US2452912A - Circuit for improving oscillator stability - Google Patents

Description

I Filed June 10, 1942 O. E. DOW

C IRGUIT F'OR IMPROVING QSGILLA TOR STABILITY 4 Sheets-Sheet 1 ANTENNA INVENTOR 0/3/4445 5. Dow.

ATTORNEY N0V.2,194& 6, 5 Dow v4.2,,452ga12f CIRCUIT FOR IMPROVING OSCILLATOR STABILITY Filed June 10. 1942' 4 sneaks-sheet 2 INVENTOR UAV/ZLE ffiam ATTORN EY Nov. 2, 1948. ,o. E. DOW

CIRCUIT FOR IMPROVING OSCILLATOR STABILITY Filed June 10, 1942 4 Sheets-Sheet} 3 Nov. 2, 1948 o E.- DOW CIRCUIT FQR IMPROVING DSCILIQATOR STABILITY Filed June 10. 1942 4 Sheets-Sheet 4 1C. 5L orrEo R. w 0. M. m 2 W R 3 U3 2. Q. s m 46 w w j 1 M M MD 1 e m T U. 5 E W 7 g 4 M3 WU L Rm fi N/ 0 M v F 85 ..M n 0 E 3 7 MM/ mm m. .MN

Fdmwo v INVENTOR ORV/ALE 5 Dow.

ATTORNEY l 'atented Nov. 2 1948 CIRCUIT FOR IMPROVING OSCILLATOR STABILITY Orville E. Dow, Port Jefferson, N. Y., ass'ignor .to Radio Corporation :of America, :a corporation of Delaware Application 'June 10, 1942 Serial ,No. 46,457

:13Ciaims. (Cl. 250-136) My present invention relates to ultra high fre- H quency circuits. One generalobject of my invention is to provide improved methods and means for controlling the flow of ultrahigh frequency waves.

In certain systems utilizing ultra high frequency waves it becomes necessary to turn off an-ultra short wave generator and subsequently turn it on as, for example, in a duplex or half duplex system where the transmitter is turned off during the receiving period. In such cases, if control of the waves is accomplished by switching on and off operating voltages applied to the electron discharge device generator, large frequency drifts of the carrier occur when the transmitter is returned to the air after an interval of inactivity. This change in frequency becomes exceedingly large and objectionable when operating at frequencies above 500 megacycles and the condition is aggravated at frequencies above 1000 megacycles.

I have found that this frequency shift or drift is due in large measure to the change in heating of the low loss frequency determining circuits-employed in the generator as it is turned on and 1 off. This change in frequency, therefore, could be substantially reduced by building low -or-zero temperature coefficient frequency determining circuits, but in most instances the cost of such circuits is not justified.

In accordance with my present invention, this difilculty is circumvented by utilizing the usual low loss resonator or tank circuit for frequency control and maintaining the power dissipated therein substantially constant inboth the on and off conditions of the oscillation generator.

In'this way, the temperature of the frequency controlling element is maintained substantially constant and, accordingly, the frequency of oscillations generated after a period of inactivity is substantially the same as the frequency of oscillations generated prior to the time the transmitter was turned ofi'..

Further in accordance with my invention, I turn off the high frequency oscillator by detuning the tank circuit. For this purpose I make use of a section of transmission line for loading and unloading the tank circuit, as will b'e-exnlained more fully hereinafter, whereby the tank circuit may easily be returned to its original frequency setting.

It will be noted that the detuning control described herein is especially useful :for frequencies above 1000 megacycles wherethe oscillator efficiency is generally low. Hence in these .cases the percent increase .in power. dissipated :inzthe oscilnot tobe construed as limiting the principles of 2 later when changed from the oscillating to the non-oscillating condition will be small.

