US2436871A - Center frequency stabilized frequency modulated oscillator - Google Patents

Description

K. RATH March 2, 1948.

CENTER FREQUENCY STABILIZED FREQUENCY MODULATED OSCILLATOR Filed Feb. Y 24, 1944 INVENTOR.

Patenied Mar. 2, 194s CENTER FREQUENCY STABILIZED FRE- QUEN CY MODULATED OSCILLATOR I Karl Hath, New York, N. Y., assignor to Radio Patents Corporation, New York, N.

poration of New York Application February 24, 1944, Serial No. 523,633

. 4 Claims. 1

My invention relates to high frequency oscillators of the type embodying a piezo-electric crystal as a stabilizing means to obtain a substantially constant oscillating frequency.

Crystal controlled oscillators known in the art utilize the crystal as an effective element of the oscillating circuit whereby the crystal forms an integral part of the oscillating mechanism. Ar-

ature.

Another disadvantage is the fact that a crystal controlled oscillator cannot be directly frequency modulated, which necessitates the use of phase modulation and other means to overcome this drawback. Furthermore, due to the extremely of the crystal, the starting or building up of the oscillationsv in many cases is not sufliciently smooth to enable rapid keying and to suit other purposes and requirements.

Accordingly, an object of my invention is to provide a single tube crystal controlled oscillator, wherein the crystal is arranged outside the oscillating circuit proper and serves merely as a stabilizing means for the oscillations generated independently of the crystal.

Another object is the provision of a single tube high frequency oscillator circuit of the regenerative or feedback type comprising a tuned oscillating circuit the frequency of which may be controlled in any desirable manner .and a piezo-electric crystal element associate'dwith the same tube or circuit and serving for stabilizing the oscillations or maintaining the carrier or center frequency of the oscillations at a substantially constant value.

These and further objects and advantagesof my invention will become more. apparent from the following detailed description taken in reference to the accompanying drawing forming part of this specification and wherein:

Figure 1 shows an oscillator circuit embodying a piezo-electric crystal as a frequency control or stabilizing element in accordance with the principles of the invention; Figures 2 and 3 are graphs explanatory of the function and operation 'of the circuit according to Figure 1; Figure 4 shows a Y., a cormodification of a single tube oscillator according to the invention, and Figure 5 illustrates a complete frequency modulated oscillator embodying a center frequency stabilized oscillator according to the invention. v

Like reference numerals identify like parts throughout the different views of the drawing.

Referring more particularly to Figure 1, I have shown at In an electronic discharge tube comprising a heater element II, a cathode l2, an inner control grid l3, an accelerating or screen grid 14, an outer control grid I5 and an anode or plate 16, all substantially arranged in the order named. A resonant oscillator or tank circuit, l1 is-;connected to the inner control grid I3 and ground or cathode and coupled with the accelerating grid [4 also known as an anode grid-through a feedback coil l8 and blocking condenser- !9. A condenser-shunted resistance 20 is inserted in the common cathode return lead for the control grid and screen grid circuits to provide proper grid operating bias potential in a manner well understood. The screen grid i4 is connected to the positive pole of a suitable high tension or space current supply source indicated by the plus symbol in series with a load resistance 2 l The circuit connected to the cathode I2 and grids l3 and I4 constitutes a standard regenerative feedback oscillator capable of generating an oscillating frequency determined by the resonant frequency of the tank circuit l1. Any other oscillator circuit known in the art and operatively associated with the cathode l2 and grids l3 and I4 may be employed for the purpose of this invention, as willbe readilyunderstood. In order to stabilize the oscillating frequency, I have shown a piezo-electric crystal 22 connected between the outer control grid !5 and ground and shunted by a high impedance element. pervious to direct current such as a choke coil or high ohmic resistance 23 as shown in thedrawing. The plate I6 is connected to the positive pole of the space current supply source and by-passed to ground for high frequency current by a condenser 24. The oscillations generated are applied to a suitable output or utilization circuit. connected to terminals 0-0 by way of a coupling condenser 25 or in any other manner known and well unfderstood.

It will be noted that the crystal 22 is connected in a circuit remote or substantially decoupled from the oscillator circuit proper and accordingly will not be subject to the drawbacks'and defects of known oscillator circuits, wherein the crystal forms an effective element or part of the oscillator circuit. The function of the frequency stabilization in a circuit of the type shown in Figure 1 will be further understood from the following:

Electrons passing from the cathode l2 to the screen or accelerating grid l4 and subjected to a fluctuation in accordance with the oscillating frequency, will in part pass through the meshes of the grid l4 and be attracted and collected by the anode or plate It. In passing from the screen [4 to the plate [6, the electrons will be decelerated due to the efiect of the negative electricfield surrounding the outer control grid l5 being at ground potential or having a potential negative with respect to ground due to the action of the biasing resistance 29.

