US2478330A - Oscillator - Google Patents

Description

Aug 9, 1949. J. R. SHONNARD OS 0 ILLATOR Filed March 21, 1946 6' cons-mm FREQUENCY saerm.

FORK 0R nzsomtoa IN V EN TOR. J. R. SHJ/VIl/ARQ t his Atforngy Patented Aug. 9, 1949 OSCILLATOR John R. Shonnard, New York, N. Y., assignor to Times Facsimile Corporation,

New York, N. Y.,

a corporation of New York Application March 21, 1946, Serial No. 655,964

3 Claims.

This invention relates to oscillators and more particularly to frequency stabilization of oscillators.

The class of oscillator to which the invention pertains includes either a mechanical vibratory system or a resonant circuit to which energy is continuously supplied to maintain continuous oscillations of constant frequency. Usually a small amount of energy is abstracted from the vibratory system, amplified and employed to provide a source for output oscillations and a source of driving energy for maintaining the vibratory system or circuit in oscillation.

The frequency stability of an oscillator of this character, which is the most important characteristic for most applications, is affected by many factors, including the physical conditions in and around the vibratory element and variations in the gain of the amplifier if the driving power and the loading of the vibratory element are afiected thereby. Even in the case of a balanced tuning fork or crystal which is compensated for temperature variations or maintained at a constant temperature, the frequency of the oscillator is affected to an undesirable degree by changes in the loading of the vibratory element caused by unavoidable changes in the electrical characteristics of the associated amplifier. These changes are the result of changes in the supply voltages to the cathodes and electrodes of the amplifier, changes in the resistance or impedance of the amplifier circuits and other changes due to ageing and the like. The effect of these changes cannot be reduced to the desired extent unless unduly complicated regulating and other auxil iary equipment are provided. The size and cost of this auxiliary equipment in many cases precludes its use even though improved stability and related characteristics of the oscillator are desired. Furthermore the complexity of such control equipment introduces problems of construction and maintenance.

It has been generally recognized that minor changes in the loading of the vibratory system or resonant circuit will affect the frequency of the oscillator, i. e. the frequency of the element or resonant circuit which is utilized to control the frequency of the current in the output circuit of the oscillator.

It has also been noted that variations in the characteristics of the amplifier referred to above will change the loading or damping sufficiently to cause a considerable change in the oscillator frequency. In the case of an oscillator having a mechanical vibratory element, the amplitude of vibration of said element will vary if the driving energy is altered and it has been proposed to reduce the frequency variations of the oscillator resulting therefrom by controlling the gain of the amplifier to maintain it substantially constant. For example, compensating arrangements have been proposed for varying the driving energy supplied to the vibratory element or system in response to changes affecting the normal oscillator frequency. However all of these methods of compensation operate in response to a change in the frequency or amplitude and thus only approach in some degree the requirement of exact frequency stabilization.

It is the object of the present invention to provide an improved method and means of stabilizing the frequency of an oscillator by preventing changes in the loading of the resonant system or frequency-determining element due to changes in the amplifier, and by limiting the feedback power so that changes in the amplitude of Vibrations or oscillations of such element are small and do not affect the natural period. Stated in another way, the object of the invention is to provide a non-dissipative coupling between the resonant system or element and the feedback amplifier so that the system or element is not clamped by the feedback circuit. In addition such an arrangement has sufficient overall gain to be dependably self-starting. It will be apparent that by controlling the damping factor and by maintaining the driving energy substantially constant, as by a limiter circuit, a constant frequency of oscillation of the frequencydetermining system is attained in spite of minor variations in either the amplitude of the vibration of the frequency-determining element or the characteristics of the associated amplifier.

Other objects and advantages of the invention will appear from the following description of the preferred embodiments thereof, shown on the accompanying drawings, wherein Fig. 1 shows a preferred embodiment of the invention, including a tuning fork for determining the frequency of the oscillator;

Fig. 2 shows a modification of the frequencydetermining element of Fig. 1; and

Fig. 3 shows another embodiment of the invention employing a different coupling between the resonator and amplifier.

Referring to Fig. 1, there is shown a tuning fork liL' provided with inductive pick-up and drive coils H and I2, respectively, adjacent the tuning fork. As is well known in the art, the vibrations of the tines of the fork induce a. small alternating voltage in the pick-up coil II, which may be amplified and impressed upon the drive coil l2 to maintain the tuning fork in vibration. The mounting and construction of the mechanical vibratory element may be similar to that shown in the patent to A. G. Cooley, No. 2,174,414 and the fork per se may be sealed in an evacuated container to reduce the effect of air damping on the tines of the fork. For thehigher frequencies, other types of mechanical vibratory elements, such as a piezo-electric crystal, may be employed, the tuning fork of the usual alloy steel composition being most suitable up to frequencies of the order of three thousand or four thousand cycles per second. As will be explained in connection with Fig. 2,"a resonant circuit or network also may be employed to determine the frequency of the oscillator.