Further, I have found that when the oscillator is turned off by detun'ing in accordance with what has 'been said above, the power dissipated in the tank or frequency controlling circuit may not be'the same as the power dissipated in the'circuit under oscillating conditions. Accordingly, my invention further contemplates the simultaneous detuning'of the low loss tank circuit togethe-rwith a change involtage away from the optimum applied to certain electrodes of the generator. In this way, adjustments may be made so that the frequency of operation is maintained substantial'ly constant despite onand off-switching of the system. i l

Further objects, features, advantages and other factors to be considered in connection with my present invention will be given with the aid of the accompanying drawings which, howevenare my present invention and wherein:

Figures 1 and la are'schem'atic diagrams of a self-oscillating Klystron oscillator as employed in a half duplex system. The drawing diagram matically illustrates means for simultaneously loading the tank-circuit by means of a section'of a transmission line and increasing the plate voltage so as to cause cessation of oscillations when theantenna is switched to the receiver;

Figure l illustrates an open-circuiting switch for line while Figure 1a illustrates a short-cir- 'Cultlng switch for line I when the system is in the receiving position;

Figure 2 diagrammatically illustrates a section of transmission line employed in-connection with the system shown in Figure 1;

Figure 3 goes into greater detail and Showsthe mechanical construction of adjustable length of coaxial line which may be used in connection with the system of Figure 1;

Figures 4 and *5 arecross-sectional and bottom views of a switch which may be used at SI in Figure 1; and

Figure 6 illustrates .;a modified arrangement of Figure 1.

Referring to Figure 1, there is illustrated schematically a :Klystron self-oscillatorand its associated equipment. TheJtulhe l,.whichis known as a velocity modulation tube, comprises a cathode a2,;a:control grid ;a dual resonator structure 4, of which a isltheinput gap; 1) the out-nut-ganand c the drift chamber. and collector electrode 5. Feedback is accomplished by means of =the courpling loop CL. Thefirst resonator nearest said cathode is often referred to as a buncher, while the second resonator nearest the collector is often referred to as a catcher.

The plate voltage supply EI subjects the oathode 2 to a negative potential with respect to the tank or plate 4 which is grounded at GI. The positive terminal of plate voltage source El is grounded through variable resistor RI. The collector voltage is diagrammatically illustrated at E2 and the control grid voltage at E3.

The carrier wave generated by the system of Figure 1 is modulated by the modulator 8 which may feed through transformer T a sub-carrier wave which may contain the signal as an amplitude modulation, a frequency modulation, a phase modulation or a constant frequency variable dot (CFVD) modulation.

Antenna 6 is connected to switch SI through the concentric transmission line ACL. The receiver RC is connected to switch SI through the concentric transmission line RCL and the transmitter or low loss tank circuit 4 is connected to the switch SI by means of loop L and line section I. The antenna 6, by means of switch SI, of course, may be switched to either the receiver or transmitter.

In the half duplex system shown in Figure 1, the transmitting and receiving frequencies are the same or very nearly the same. Usually, because of its cost, elaborate shielding is not provided to prevent radiation directly from the transmitting oscillator to the receiver. Hence, it will be necessary to prevent the velocity modulation tube from oscillating during the receiving interval. Later, when transmitting it Will be desirable to turn the velocity modulation tube on so that it operates immediately at its assigned or desired frequency.

In accordance with my invention, oscillations are stopped when the switch SI is open-circuited.

at its upper terminal TI, and the antenna 6 connected to the-lower or receiving terminal T2, in turn connected to the receiver. Inother words, when the switch SI is in the receiving position the transmission line section I is open-circuited at point TI. This places a reactive load on the oscillator instead of the resistive load which is present when the line I is connected to and matched to the antenna 6.

With point TI of transmission line section 1 open, transmission line section 1 will change the tuning of the resonator 4. The amount of detuning will depend upon the value of the reactive load which, in turn, will depend upon the length ofline 1 between the loop L and point TI. As this length of line or section of line between L and TI is varied, the reactive load which this line section places upon resonator 4 will change continuously from a high (depending on the Q of the line) negative reactance through zero to a high positive reactance. At some point the resulting detuning of the resonator 4 will stop the oscillations. In Fig. 1, the length of line I from the tank circuit to point TI is given by the relation where A is the operating wavelength and N is an integer. This length is further mentioned hereinafter.

