US2404568A - Automatic frequency control - Google Patents

Description

July 23, 1946 Aur Filed July 21, 1942 3 Sheets-Sheet l CENT/Pa; 7o

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ATTORNEY Jul 23,1946. o. E. DOW 2,404,568

AUTOMATIC FREQUENCY CONTROL Filed July 21-, 1942 s Sheets-Sheet 2 AMP; 7-005 Cow/P04 Mom INVENTOR Q/PV/LLE E. aw.

ATTORNEY Jul 23, 1946.

Filed July 21, 1942 0.1:. Dow

- AUTOMATIC FREQUENCY CONTROL 3 Sheeis-Sheet 3- INVENTOR ORV/a5 E. Dam

BY )zfi m ATTORNEY Patented July 23, 1946 AUTOMATIC FREQUENCY CONTROL Orville E. Dow, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 21, 1942, Serial No. 451,717

21 Claims. 1

This invention relates to automatic frequency control and more particularly to automatic frequency control for generators operating at frequencies above 500 megacycles.

One method presently employed to control the frequency of a generator generating waves having a frequency above 500 megacycles is to compare the operating frequency of the generator with the resonant frequency of a, low loss or high Q circuit. In accordance with this method some of the energy from the generator is impressed on the high Q circuit. Part of th oscillatory energy is then taken from the circuit, detected and the amplitude of the detected output is observed. Variations in the amplitude of the detected output represent changes in frequency away from the frequency of the resonant circuit. This arrangement, however, leaves much to be desired since it indicates only the amount of frequency change and not the direction of that change.

One way of determining the extent, as well as direction, of the frequency change would be by the use of two high Q circuits, one tuned above and one tuned only below the desired operating frequency of the generator. Since such low loss or high Q circuits must be very stable, they are quite expensive in view of the precision required in their construction.

In order to avoid the use of two such low loss circuits, and thereby reduce the cost of the apparatus, it has been proposed to continuously and cyclically vary the tuning .of a single low loss or high Q circuit about the desired operating frequency. This can be done by means of a properly shaped rotating condenser plate coupled to the low loss circuit.

Heretofore, a detector coupled to the output of this resonant low loss circuit, whose tuning is varied as indicated, fed its resulting output voltage to a commutator arranged to be integral with the cyclically varied tuning condenser to thereby generate or produce a voltage whose polarity is of one sign when the operating frequency is above the tuning of the low loss circuit and of the oppo site sign when below the assigned or desired frequency. One object of my invention is to provide a system which does not mak use of a commutator and thereby avoids the difiiculties concomitant to its use. In carrying out this object, I make use of a balanced detector system, as will be described more fully hereinafter, in place of the commutator.

Use of my balanced detector arrangement requires another voltage synchronous with the voltage generated by the modulating or continuously varied tuning condenser. Heretofore, with the commutator arrangement, a synchronous voltage for use in detecting frequency departures has been generated by means of an alternator mounted on the same shaft as the modulating or tuning condenser. In accordance with my present invention, this alternator is eliminated and the synchronous voltage used in my detector system is derived in a much more satisfactory and practical manner.

Other objects, advantages and features orm present invention will be apparent as the more detailed description thereof proceeds. The latter will be given with the aid of the accompanying drawings wherein:

Figure 1 illustrates one modification of my present invention;

Figures la, 1b, 1c and 1d are voltage-time graplhs used in describing the operation of Figure Figure 2 is a variation of Figure 1;

Figure 3 is a modified form of Figure l and is characterized by the fact that a voltage is changed in order to correct the frequency of oscillations generated by a generator rather than, as illustrated in Figure 1, change the tuning 'of a generator in order to correct the frequency of oscillations generated thereby; and

Figure 4 is still another modification or variation of Figure 1 in which a reversible motor for frequency control is operated directly from electronic tubes.

Referring to Figure 1, I have illustrated at I a low loss high Q resonant circuit accurately tuned to a desired operating frequency. Low loss circuit I consists of a cylindrical outer conductor IA and an inner concentric conductor I B. The resonant frequency of the resonant element I is varied above and below the assigned or desired operating frequency by means of rotating disc 2 mounted on the shaft of and driven by synchronous motor 3. Disc 2 is cut away or notched, as shown at 2E, 2F, 2G and 2H, so as to have four condenser poles 2A, 2B, 2C and 2D. As illustrated, these condenser poles rotate through a slot in the outer conductor IA of the concentric transmission line I and cause the resonant frequency of the low loss device I to vary equally above and below the desired operating frequency.

Radio frequency oscillations are fed from the high frequency generator HFG to the concentric transmission line 5A5 to a suitable circuit such as an antenna (not shown). By means of the transmission line section 6 having an internal conductor BA and coupling loops 6B and 60, high frequency oscillations from the generator are also fed through the action of loop BB, conductor 6A and loop BC to the resonant low loss circuit i.

By means of loop 4A and the inner conductor 4B of transmission line section 40, wave energy is picked up from the low los resonant circuit i and fed to high frequency crystal detector 4 provided With a suitable by-pass condenser 49.

The rectified output of crystal detector 4 is fed through line 4F and transformer T to the control grid of the amplifier-limiter tube 9, The output of limiter 9 is impressed through transformer T3 in push-pull to the control grids of vacuum tubes VT1 and VT2 forming a balanced detector Ill.

The source of constant relatively low frequency waves derived from a source LFS, which drives the synchronous motor 3, also supplies transformer T3 feeding the full wave rectifier 1,. Unfiltered rectified voltage from rectifier I is fed through condenser Cl and conductor C2 to the plates of vacuum tubes VTl and VT2.