As a result of this decelerating action, a dense electron cloud or concentrated space charge also mal plate current Io. The average or steady current passing to the screen grid [4 undergoes a similar change as shown by the dotted line curve isg, that is, with the phase or sense of the variknown as a virtual cathode will be formed adjacent to the grid l5, said space charge having a charge density which varies in accordance with the oscillating frequency determined by the resonant frequency of the tank circuit l1. As a result of this variable space charge, an oscillating current will be induced by electrostatic induction in the outer circuit connected to the grid is and cathode l2 and including the crystal 22. The phase of the induced current depends on the character of the impedence offered by the crystal and accordingly, will be dependent upon the relative frequency departure between the oscillating frequency and the fixed resonating frequency of the crystal.

If, under normal conditions, the oscillating frequency or resonant frequency of the tank circult l1 corresponds to the resonant frequency of the crystal T2, the induced current passing through the crystal will be exactly 90 out of phase with the oscillating frequency due to the fact that for this frequency the crystal offers pure resistive impedance and that the induced current varies in accordance with the rate of change or derivative of the space charge fluctuations which latter are in phase with the oscillating frequency. If the oscillating frequency deviates in either sense from the resonating frequency of the crystal 22, the impedance offered by the crystal to the induced current will become either capacitative or inductive, resulting in a phase shift of the potential of grid l5 being greater or smaller, as the case may be, than the normal 90 or quadrature phase relation. Accordingly, the potential upon the grid l5 caused by the drop of the induced current through the crystal will have a phase with respect to the phase of the potential on the grid l3 determined by the oscil ations, which varies both in sense and magnitude depending on the departure of the. oscillating frequency from the resonating frequency of the crystal 22.

As a result of the dual control of the electron stream passing to the plate It by the potentials on the grids l3 and of varying relative phase, the average or steady plate or output current varies in a manner shown bythe curve is. in Figure 2 as a function of the frequency f of the oscillations, i. e. in turn the resonant frequency of the tank circuit H. In case of equality between the resonanting frequencies of the tank circuit I1 and the crystal 22 the average plate current has a value Io equal to the plate current if one of the grids 13 or l5 were omitted or disconnected, i. e. determined solely by the steady operating, and bias potentials of the tube. This condition corresponds to a normal operating frequency identified as is in Figure 2. If the osciloperating potential of the screen grid 14.

ations reversed with respect to those of the anode current ia.

According to the present invention, the average plate or screen current changes as a function of an initial deviation of the oscillating frequency from its desired or normal value f0 determined by the resonating frequency of the crystal 22, are utilized to effect a frequency control to counteract the initial frequency deviation in a manner to restore the original or normal oscillating frequency. In the embodiment of the invention shown in Figure 1, there is inserted for this purpose in the screen grid circuit a load impedance in the form of an ohmic resistance 2| of suitable resistance value to produce a potential drop by the screen grid current isg causing a variation of the steady screen grid potential. The latter reacts upon the tank circuit 11 to vary the reflected reactance thereof in such a manner as to maintain a constant oscillating frequency independent of drift and other causes tending to vary the resonating frequency of the tank circuit 11. This function will be further understood from the following.

It is well'known in vacuum tube circuits of this type that a reactance, usually a capacitative reactance, is reflected from the plate to the grid due to inter-electrode coupling (Miller effect), which reflected reactance is proportional to the output potential i. e. in the present case to the Since the latter varies in accordance with the deviations of the oscillating frequency from the predetermined resonating frequency of the crystal 22, the variations of the reflected reactance, by the proper design of the circuit constants and parameters, will be such as to compensate for or counteract an initial frequency variation in a manner to restore and maintain a substantially constant oscillating frequency.