When the switch SI is placed back in the transmit position so as to connect the antenna 6 through line AOL and section 'I to the resonator 4, the loading on the oscillator will be normal, assuming the transmission line 'I and AOL to be 4 matched to the antenna load. In this Way I have provided a means for stopping the oscillator by detuning the resonator 4 and of easily and accurately returning it to its original tuning point.

It is to be noted that the condition of constant heating of the resonator 4 for the on and oil? periods willdepend upon certain characteristics of the oscillation generating system. In a particular Klystron oscillator which I have employed, I have found that the plate efficiency was about 5%. The plate input power (100 watts) was the same for the oscillating or non-oscillating condition when using the detuning method for stopping oscillations, as explained in connection with Figure 1. Thus, the plate input power dissipated in the resonator changed from watts to watts when oscillations were stopped. I also found that when the transmitter was started again the frequency'was high. Hence, although the detuning method described stopped oscillations it did not in and of itself provide all of the frequency stability which was desired.

In accordance with another feature of my invention, frequency stability is enhanced by simultaneously increasing the plate voltage during the off period. In this way a condition is obtained whereby the frequency at the beginning of the transmitting period is very nearly the same as that at the end of the previous transmitting period.

schematically, the system for increasing the plate voltage when a section of line is used to detune the tank is shown in Figure 1. Switch S2 is uni-controlled with switch SI so that when point TI is open-circuited, RI is short-circuited by switch S2, thereby increasing the voltage impressed upon the tank or resonator 4 relative to the cathode. The value of RI in series with the plate voltage source El can be adjusted to give the desired compensation. In a practical circuit the source of voltage EI may comprise a conventional rectifier and RI may be placed in the primary winding of the rectifier plate transformer.

The action of the increased voltage to enhance frequency stability may be explained when it is considered that not only the total power dissipated in the resonator 4, but the distribution of this power in the resonator and the consequent thermal condition of the resonator will determine its resonant frequency. The paths of the electrons will be changed in the oscillating conditions from what they are in the non-oscillating condition due to the radio frequency voltages set up across the gaps a and b. This will result in a redistribution of heating in the resonator from the on condition to the off condition. I have found, as eXplained above, that this re-distribution of heating can be in part compensated for by a change in amount of heating and this amount of heating may be varied by varying the voltage to which the plate and cathode are subjected in the off and on periods of the oscillator.

For one particular setting of the oscillator I have found the following plate voltages to maintaint the carrier frequency substantially the same at the beginning of a transmission period as it was at the end of the previous transmissionperiod with, of course, the use of a transmission line section for detuning:

Oscillator on Plato voltage=16fl0 Plate current=67 MA Oscillator ofi Plato voltagc=l775 Plato currcnt=67 MA It is to be noted that an additional advantage of changing the voltage E I to which a pair of electrodes of the oscillator are subjected, results from a characteristic of the velocity modulation tube to the effect that for a particular operating frequency there is a certain value of plate volt age at which the Klystron operates best. If this voltage is changed from this optimum value, oscillations may be more easily stopped by detuning. The direction of the voltage change and the detuning should be such as to aid rather than oppose each other.

