US2490007A - Frequency controllable magnetron system - Google Patents

Description

Nov. 29, 1949 P. H. PETERS, JR

FREQUENCY GONTROLLABLE MAGNE'I'RON sys'rma Filed March 15, 1947 LIMITER D.c AMPLITUDE M'XER DISCRIMINATOR AMPLIFIER MODULATOR I I. l f I 20 2| Z3 %,,5 %,|6 1: l:

1 LQCAL ONT LL OSCILLATOR M'XER ggggiflgg LIHITER OISCRIMINA ads TIME DELAY 29 LOCAL L OSCILLATOR FREQUENCY MODULATOR v 5' Inventor:

Philip HPeters Jn,

by His Attorney.

Patented Nov. 29, 1949;

FREQUENCY CONTBOLLABLE MAGNETRON SYSTEM Philip H. Peters. Jr., Schenectady, N. Y., asslgnor to General Electric Company, a corporation of New York Application March 15, 1947, Serial No. 734,920

' .0 Claims. (61.332-) This invention relates to high frequency systems and more particularly to a novel method and apparatus, especially useful with magnetron type oscillators for the generation of frequency modulated oscillations or for the generation of electromagnetic oscillations of stabilized frequency.

As is well known in the communications and related arts, it is customary to transmit intelligence by modulating a high frequency electromagnetic carrier wave capable of being pro- Jected into space with a decipherable signal component which is an accurate and faithful representation of the intelligence to be transmitted. Such modulation may be accomplished by causing any one of the parameters of the electromagnetic wave (1. e., amplitude, frequency or phase) to vary in continuous correspondence with the desired intelligence while the others are held constant, the resultant variations constituting the mentioned signal component. If, for any of numerous possible reasons, distorting factors are introduced by the apparatus employed, the modulated wave will be correspondingly distorted with the result that the desired high fidelity may not be obtained and consequently the intelligibility of the transmission may be impaired. If the intelligence is speech, music or television signals the degree of impairment may be such as to render the transmission completely useless in'practice.

For accurate and faithful transmission by the aforesaid methods, and for numerous other applications, it is highly desirable to have available high or ultra high frequency apparatus which is capable of generating oscillations of highly stable frequency or oscillations the frequency of which may be controllable varied at very high rates. With such apparatus, signal distortion caused by undesired and uncontrollable frequency fluctuations may be minimized. Moreover, in order to meet the heretofore unsatisfied demand of the art for intelligence broadcasting facilities capable of operating both at ultra high carrier frequenciesie. g. above about 500 megacycles) and at power levels sufficiently high to afford practically detectable signals at substantial distances, it is alsohighly desirable that such apparatus also be capable of developing the highest possible power output.

The magnetron type of discharge device is presently the only available type capable of generating at both satisfactorily high power output levels and ultra high frequencies. However, its usefulness as a source of oscillations in such apparatus as the foregoing has heretofore been limited by certain inherent magnetron characteristics which render it difficult to produce therein distortion free modulation. The reason therefor may be attributed largely to an inherent frequency instability characteristic of most ma netrons. As is known, such instability may manifest itself as sudden random changes of the oscillation frequency throughout a frequency spectrum much wider (e. g. 500 kilocycles) and more discontinuous than that experienced with other types of oscillators. Generally, the results thereof have been that where either amplitude er frequency modulation has been attempted heretofore with magnetrons, a large amount of undesirable and uncontrollable frequency fluctuations has been found to be present as a signal distorting factor. Because of the wider range of frequency fluctuations, and to some extent because ofthe discontinuous nature thereof, it has not heretofore been possible to employ satisfactorily the frequency modulation and frequency control methods heretofore employed with other types of frequency modulated generators.

Accordingly, it is the general object of this invention to provide a system capable of transmitting distortion free intelligence by means of modulated electromagnetic waves of ultra high frequency and practically useful high power level.

It is a further object of this invention to provide a method and apparatus particularly although not exclusively, useful with magnetrons for the generation of oscillations either of readily controllable and rapidly variable frequency or of highly stabilized frequency.