The reactance reflected into the grid or tank circuit I! usually of a capacitative nature, in some cases may be of an inductive type and may vary in either sense in proportion to the potential on the screen grid I4. In order to obtain the proper phase of the reflected reactance variations to counteract the initial changes of the oscillating frequency, various methods may be utilized depending on the particular oscillating circuit employed. Thus, in Figure 4 there is shown a circuit wherein the operating potential for the screen grid l4 serving as output electrode for the. oscillator is derived from the plate current is, having a phase opposite to the screen current changes as shown in Figure 2. Alternatively, the crystal 22 may be utilized at either its parallel-resonant or its series-resonant frequency, in which case again the plate and screen current changes will be reversed. Figure 2 corresponds to the operation of the crystal at its parallel-resonant frequency, while Figure. 3 shows the plate and screen currents if the crystal 2'2 is employed at its series-resonance. In the latter case,.as is seen, the phase of the plate and ing either screen grid or plate current changes for producing the operating potential for the screen l4, it will be possible to provide a screen grid potential varying in the proper sense as a result of an initial deviation of the oscillating frequency from its assigned value, whereby to counteract said deviation and to restore and maintain a constant oscillating frequency.

Referring to Figure 4, there is shown a circuit similar to Figure l with a somewhat different oscillator circuit connected to thev cathode l2 and grids l3 and 14. This oscillator differs from Figure l in that the oscillating or tank circuit I1 is connected to the screen [4 and the feedback coil I8 is connected to the control grid I3 in connection with a coupling condenser 21 and grid leak resistance 28. The plate circuit in this case includes a load impedance 30 to produce a plate potential varying in accordance with the relative frequency departure of the oscillating frequency from the assigned frequency of the crystal 22 as shown by the curve is. in Figures 2 and 3, depending on whether the crystal is utilized at its parallel-resonant or at its series-resonant frequency, respectively. The varying plate po tential serves for energizing the screen grid or output electrode l4 of the oscillator; whereby to control the reflected reactance appearing in the grid circuit and affecting the resonant frequency of the tank circuit I! by virtue of the inductive coupling between the latter and the grid circuit.

Referring to Figure 5 I have shown a center frequency stabilized oscillator similar to Figure 1 associated with a separate reactance control tube for controlling the oscillating frequency in accordance with the variations of a signal source such as a microphone or the like to obtain a frequency modulated oscillation having a substantially constant center or carrier frequency and an instantaneous frequency varying in proportion to the signal current variations.

There is connected for this purpose in parallel to the oscillator tank circuit I! a reactance control tube 35 comprising a heater 36, a cathode 31, a control grid 38, a screen grid 4! a suppressor grid 4| and an anode 42. The control grid 38 is excited by a quadrature potential derived from the oscillating circuit by way of a phase shift network comprising a resistance 44 and a condenser 45 and connected across the tank circuit through a large blocking condenser 43. The screen grid 40and anode 42 of the control tube are connected to a suitable space current supply source by way of potential drop resistors 46 and 47, respectively. The control grid 38 is furthermore excited by a signal potential by way of an audio transformer 50 having a pri mary connected to a battery 5| and a microphone or other signal device 52.

The function of the control tube 35 is well known. By varying the control potential in accordance with the signals a variable reactance represented by the cathode-anode path of the tube is connected across the tank circuit ll, resulting in a corresponding variation or modulation of the oscillating frequency. In order to prevent potential variations of the anode [6 in accordance with the signal frequencies from being impressed .upon the grid M, a choke coil 3| is inserted between the grid l4 and the load resistance 30 in the anode circuit of the oscillator. In this manner, only slow variations of the oscillating frequency due to drift and other causes will be impressed upon the screen grid I4 and neutralized by the compensating or frequency control action 'in the manner described.

-If desirable, the'choke coil 3| or an equivalent linearity of the modulation, 'reducingnoise and obtaining other advantages of inverse feedback well known.

From the foregoing and the diagrams shown in Figures 2 and 3, it is found, in the case of the standard triode oscillator associated with the electrodes I2, l3 and I4 wherein the reflected reactance in the grid circuit is of a capacitative nature in accordance with the well-known Miller effect, that in order to counteract an initial o'scillator frequency deviation from the crystal frequency, the crystal should be operated at series resonance when utilizing the screen grid current to produce the frequency responsive oscillator anode voltage upon grid 43 such as shown in Figure 1, and that the crystal should be operated at parallelresonance when utilizing the plate cur counteract this tendency by reducing the oscillating frequency. This in turn necessitates an increase of the oscillator anode potential upon grid l4 or, in other words, a decrease of the anode and screen current, whichever is used for producing the frequency-responsive oscillator anode potential. 1 7

Hence, if the crystal is operated at its parallel-resonant frequency, corresponding to Figure 2, it will be the anode current in which'decreases with increasing oscillating frequency f, while if the crystal is operated-at its series-resonant frequency, corresponding to Figure 3, it will be the screen grid current in which decreases with increasing oscillating frequency.