It should also be noted that switch SI, as will bedescribed more fully later in connection with Figures 4 and 5, can be so constructed that the output end of line section I is short-circuited during the receiving period. The length of the transmission line section 1 in this case should be made one-quarter wave length longer or shorter when short-circuited at TI than when open-circuited at Tl. Such an arrangement is shown in Fig. 1a. If it is desired to impress a maximum load upon the tank circuit 4 by means of the line section 1, the section I should be effectively an odd number, including one, of quarter wave lengths long when point TI is open-circuited and an odd number, including one, of half wave lengths long when point TI on the inner conductor IC is short-circuited to the outer conductor C. In Fig. 1a the switch S is similar to the one shown in Figs. 4 and 5 of my United States Patent 2,426,186 granted August 26, 1947. The cylinder is the same as the correspondingly numbered part in Fig. 4. The switches S2 and SI are shown in the receive position. The antenna is connected to the receiver and transmission line I is shorted at Tl by means of the cylinder 35 which makes contact to'the shell of the switch. In the transmitting position, the elbow in the cylinder 35 is rotated 180 so the antenna is connected to the terminal TI of line I, and switch S2 is opened. The receiver-transmission ,line terminal T2 will then be shorted. As explained herein, the length of line I will be different with this switch than with the switch shown in Fig. 1.

Transmission line 1 should be adjustable in length to obtain the best performance. A transmission line whose length is adjustable may be constructed as shown in Figure 2. The outer conductors are 2! and 23, the inner conductors are 22 and 24. The transmission of R.-F. energy is from 24 to the junction J of 24 and 22 and then to the right on 22. Metal rods 26 and metal sleeve MS are supported by and mounted upon metal sleeve 25 and make sliding contact to inner conductor 22 and are spaced one-quarter wave length from the sliding junction of 22 and 24.

It is to be noted that 26 may be ametal disc with ears protruding through the slots in the hollow conductors. Sleeve 25 is connected to the outer conductor 23 and makes sliding contact with outer conductor 2|. Outer conductor 2| is slotted at SI for accommodating the ears on disc 26 or for the bars or rods 26 and also slotted for inner conductor 24 so that the adjustment in line length may be slightly more than one-half wave length. Sleeve 25 is long enough to completely cover the slots SL in 2! at any position. The location of short-circuiting rods 26 with respect to 24 may be adjustable so the line may be used over a range of frequencies. Insulators 21 are spaced one-quarter wave length apart to compensate the reflections caused by them individually.

An adjustable length coaxial line of difierent construction is shown in Figure 3. This was built to operate at a frequency of about 3000 me. Both the inner IC and outer OC conductors are of telescoping tubing. The Victron insulators are in pairs spaced one-quarter wave length apart to compensate for reflections. This type of construction does not result in a uniform line, but the variations in characteristic impedance along its length is not generally serious. This adjustable section was used between two lengths of flexible coaxial cable.

Figures 4 and 5 show a form the switch Si of Figure 1 may take. Figure 4 is a cross-section of the switch and Figure 5 is a bottom plan view.

Care has been taken to keep the coupling low between coaxial fittings 3|, 32 and 33. :3I may be the coaxial cable or line section I to the transmitter of Figure l, 32 goes to the receiver and 34 is the connection to the antenna. The fitting 33 (see Figure 5) is a spare. This switch was constructed for operation at about 3000 me. The insulators are in pairs spaced one-quarter wave length apart. The plug or piston 35 may be rotated by means of handle 36 within cylinder 31. The spring Washer 39 insures good contact of the ends of piston 35 with the bottom of cylinder 31 and its cover plate 38. The outer conductors 0C or fittings 3|, 32, 33 and 34 are soldered into cylinder 31. The inner conductors IQ of 3|, 32 and 33 are held rigidly in place by their insulators and have spring chucks at their outer ends to receive #13 gauge wire and have slots at their inner ends to receive the blade at the end. of the inner conductor of 34. The inner conductor of 34 is held by insulators 40, 42 rigidly within the elbow 44 drilled in piston 35. Hence, when 35 is rotated the inner conductor of 34 rotates and engages in turn the inner conductors of 3|, 32 and 33. There is a hearing or rotating contact between the inner conductor of 34 and the inner conductor of the cable or transmission line connected to 34. About the circumference of 35 is a groove 48 the width of which is equal to the diameter of the inner conductors of 3i, 32 and 33. The sides of this groove make contact with and ground the inner conductors of the two coaxial lines not connected to 34. i

In Figure 6 the transmission line section 50 used to "turn the oscillator on and on" is independent of the transmission line 52 leading to antenna 6. In the oscillating condition shown, with the far end FE of the line section grounded by switch SI, the effective length of line 50 from loop L to the grounding point FE should be an odd number, including one, of quarter wave lengths. Then, when switch 5! is opened, the loading will be found to increase to such an extent as to stop the generation of oscillations.