In the embodiment shown in Fig. 1, the alterhating voltage derived from the pick-up coil H is impressed upon the control grid I5 of a space discharge device 16 through the conductor ll. As shown, the space-discharge device I6 is a double triode having a cathode I8 and an anode I 9 associated with the control-grid element l5; and a second section consisting of control-grid 2 I, cathode 22 and an anode 23 connected as an amplifier to supply energy to the drive coil I2 of the mechanical vibratory element.

It has been found heretofore that the use of a stable amplifier arranged to furnish a substantially constant driving energy to a tuning fork or equivalent element provides a relatively stable oscillator, if the fork is properly aged and insulated against substantial variations of temperature and ambient pressure. It is also known that changes in frequency resulting from shaking or changing the position of the fork may be minimized by using a small, compact tuning fork with the tines carefully matched as to size and mass. However, it has been found that even after taking all of these precautions, the freqency of'the oscillator varies appreciably, say one or two parts or more per hundred thousand, if the characteristics of the amplifier change to a slight extent that is impossible to avoid. Such minor changes may result, for example, from variations in the components of the amplifier due to humidity variations, ageing and so forth, and particularly from variations in the damping of the fork or other frequency-determining element. Heretofore as pointed out above, it has been proposed to maintain the output of the amplifier or the driving energy supplied to the fork substantially'constant, on the theory that if the driving energy were augmented, the amplitude of vibration of the fork would increase and this would result in greater damping of the fork itself, with decreased period of vibration. It is obvious that increased amplitude results from increased driving energy but it has not been recognized that the damping effect is primarily the result of the coupling heretofore employed between the pickup coil of the fork and the amplifier. I have found that by increasing the impedance of the input circuit to the amplifier substantially to infinity, the stability of the oscillator is markedly increased even if the driving energy and amplitude of vibration of the fork are permitted to vary somewhat. The improvement may be realized by input impedances which are relatively high, compared to the impedance of the inductive pick-up coil measured at the frequency of the oscillator, provided the phase relation between the potentials at the terminals of the drive element i2 and the input electrode l5 of the amplifier is held constant. If the input impedance of the amplifier is of the order of one megohm, excellent results are obtained with respect to increased frequency stability. Obviously the higher the input impedance, the less the damping of the fork.

One method of accomplishing this result which has been found to'work well in actual practice is to employ an isolating stage having substantially infinite input impedance, as by connecting the pick-up coil directly-to the control grid [5 of the space-discharge device connected to the amplifier; and an unbypassed cathode-bias resistor 25 is connected to the cathode l8 of the space-discharge device. With this arrangement in a tube of the SP8 or 7N7 type, for example, the input impedance of the tube composed essentially of the inter-electrode capacities is of the order of several megohms at a frequency of one or two thousand cycles. Many other types of tubes are also suitable for this application, of course. Under these conditions, no grid current flows in the control-grid circuit under operating conditions and the damping effect of the amplifier upon the tuning fork is exceedingly small, practically zero, even if the voltage across the terminals of the pick-up coil increases as a result of increased amplitude of the fork. It is to be noted that the gain in the first or isolating stage of the amplifier is negligible under these conditions, the purpose of the first stage represented by the electrodes I5, l8 and I9 being merely to provide a non-dissipative coupling between the fork and the subsequent stage or stages of the amplifier so that the alternating voltage of the pick-up coil H is coupled to the amplifier without the possibility of a change in the damping effect or absorption of energy from the fork element, if the sources of cathode heater and space current of the amplifier vary.

As shown, the output of the first stage of the amplifier is coupled through the couplin condenser 26 to the control-grid electrode of an amplifier 21, which is shown as a high gain pentode tube. The tube 21 is arranged to act as a limiter by adjusting the circuit constants so that the signal impressed thereon causes grid saturation. The constant-level output of the tube 21 is impressed upon the control grid 2! of the second section of the tube I 6 through a coupling condenser 28 and resistor 29. The amplified constant-level output current of said section is impressed upon the drive coil [2 of the fork element through the coupling condenser 3 I.

With this arrangement it is found that substantially perfect stabilization of the fork frequency is obtained even if the characteristics of the amplifier vary to such a degree that the frequency of the oscillator as constructed heretofore would be affected adversely. The amplifier is particularly sensitive to changes in cathode heater current, and in actual practice it is found that such variations that would cause a change in oscillator frequency of one or two parts or more per one hundred thousand in the prior oscillators under favorable conditions, may cause a change of only approximately one part per million as a result of the improved coupling between the pick-up coil of the fork element and the amplifier. H

In order to provide a constant-level output current of constant frequency from the oscillator, a connection is made from the output circuit of the limiter tube 21 through a resistor 32 to an auxiliary amplifier 33. As shown the tube 33 is of the double triode type having its input circuits connected in parallel, thus providing two output circuits 34 for supplying two separate load circuits. The voltage divider including resistor 35 connected in series relation with one of the input circuits of the tube 33 reduces the amplitude of the current in the associated output circuit. The additional amplifier 33 prevents a varying load circuit from affecting the frequency stabilization of the oscillator but other modifications of the output circuit will occur to those skilled in the art and may be used if desired.