Accordingly, in the case of Figure 1 where the varying screen'current is used to control the oscillator anode potential, the crystal 22 should be operated at its series-resonant frequency, while in the case of Figure 5 where the varying anode current is used to control the oscillator anode potential, the crystal should be operated'at its parallel-resonant frequency in order to counteract the initial oscillator frequency deviations and to stabilize the oscillator.

Since both paralleland. series-resonant fre quencies of a crystal are closely adjacent to each other, it may be easily determined, by experiment and from the relationship shown in Figures 2 and 3, which operating mode should be chosen for a given oscillator circuit and a given relation between the reflected reactance and the anode disclosed herein for illustration, but that the underlying thought and principle are susceptible of numerous modifications and variations coming within the broader scope and spirit of the in-- vention asdefined inthe appended claims. The specification and drawing arev accordingly to be regarded in an illustrative rather thanin a limiting sense.

I claim:

1. An oscillator comprising, an electronv discharge tube having a cathode, a first control grid, a screen grid, a second control grid and an anode, all arranged. substantially in the order named, a regenerative oscillating circuit comprising a resonant tank circuit connected. between said first grid and said cathode and feedback coupling means therebetween and said screen grid to generate sustained electrical oscillations having a frequency determined by the resonant frequency of said tank circuit, a piezo-electric crystal shunted by ahighimpedance pervious to direct current. and connected between said second control grid and said cathode, said second. control grid having a steady bias. to produce a concentrated electron space charge adjacent thereto and fluctuating at oscillating frequency, whereby to cause the steady electron current to vary in sense and magnitude in proportion to relative frequency departure between the generated oscillotions and the resonant frequency of said crystal, a load resistance connected to said anode, means for applying varying operating'potential to said screen grid from said load resistance to thereby control, the reactance reflectedupon said tank circuit and to vary the oscillating frequency to counteract initial deviations thereof from the crystal frequency, means to modulate the frequency of the generated, oscillations in accordance with a modulating signal amplitude, and means inthe screen grid circuit to prevent anode current variations in accordance with the modulating signal frequency from aifecting the steady potential of said screen grid.

2. An oscillator comprising an electron discharge tube having a cathode, a first control grid, a screen grid, a second control grid and an anode, all arranged substantially in the order named, a regenerative oscillating system comprising a resonant. tank circuit connected between said first control grid andv said cathode and feedback means therebetween and said screen grid to generate sustained oscillations having a frequency determined by the resonant frequency of said tank circuit, a piezo-electriccrystal shunted by a high impedance pervious to direct current and connected between said second control grid and said cathode, a load resistance connected to said anode, a conductive circuit connection from said anode to said screen grid for applying varying operating potential thereto, an output circuit connected to said tank circuit, further means. for controlling the frequency of the generated. oscillations in, accordance with a modulating signal, and means in the screen grid circuit to substantially prevent anode current variations in accordance with said modulating signal from aifecting the screen grid potential.

. 3. An oscillator comprising an electron discharge tube having a cathode, a first control grid, a, screen grid, a second control grid and an anode, all arranged substantially in the order named, a regenerative oscillating system comprising a resonant tank circuit connected between said first control grid and said cathode and feedback means therebetween and said screen grid to generate sustained oscillations having a frequency determined by the resonant frequency of said tank circuit, a piezo-electric crystal shunted by a high ohmic resistance and connected between said second control grid and said cathode, a load resistance connected to said anode, and means for applying varying operating potential to said screen grid from said load resistance, an output circuit connected to said tank circuit, further means for controlling the frequency of the generated oscillations in accordance with a modulating signal, and audio frequency choke means inserted in the screen grid circuit to substantially prevent anode current variations in accordance with said modulating signal from affecting the screen grid potential.

4. An oscillator comprising an electron discharge tube having a cathode, a first control grid,

a screen grid, a second control grid, and an anode, all arranged substantialiy in the order named, a regenerative oscillating system comprising a resonant tank circuit and connected between said first control grid and said cathode, and feedback coupling means therebetween and said screen grid to generate sustained electrical oscillations having a frequency determined by the resonant frequency of said tank circuit, a piezo-electric crystal shunted by a high impedance pervious to direct current and connected between said second control grid and said cathode, a load resistance connected to said anode, means for applying varying operating potential to said screen grid from said load resistance, further means for controlling the frequency of the generated oscillations in accordance with a modulating signal, an output circuit connected to said tank circuit, and means in the screen grid circuit to prevent anode current variations in accordance with said modulating signal from affecting the screen grid potential.

KARL RA'IH.

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

UNITED STATES PATENTS France NOV. 28, 1938

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