As an alternative, oscillation generation may be made to stop with the switch Si closed and S2 closed by making the effective length of the line section between L and FE effectively equal to an odd number, including one, of half wave lengths, such lengths being taken at the desired operating frequency.

Having thus described my invention, what I claim is:

1; High frequency apparatus comprising a multi-electrode electron discharge device and a resonator circuit therefor electrically and thermally connected thereto, a section of transmission line coupled to said resonator, means for applying a potential to a pair of electrodes of said device cause sustained oscillations in said resonator circuit with said section of line coupled thereto at a resonant frequency dependent on the thermal condition of said resonator circuit, and means si multaneouslyto terminate saidsection of transmission line with ,an impedance and at a 11101131) chosen to load said resonator circuit tonon-resonanceto prevent oscillations therein and to apply a potential to said pair of electrodes to improve the stability of the thermal condition ,of said resonator circuit whereby the frequency stability of said. apparatus upon the generational oscillations immediately after periodsof ;non.oscillation is improved.

2. High frequency apparatus comprising a multi-electrode electron discharge device and a resonator circuit therefor electrically and thermally connected thereto, means for applyinga potential to a pair of electrodes of said deviceito cause sustained oscillations in said resonator circuit ata resonant frequency dependent on the thermal condition of said resonator circuit, a section of transmission linecoupled to said resonator for coupling said resonator to a utilization circuit, and means simultaneously to terminate said sec.-

tion of transmission line with an impedanceand at a point chosen b0 load said resonator circuit 'to non-resonance to prevent oscillations therein and to apply a potential to said pair of electrodes to improve the stability of the thermal condition of said resonator circuit whereby the frequency stability of said apparatus upon the generation of oscillations immediately after periods of non-oscillation is improved.

3. The apparatus claimed in claim 2, said point a being an integral number of electrical quarter wave lengths distant from the pointof electrical coupling of-said transmission line tosaid tank circuit at said resonant frequency.

4. The apparatus claimed in claim 2, said point being an odd integral number of electrical quarter-Wave lengths distant from the point of electrical coupling of said transmission line to said tank circuit at said resonant frequency, said terminating impedance being a short circuit.

5. The apparatus claimed in claim 2, said point being an even integral number of electrical quarter Wave lengths distant from the point of elec:

trical coupling of said transmission line to said tank circuit at said resonant frequency, said terminating impedance being an open circuit.

6. High frequency apparatus comprising a multi-electrode electron discharge device and a resonator circuit therefor electricallyand thermally connected thereto, means for applying a first potential to a pair of electrodes of said device to cause sustained oscillations in said resonator circuit at a resonant frequency dependent on the thermal condition of said resonator circuit, a section of transmission line coupled to said resonator for coupling said resonator to a utilization circuit, and means simultaneously to terminate said section of transmission line with an impedance and at apoint chosen to load said resonator circuit to non-resonance to prevent oscillations therein and to apply a second potential to said pair of electrodes greater than said first potential to improve the stability of the thermal condition of said resonator circuit whereby the frequency stability of said apparatus upon the generation of oscillations immediately after periods of non-oscillation is improved. I