. It is a still further object of the invention to provide a method and apparatus which may be used to effect a desired amount of frequency or amplitude modulation of a magnetron oscillator with a minimum amount of distortion attributable to random frequency fluctuations and with simlating the standard frequency in accordance with desired signals introduced into the second circuit, it thereby becomes possible to frequency modulate the magnetron oscillator in accordance with the desired signals. By virtue of a relative independence of the first and second mentioned circuits, desirable frequency changes attributable to the frequency modulation are unaffected by the corrective action of the first circuit while the undesirable frequency changes attributable to random frequency fluctuations of the magnetron oscillator itself are so affected and their effect thereby eliminated. The aforementioned first circuit may advantageously employ a novel magnetron tube structure (such as that described and broadly claimed in copending application Ser. No. 719,704, filed January 2, 194 7, by Philip H. Peters, Jr., and John P. Blewett and assigned to the same assignee as the present invention) in which two or more magnetron electrode configurations are coupled together in such fashion that one behaves as a conventional oscillator generating high frequency electromagnetic waves while the other behaves as a controllable variable electronic impedance reacting with the oscillator to produce controllable changes in oscillation frequency.

The features of the invention desired to be protected are pointed out in the appended claims. The invention itself, together with its further objects and advantages, may best be understood by reference to the following description and to the appended drawing which represents schematically a circuit illustrating the principles of the invention.

Referring now to the drawing there is shown a magnetron oscillatior l which may, for example, comprise an electron discharge device of the magnetron type such as that described and claimed in the aforementioned application of Philip H. Peters, Jr., and John P. Blewett. As shown, the oscillator includes a plurality of anodes or electrodes 2, 3 and 4 arranged to excite and sustain oscillations in a resonant circuit such as the parallel wire transmission line 5 comprising parallel wires 6 and and which is preferably tunable by suitable means such as the short circuiting conductor 8 adjustablypositionable lon gitudinally of the transmission line by the rod 9. As described in the aforementioned application, the electrodes 2, 3 and 4 have opposed curved surfaces defining generally cylindrical electron discharge chamber Ill in which a rotating electron space charge of the magnetron type may be developed concentrically with the cathode II. The latter electrodes together with the associated cathode H may be viewed as providing an oscillator section, that is, the section which establishes and sustains oscillations in the resonant circuit comprising transmission line 5. For the purpose of controlling the frequency of such oscillations in supplement to the tuning effect of the conductor 8, there may be provided a variable reactance section formed about a second generally cylindrical discharge chamber l2 in which a rotating electron space charge of the magnetron type may be arranged to rotate about the second cathode l3. As described in the aforementioned application, chamber l2 is formed by opposed semlcylindrlcal surfaces of electrodes 2 and 3. The electrode 4 of the oscillator section may be connected directly to the midpoint of the U-shaped portion l4 terminating the one end ofv the transmission line 5. It will be understood from the aforementioned application that when suitable energizing potentials, such as those supplied by the direct current sources comprising batteries l5 and I8, are applied between the respective cathodes and anodes oscillations may be generated and sustained in the oscillator, and the frequency thereof may be controlled by controlling voltage between cathode l3 and electrodes 2 and 3 of the reactance section. For the purpose of deriving useful energy from the oscillator and energizing suitable utilization therewith, any suitable load connection to the transmission line 5 may be provided, for example the means indicated schematically as the resistor I! which may be an antenna coupling cable connected across the line at a suitable point.

For the purpose of controlling the frequency in the oscillator and to minimize the effect of inherent random frequency fluctuations therein, there may be provided what may be termed a feed back loop circuit comprising any suitable energy pickup device such as a coaxial line having inner conductor l8 and outer conductor IS, a mixer 20, the limiter-discriminator 2| and the unidirectional potential amplifier 22, all of the latter elements being of the types well known in the art. The function of the mixer 20 is to combine a voltage component of frequency component f0 derived inductively from the oscillator I through the coaxial line l8l9 with a voltage component of standard frequency f1 derived from the local oscillator 25 hereinafter to be described. From the mixer 20 there is obtained thereby a voltage component of frequency equal to the difference in frequency between the two frequencies f1 and in. That is to say, it produces a component of differential or beat frequency delta f equal to fo-fi. This voltage component of frequency delta f is applied to the limiter-discriminator 2| which has the function of providing at tial frequency component and, secondly, the differential frequency component, now of fixed amplitude, is so rectified as to provide a unidirectional voltage directly proportional to the differential frequency delta f. By the two steps there is provided at the output of the limiter-discriminator 2| a voltage which is solely a measure of the frequency difference delta f.

