 16, 1943 (Ittorneg Patented Mar. 12, 1946 UNITED STATES PATENT OFFICE oso Lm'rroN GENERATOR Maurice Artzt, Princeton, N.'J., minimto Radio 7 Corporation of America, a corporation of Dela- Claims.

This invention relates to oscillation generators. and more particularly to improvements which facilitate the maintenance of a high degree of stability in an oscillation generator.

My invention is applicableto oscillation generators in which resonance is obtained by a combination of resistance and capacitance. The invention is also applicable to generators in which resonance is obtained by means of a tuning fork, or a tank circuit consisting of parallel-connected inductance and capacitance elements, or still again by means of a piezo-electric crystal or other electromechanical vibrator.

when investigating the properties of oscillation generators of the prior art, it has been found that those of the type having resistance-capacitance resonators, hereinafter called RC oscillators, are particularly subject to adverse influences unless they are properly shielded. For example, if an oscillator is to deliver a frequency in the low range, say between and 200 cycles, and if a high value of grid leak is used, say 8 megohms or more, the system is quite sensitive to hum pick-up. If the oscillator is set near 30, 60, 90, 120. or 180 cycles, it will tend to lock up or synchronize itself to hum. This is not necessarily true of fixed frequency oscillators, or where the network resistors are of less than one megohm in value. However, oscillators of the type having inductive and capacitive tanks, hereinafter called LC oscillators, are not so susceptible to adverse external influences. In the RC oscillators of my invention as herein disclosed, the ease with which they lock in or synchronize with a control frequency is turned to advantage, instead of being a nuisance diflicult to eliminate;

It is an object of my invention to provide an oscillator having improved frequency stability.

Another object of my invention is to provide an improved tuning fork controlled electronic oscillator.

Still another object is to provide an oscillator in which improved frequency stability is obtained by means of a piezo-electric crystal.

. 4 A further object of my invention is to simplify the design of a tuning fork in association with an oscillation generator.

One feature of simplification to be found in my Conventional circuits as used to drive a tuning fork employ two magnetically polarized coils, one in the plate circuit of a tubeand the other in the grid circuit. The fork acts as a mechanical coupling between grid and plate, .and is sustained in oscillation by taking drive power from thetube. In any tuning fork, the best frequency constancy is reached at very low amplitudes, and the true fundamental frequency exists only when the amplitude of oscillation is infinitesimal. Any excess of drive power will force the fork to a lower frequency, just as overswlnging a pendulum will lengthen the period of oscillation. With the usual circuits for driving a fork, considerable overdrive is needed to make the system self-starting 'and, as a result, the frequency varies with changes in drive effort. Nearly all previous work on fork drive amplifiers has been directed towards making this drive power constant.

My invention will now be described in more detail, reference being made to the accompanying drawing, in which:

Fig. 1 shows an oscillation generator circuit arrangement having tuning fork control and em- :iodying the fundamental features of my inven- Fig. 2 shows an embodiment in which the resonant unit comprises a parallel tuned inductance.

and capacitance; and

Fig. 3 shows still another modification of the invention in which the resonant element is a piezo-electric crystal.

Referring first to Fig. 1, I show therein, by way of illustration, two electron discharge tubes l and 2, the cathodes of which are interconnected and are provided with a common cathode resistor 3 leading to ground. The tubes l and 2 also contain grids 4 and 5 respectively and anodes 6 and 1 respectively.

A source of direct current operating potential indicated as +B is connected between ground and the anode 6. This same source is connected through a potentiometer 8 to the anode 1 in tube 2.

The input circuit for tube I includes two resisters, 9 and ID in series between the grid 4 and ground. A capacitor H is in shunt with resistors 9 and i0. A feedback circuit includes blocking condenser l2 which is connected between a tap on potentiometer 8 and the junction between resistors 9 and Ill.

The grid 5 in tube 2 is directly connected to ground and derives its bias potential by virtue of a potential drop developed in resistor 3.

As shown in Fig. 1, the oscillator derives its resonance stability from the operation of a tunthe anode of the first tube and the grid of the second tube, a direct ground connection for the grid of the first tube, a common cathode resistor connecting the cathodes of both tubes to ground, a series resonant circuit in shunt with said cathode resistor and having said tuning fork disposed as an armature to a polarized inductive element therein, means in said feedback circuit having a time constant such that the oscillations generated in said tubes tend. to drift toward a frequency slightly higher than the natural frequency of said tuning fork when driven at low amplitude, the interaction between said tuning fork and the inductive element of said resonant circuit being effective to drive the fork and to maintain said oscillator in synchronism therewith.

6. An RC-oscillator comprising two discharge tubes each having a cathode, an anode and a grid and operating circuits therefor, including a resistor connecting both cathodes to ground, a potentiometer in the output circuit of one of said tubes, an adjustable tap on said potentiometer, a feedback circuit connected betweensaid tap and ground and constituted as a series capacitance and resistance, the grid of the second tube being coupled to said output circuit through said capacitance, a direct ground connection for the grid of the first tube, a frequenc stabilizing device having a series-resonant circuit in shunt with said cathode resistor, and a tuning fork constituted as an armature for a magnetic circuit which includes the core of an inductive winding in said series resonant circuit,

7. An RC-oscillator according to claim 6 and further characterized that said feedback circuit possesses a time constant equivalent to a frequency slightly above the natural frequency of said tuning fork when the latter is driven at low amplitude.

8. In combination, an oscillation generator comprising two electron discharge devices of the discharge device for causing the frequenc of the generated oscillations to exceed slightly the natural frequency of said vibrator element, if uncontrolled thereby, and means for so limiting the amplitude of electrical output energy derived from said generator and applied to said magnetic device for driving said vibratory element that the amplitude of mechanical vibrations produced in said vibratory element is maintained at an optimum value for frequency stabilization, and said generator is locked in step with said vibratory element.

9. An RC-oscillator comprising two discharge devices, means including a feedback circuit having a time constant value determined by a suitable choice of capacitive and resistive elements in series, said feedback circuit being arranged to derive control potentials from the output of one said device and to apply them to the grid of the other said device for maintaining phase opposition between oscillations to be generated in one device relative to those in the other, a direct ground connection for the grid in the one said device, a cathode resistor connecting the cathodes of both said devices to ground, a tuning fork, and means having series inductance and capacitance in shunt with said cathode resistor for providing a coupling between said oscillator and the mechanical motion of said tuning fork, said tuning fork being disposed in the relation of an armature to the field of said inductance for causing the fork to vibrate and for causing the fork to control the frequency of the oscillations generated.

10. An RC-oscillator comprising two discharge tubes and operating circuits therefor, including a. resistor connecting the cathodes of both tubes to ground, a piezo-electric device connected in shunt with said resistor and chosen to produce resonance at the frequency of the oscillations to be generated, a potentiometer in the output circuit of one of said tubes, an adjustable tap on said potentiometer, a feedback circuit containing series capacitance and resistance connected between said tap and ground, the grid of the second tube being coupled to said output circuit through said capacitance, said feedback circuit having a time constant value which tends to develop oscillations in said tubes at a frequency slightly-higher than the natural frequency of said piezo-electric device, and a direct ground connection for the grid of the first tube.

MAURICE ARTZT.

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