7. The apparatus'claimed in claim 2, said resonator circuit comprising a cavity resonator,

8. High frequency apparatus comprising amulti-electrode electron discharge device and a resonator circuit therefor electrically and thermally connected thereto, means for applying a potential V to a pair of electrodes of said device to cause sustained oscillations ,in said resonator circuit ata 1 resonant frequency dependent on the-thermal conditionof said resonator circuit, {a utilization circuit, a section of transmission line coupling said resonator {circuit to said utilization circuit. .and means simultaneously toterminate said-sec.- tion of trans-missioniinewith animpedance-and at a pointchosen to loadsaid-resonator circuit .ato non-resonance to prevent oscillations.thereirnand to applyia potential-to said pair of electrodes to improve thestability of the :thermal condition sof said resonator circuit whereby :the frequencystaa .bility ofsaid apparatus upon the generaticnsof .auscillations immediately .after periods of anon-psicillation is improved.

9. The apparatus claimed in claim 8, said sresonator circuit comprising acavity resonator-asan integral part of said electron discharge. device.

10.,High frequency apparatus comprising a multi-electrode,electron discharge :deil-ice and :a cavity resonator therefor electri ally and :thermally connected thereto as .an integral part thereof, means for applyinga potential tea-pair of electrodesof said device'to cause sustained oscillations insaid cavity resonator at a resonant frequency dependent on the thermal condition of said cavity resonator, a utilization circuit, .a section of transmission line coupling said cavity resonator to said utilization circuit, a short-.cin cuiting element for short-circuiting saidtransmission line, and-a switch mechanically coupled tosaid short-circuitingelementto terminate said section of transmission line with a short-circuit at a-point chosen to load said :resona tor circuit to non-resonance to prevent oscillation thereof and simultaneously to apply ,a potential to said pair of electrodes to improve the stability of the thermal condition of said resonator circuit whereby-the frequency stability of said apparatus, upon the generation of oscillations immediatelyafter periods of non-oscillation :is improved.

1'1. High frequency apparatus comprising -a multi-electrode electron discharge device .and a resonator circuit therefor electrically and :thermally connected thereto, a resistor, a source 50f potential, said source, said resistor and a pair a of said electrodes being serially connected to apply a potential to said pair of electrodes to cause sustained oscillationsin said resonator circuit at a resonant frequency dependent on the thermal condition of said resonator circuit, a utilization circuit, a section of transmission line coupling said resonator circuit tosaid utilization circuit, and-means simultaneously to terminatesaid section of transmission line with an impedance and at a point chosen ,to load said resonator=circuit -to non-resonance .to preventoscillation thereof and to short-circuit said resistance to apply a secondpotential to said pair of electrodes greater than said first potential to improve :the stability of the thermal condition 0f said resonator :cir-

cuit whereby gthefrequency stabilityof said apparatus upon the generation of oscillations immediately after periods of non-oscillation ris gimproved.

1'2. The apparatus claimed in claim 1d, said resonator circuit "being (a cavity resonator which is an integral part of said electron discharge :de- -vice. 1

l3. {Ihe apparatus claimed :in claim 11 said electron discharge device comprisingifirs't andsec- 0nd cavity resonators each having a gap therein,.a cathode positioned outside the field of-said-first cavity -re sonator, a collector positioned outside the. field of said second resonator, said resonator circuit comprising said .cavity resonators, said pair of electrodes being said cathode and said collector, said transmission line being coupled to the interior of said second cavity resonator.

ORVILLE E. DOW.

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

UNITED STATES PATENTS Conklin Aug. 13, 1940 Number Number 358,917 1% 187,457

Name Date Gossel Oct. 29, 1940 Christ Nov. 20, 1940 Ramo Dec. 3, 1940 Chaffee Mar, 18, 1941 Davies Feb. 18, 1941 Robb Sept. 16, 1941 Pawsey et a1. Aug. 25, 1942 Hansen et a1 Feb. 23, 1943 Litton Mar. 2, 1943 FOREIGN PATENTS Country Date Great Britain Oct. 1931 Great Britain Oct. 26, 1922

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