The output of the limiter-discriminator 2i may next be applied to the direct current amplifier 22, if amplification is desirable or necessary and, in turn, the amplified voltage may be applied to the anode-cathode circuit of the reactance section of the magnetron oscillator in such manner as to control the frequency thereof in accordance with the value of delta 1'. As explained in the aforementioned application, changes in the voltage between the anodes 2 and 3 and the cathode l3 will efiect certain changes in the reactance properties of the device and thereby change the frequency of the oscillator. As shown, the correction voltage derived from the limiter-discriminator and amplifier is connected in series with the principal energizing voltage of the battery l5.

It will be understood by those skilled in the art that the circuit as thus far described may be made to operate to stabilize the frequency of oscillation of oscillator I against any random fluctuations in the following manner: The circuit may be adjusted to be balanced at a desired mean frequency of the oscillator by appropriate choice of the circuit parameters. Generally, the frequency {1 of local oscillator 25 will be so chosen with respect to the desired mean frequency In of oscillator I as to provide a conveniently handled predetermined intermediate frequency delta 1' (for example, delta I equal to 30 megacycles) when the oscillator I is oscillating at the desired mean frequency, and the limiter-discriminator will be so designed that it develops zero output voltage at delta f equal to the predetermined intermediate frequency (30 megacycles). Undesired frequency deviations, random or otherwise, of oscillator I will then vary delta 1 in such manner as to cause the limiter-discriminator to develop a unidirectional output voltage proportional to the deviation and of polarity determined by the direction of the I deviation. Assuming the frequency 11 of the local oscillator 25 to be constant, then it will be apparent that any changes in the frequency Io of the magnetron oscillator I which result in a delta f different from the balanced value will give rise to a discriminator output voltage which may be applied to the reactance section between the cathode l3 and the anodes 2 and 3 in such manner as to change the reactance thereof and thereby introduce a reactive component tending to correct or compensate for the frequency fluctuations which caused the unbalance. That is to say, the oscillator is returned to its desired mean frequency by the change in reactance. Y In principle, so much of the system thus far described is functionally analogous to similar techniques already known in the art of frequency modulated transmitters wherein such techniques are employed for center frequency stabilization i. e. stabilization against undesirable tendencies of the mean oscillator frequency to deviate or drift slightly at relatively low time rates. It differs however, in one important respect, namely in that the circuit in the present case must be designed to accommodate the relatively much larger undesirable frequency changes and relatively much higher time rates of such changes characteristic of the magnetron, both of which represent distortion components having frequencies within the same frequency range as that of -the desired frequency changes due to the modulation signal of the intelligence 1. e. the frequency changes due to random fluctuation are within. the portions of the audio or video spectrum useful for modulation purposes. Because both -the desired and undesired frequency changes are in the same range, the frequency corrective action of the system thus far described can not be made selective as between desired and undesired frequency changes. In contrast, in the analogous prior art systems, the undesirable frequency changes represent distortion components having frequencies outside the range of the desired frequency changes due to the modulation signal i. e. the frequency changes due to random fluctua- -.tion or drift are below the audio or video spec- .trum useful for modulation purposes. Thus the frequency corrective action of the prior art techniques can be made selective as between desired and undesired frequency changes. An example of the latter selective action will incidentally be illustrated hereinafter in connection with the circuit for modulating the frequency of local oscillator 28.

It is because of the necessary difference in de-' sign that it becomes impossible to employ the circuit as thus far described both as the frequency stabilizer for the magnetron oscillator and a circuit for producing or introducing a frequency modulation signal. If one attempted to introduce a frequency modulation signal directly within this circuit, for example, by applying it directly to the reactance section of the oscillator (or as is done with analogous prior art control circuits designed for selective action between the modulation signal and undesired frequency change due to slow and small frequency drifts), it would be foundthat the frequency changes due to the frequency modulation signal were affected in the same manner as the frequency changes due to the random frequency shift, i. e. they too would be attenuated by the compensating action of the circult. Such a situation does not obtain in the conventional type of frequency modulated transmitter because only the frequency changes attributable to random shift or drift in frequency are so attenuated. In order to be effective against changes due to drift, the described circuit must therefore have wide band characteristics and thereby the insertion of modulation directly within the circuit in the conventional manner becomes impractical since it would be attenuated equally with the noise due to frequencyshift.

In order to eliminate random frequency fluctuations without attenuating the modulation signal, the modulation function may in accordance with the essence of this invention, be performed outside of the circuit thus far described, for example, by the circuit designated generally as 23 in the case of frequency modulation and by the modulator 24 in the case of amplitude modulation. It will be noted that the circuit 23 includes a local oscillator 25 against which the frequency of the magnetron oscillator i is to be compared by the mixer-limiter-discriminator circuit already described, and suitable control elements for controlling and modulating the frequency of. oscillator 25.