Fig. 2 illustrates a resonant circuit or phaseshifting network, which may be substituted for the fork element shown in Fig. 1 by connecting the same to the terminals lit. The phase-shifting network of Fig. 2 of conventional design comprises a plurality of series condensers 31, 38 and 39 having shunt resistors 4|, 42 and 43 connected as shown to effect a 180-degree phase shift between the input and output voltages at the desired oscillator frequency. The resistor 41 may be adjustable in order to permit the oscillator frequency to be adjusted to the desired value. Heretofore the use of a resonant circuit has been considered impractical in an oscillator where a high degree of stability is desired because of the efi'ect upon the frequency of the oscillator produced by slight variations in the characteristics of the associated amplifier. However it is found that a high degree of stability is obtained in such an oscillator if the resonant circuit is coupled to the amplifier through a non-dissipative coupling as described.

Fig. 3 shows a modification in which the signal from the frequency-determining element or oil'- cuit 5! is impressed on the control electrode 52 of a space-discharge device 53. The device 53 is arranged in such manner that the potential of the cathode 5t varies directly with the potential applied to the control electrode, such as in a conventional cathode follower, in which the flow of D, 0. grid current is obviated. Hence the input impedance is extremely high at low frequencies, being solely dependent upon electrode and circuit capacitances. The output from the cathode follower 53 is coupled through condenser 55 to the control electrode 55 of an amplifier 5'! constituting a conventional limiter stage. The output of this stage, which as shown is identical with the limiter stage of Fig. 1, is coupled directly or through a coupling condenser to another cathode follower 58, which provides a low-impedance source of power to feed the frequency-determining element or resonant circuit 5] in order to obtain a high order of regulation. Thus the output of the cathode follower is independent of changes in the load imposed by the frequencydetermining element or resonant circuit. A 100- tentiometer 6|] or other amplitude control device may be employed to adjust the feedback power to such a value as to produce satisfactory selfstarting characteristics and optimum stability.

Conductor 6| may be connected to the circuit or, device to be supplied by the oscillator.

At frequencies where the pick-up tube input impedance becomes a substantial shunt on the frequency-determining element or resonant circuit, load and phase shift may become large enough to be detrimental to frequency stability. In accordance with the invention, selection of the pick-up tube type and design of the frequencydetermining element or resonant circuit to present a relatively low impedance to the pick-up tube should be such as to minimize the damping effect of the coupling to the amplifier, as explained above.

While several preferred embodiments of the invention have been described in detail for the purpose of explaining the principles involved, various modifications in the circuit arrangement and components may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

1. An oscillator comprising a frequency-determining system or element adapted to resonate at a predetermined frequency, means to supply energy to said system or element to maintain continuous oscillations thereof, said means including a multi-stage amplifier, the first stage of which embodies a space-discharge device having a cathode, an anode and a control electrode conductively connected in potential-transferring relation to the frequency-determining system or element, biasing means connected to the elec trodes of said space-discharge device to maintain the control electrode at a negative potential with respect to the cathode and at a value sufiicient to prevent the flow of current in the control electrode circuit under oscillating conditions to minimize changes in the loading on the frequency determining System or resonant circuit, and means for maintaining the driving energy supplied to said system or element substantially constant.

2. An oscillator comprising a frequency-determining system adapted to resonate at a predetermined frequency, pick-up and drive elements therefor and means to supply energy to the drive element of said system to maintain continuous oscillations thereof, said means including an electronic tube amplifier coupled to the pick-up and drive elements of said frequency-determining system, said means also including an isolating stage for said pick-up element consisting of a space-discharge device embodying a cathode, an anode and a control electrode, said control electrode being condu'ctively connected to said pickup element, and means including an unbypassed resistor connectedi n a common portion of the cathode-anode and cathode-control electrode circuits of said discharge device for maintaining said control electrode at a negative potential with respect to said cathode and of a value sufiicient to prevent the flow of direct current in the control electrode circuit under oscillating conditions.

3. An oscillator comprising a frequency-determining system having a natural period of vibration or oscillation, a pick-up element therefor, a feedback circuit arranged to supply energy to said system to maintain continuous oscillations thereof and a non-dissipative coupling between the pick-up element and said feedback circuit having an impedance of the order of a megohm or more at the oscillator frequency, said coupling consisting of a space-discharge device embodying a cathode, an anode and a control electrode, said JOHN R. SHONNARD.

c onductively connected 8, REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Stallard July 23, 1936 Norrman May 9, 1939 Wallace Sept. 26, 1939 Millar Aug. 11, 1942 Whitaker Oct. 27, 1942 Root Nov. 24, 1942 T

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