Since its function is solely to supply frequency modulatable or frequency stabilized reference oscillations for the magnetron oscillator, the entire circuit 23 may be either a conventional frequency modulation circuit or a conventional constant frequency oscillator of the types well known in the art depending on whether it is desired to frequency or amplitude modulate the magnetron oscillator. For example, in order to control the frequency of the local oscillator 25 and to modulate its frequency in accordance with a desired signal, the circuit 23 may include a reference oscillator 26, a mixer 21, a local limiter discriminator 28, a time delay circuit 29 and a local oscillator frequency modulator 3i) driven by any source of intelligence signals such as microphone 3. All of these including the local oscillator 25 itself may be of the forms well known in the art. For example, oscillator 26 may be a crystal controlled oscillator of highly constant frequency. Mixer 2! may be similar to mixer 20 and will serve to derive a differential frequency constituting the difference between the center frequency of oscillators 25 and 26. Limiter-discriminator 28 will function to derive from the mixer 21 a unidirectionalvoltage which is solely a measure of the latter differential frequency and which may be used to stabilize the center frequency of oscillator 25. Local oscillator modulator 30 may include a reactance tube circuit which controls the frequency of oscillator 25 in response to the unidirectional voltage output of discriminator 28 and also in response to frequency modulation signals from microphone 3|. It will be understood of course, that the time delay circuit 29 may be designed in numerous ways so that it functions to attenuate rapid frequency changes while leaving slow frequency changes unaffected. Thereby the small and slow frequency changes below the lowest modulation frequency (e. g. 40 cycles) and due to undesirable drift away from the center frequency are corrected by the mixerlimiter-discriminator 21-28 while those due to desired frequency modulation signals are not so corrected and the circuit becomes selective in the manner heretofore indicated.

It will be apparent that in operation the mixerlimiter-discriminator -2I will tend always to stabilize the frequency of the magnetron oscillator I at a fixed frequency from that of local oscillator 25, i. e. at a frequency differing from that of oscillator 25 by the intermediate frequency output of mixer 20 (e. g. megacycles) Therefore, if oscillator 25 be frequency modulated, the magnetron oscillator I will likewise be modulated. But because the frequency stabilization feedback circuit 20-2 I-22 for the magnetron has no eitect on the frequency modulation signal, it can be designed to give the necessary wide band control of the magnetron center frequency without interfering with the intelligence.

It may also be noted that frequency drifts caused by conditions within the feedback circuit 20-2I-22, such as drift of the quiescent level of amplifier 22 caused by supply voltage variations, are also corrected. Moreover, nonlinearity of the magnetron reactance section and of the limiter discriminator 2| becomes of little importance.

By way of more detailed analysis of the operating conditions of the circuit, it is pointed out that the extent of the possible frequency swing or deviation of the magnetron oscillator is confined to the band pass of the limiter discriminator 2| times the frequency gain of the feedback loop circuit comprising the magnetron and reactance section, the mixer 20, the limiter discriminator 2| and the amplifier 22. This gain in cycles of correction produced for each cycle of deviation may be viewed as the product of the discriminator gain in volts per cycle times the gain of the amplifier 22, t mes the gain of the reactance sec-' tion of the magnetron in cycles per volt. The

percentage correction for any disturbance of volt-- age or frequency within the loop circuit can be shown to be given by the expression:

Percentage correction m X 100 delta f G delta rm- G 1 Thus, attenuat on of the noise in the loop and not the modulation outside of it greatly improves the signal-to-noise ratio of the system. This improvement results in noise reduction of the noise ratio by the factor l/G. It may be emphasized that by utilizing a low power conventional frequency modulated local oscillator, such as a low power triode, for the generation of the frequency modulated signal components and thereafter employing that local oscillator to control a magnetron, one is able to take advantage of the higher power handling possibilities of the magnetron and yet utilize the high fidelity modulation characteristics of other types of tubes. It will be recognized that at low power levels to which conventional oscillator tubes are limited, high fidelity frequency modulation can readily be obtained with a local oscillator 25 without the serious frequency stability and distortion problems to raise the power level of the frequency modulated electromagnetic waves thus created to a power level which is more practically useful in the transmission of intelligence. one may employ the present invention and thereby also render the magnetron useful for a purpose for which it was not heretofore practicable, namely, for conventional intelligence transmission. With this system I have been able to raise the signal to noise ratio in magnetron oscillators to levels comparable with that of conventional triode oscillators. Such performance has hitherto not been obtainable with magnetrons.

It will be readily apparent that the circuit thus described is useful not only for frequency modulation transmission but also for amplitude modulation. If it be desired to employ the system for amplitude modulation it will suffice that the local oscillator 25 be maintained at a fixed frequency rather than frequency modulated, that is to say it is employed simply as a reference oscillator of highly constant frequency; In that case, the center frequency of the magnetron oscillator I will be caused to be stabilized near the frequency of the local oscillator in the manner discussed. Amplitude modulation may then be eifected by varying the amplitude of the output of the magnetron oscillator l by suitable manipulation of the electrode potentials. For example, I have indicated a possible means for effecting amplitude modulation as comprising a signal source such as modulator 24 which varies the voltage on the electrodes of the oscillator section comprising the electrodes 2, 3 and 4 and the cathode II in accordance with the desired signal, the modulator 24 being inserted in series with the battery It in the manner 1 indicated. It will be understood that any source of intelligence such as a microphone 32 may be connected to the modulator 24 for the purpose. It will also be recognized by those skilled in the art that the feedback circuit will also serve to correct or compensate for frequency changes ordinarily caused by changes in the electrode voltage necessary to the amplitude modulation. Heretofore attempts to amplitude modulate magnetrons by varying the electrode voltages in this manner have been unsatisfactory for the reason that suchvoltage variations also introduced a certain amount of frequency modulation which further added to the distortion due to random" frequency shift.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects and I, therefore, aim in the appended claims to cover reference oscillations of precisely controllable frequency, means for maintaining the frequency of said oscillator at a fixed relation with respect to the frequency of said source comprising means for deriving a voltage proportional to the difference in frequency between the frequency of said oscillator and the frequency of said source and for applying said voltage to said first mentioned means to effect an adjustment of the frequency of said oscillator to maintain said fixed relation, and means for modulating the frequency of said source whereby the frequency of said oscillator is likewise modulated.

2. A system as in claim 1 in which said oscillator comprises a magnetron type discharge device having a resonant circuit and spaced electrodes for sustaining said electromagnetic oscillations therein and said first mentioned means comprises electrode means operatively connected to said spaced electrodes and constituting a variable electronic impedance effective to adjust the resonant frequency of said resonant circuit.

3. A system for generating electromagnetic oscillations and modulating the frequency thereof comprising an oscillator including means effective to adjust the frequency thereof, a source of reference oscillations of precisely controllable frequency, means for maintaining the frequency of said oscillator at a fixed relation with respect to the frequency of said source comprising means for deriving an electrical voltage of differential frequency equal to the difference between the frequency of said oscillator and the frequency of said source, means for deriving a unidirectional voltage proportional to said diflerential frequency and means for applying said unidirectional voltage to said first mentioned means to effect an adjustment of the frequency of said oscillator to maintain said fixed relation, and means for modulating the frequency of said source whereby the frequency of said oscillator is likewise modulated.

4. A system as in claim 3 in which said oscillator comprises a magnetron type discharge device having a resonant circuit and spaced electrodes for sustaining said electromagnetic oscillations therein and said first mentioned means comprises electrode means operatively connected to said spaced electrodes and constituting a variable electronic impedance effective to adjust the resonant frequency of said resonant circuit.

5. A system for generating electromagnetic oscillations and modulating the frequency thereof in accordance with a predetermined signal comprising an oscillator including means effective to adjust the frequency thereof, a source of reference oscillations of precisely controllable fre quency, means for maintaining the frequency of said oscillator at a fixed relation with respect to the frequency of said source comprising means for deriving an electrical voltage of differential frequency equal to the difference between the frequency of said oscillator and the frequency of said source, means for deriving a unidirectional voltage proportional to said differential frequency and means for applying said unidirectional voltage to said first mentioned means to effect an adjustment of the frequency of said oscillator to maintain said fixed relation, and means for modu- REFERENCES orrEn The following references are of record in thefile of this patent:'

UNITED STATES PATENTS 0 Number Name Date 2,113,225 Wolff Apr. 5,1938 2,241,976 Blewett et al. May 13, 1944 2,296,962 Tunick Sept. 29, 1942 2,337,214 Tunick Dec. 21. 1943 ll 2,406,125 Ziegler et al. Aug. 20, 1948

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