The cathodes of detector tubes VT1 .and VTz are connected to and actuate polarized relay H, in turn actuating relays l2, in turn controlling a reversible motor RM used to control the frequency of the high frequency generator HFG. As illustrated, this is accomplished by increasing or decreasing the capacity of the frequency controlling condenser FCO which may form part of the tank circuit of the vacuum tube generator HFG.

The synchronous motor 3 may be a .e-pole motor with a speed of 1800 R. P. M. The rotating condenser pole may be a ll-pole condenser. In operation any one pole of the rotor of the synchronous motor may line up with any of the four poles of the stator. The synchronous:motor 3 may take any one of four positions. For this reason the condenser 2 must have the same number of poles as the .motor 3 so that the phase of the output voltage from crystal detector 4 will be independent of the position of the synchronous motor 3.

The voltage fed to the plates of balanced detector I is rectified full wave, 60 cycles and is represented in Figures 1a, 1b and by Ep. Figure 1d represents a resonance curve of the high Q circuit I. Thus, if the generator frequency is at F0 the rotating condenser will vary the frequency of ,1 between the limits of F1 and F1 and the resulting output voltage in crystal 4 will be as represented in Figure 1a by E91 and Eg Egi is the voltage impressed on the control grid of tube VTi and Egz is the voltage impressed on the control grid of VTz. An examination of Figure la will show that the average current drawn by VTi will be equal to the average current drawn by VT2 and the polarized relay M will not be operated.

However, if the frequency of the generator shifts to E1 the modulating condenser 2 will vary the frequency of the circuit 1 between the limits of F3 and F0. The output voltage on crystal 4 will be half the frequency that it was for the condition when the generator frequency was F0. The resulting voltages impressed on thecontrol grids of VTl and VI'z are represented in Figure lb by E91 andEgz. Examination of this curve shows that VTl will draw more current than VT2 because the grid is plus during the time that the plate voltage i also plus whereas the grid of VTz is negative when the plate voltage is plus. Thi will operate the polarized relay H, which will operate relay l2 and start the control motor RM in the proper direction to correct the generator frequency.

If the generator frequency shifts to F2 the conditions will be as illustrated in Figure 10. Now the control grid of VTz is plus when the plate voltage is plus, while the 'control grid of VTl is negative and, therefore, VTz will draw more current than VT1 and the polarized relay II will be operated in the opposite direction, causing the control motor RM to operate in the proper direction to correct the frequency.

In the above the phase of the voltage Ep may be adjusted with respect to the phase of voltages Em and Eye by locating condenser 2 in the proper position on the synchronous motor shaft. Also, amplifier 9 is operated so that it is limiting. This results in the amplitude of the voltages Em and Egg to balanced detector iii being independent of the amplitude of radio frequency voltage coupled into high "Q circuit 1 and, therefore, substantially responsive only to changes in frequency.

The function of condensers l3 in Figure 1 is to smooth out the pulsating currents which flow through the tubes VT1 and VTz and maintain a more nearly constant current through the D.-C. polarized relay ii. That is, the high peak currents through the tubes VT1 and VT2 charge up the condensers i3 which discharge through the relay H and the resistor {3A. The average current through this resistor produces a bias for the tubes W1 and VT2.

The above description relates to a l-pole synchronous motor, but also it would be possible to use a 2-pole synchronous motor. In that case the rotating condenser would have to have two poles P1 and P2, as illustrated in Figure 2. Also, if the output from crystal detector 4 were large enough it may be possible to omit amplifier 9 and impress the crystal output voltage directly on the balanced detector i6, as shown in Figure 2.

The output at the balanced detector may be made to control a voltage which will correct the frequency drift of the generator. lhis may be done for such generators as the Klystron oscillator or other oscillators whose frequency is a function of the supply voltage. Specifically, the frequency increases with an increase in plate voltage (increase in electron velocity in beam) in a Klystronor similar type of oscillator, and decreases with a decrease in voltage.

Figure 3 shows a circuit for controlling the frequency in this way. The operation and arrangement of the balanced detector iii are as given in connection with Figure 1. However, it is to be noted that the cathode output circuits of the detector tubes VTi, VT2 of Figure 3 are connected in series with one cathode, namely, cathode MIC of vacuum tube V'I2, grounded.

As a consequence, in Figure 3 a D. C. output voltage at terminal I? will appear which will be negative or positive with respect to ground de pending upon whether the frequency of the high frequency generator I-IFG (see Figure 1) is above or below its assigned Value.

The voltage at terminal ll of Figure 3 is impressed through a suitable resistance and condenser filter on the control grid of vacuum tube amplifier T4. Vacuum tubes T4, T5 and To serve to regulate the power supply at terminals a, I) connected to the oscillation generator.

The voltage impressed on the control grid g4 .of electron discharge device T4 from potentiometer l6 by way of tap 3H5 is amplified by tubes T4 and'Ta and impressed on the grid g6 of vacuum tube T6 in such a way as to maintain the voltage across the output terminals a and b constant.

The function of the glow tubes GT1 and GT2 of Figure 3 is to maintain the cathodes C4 and C5 of vacuum tubes T4 and T5 at a fixed potential below ground. Thus an increase in voltage across ab will increase the voltage from a to the tap 3I6 on the potentiometer Hi. This means the grid voltage of T4 will be made more negative with respect to a or ground and, hence with respect to its cathode C4. This will reduce the current in the plate resistor R4 of tube T4 and, hence, decrease the negative potential on the grid Gs of T5. This will result in an increase in current through the plate resistor R5 of tube T5 and an increase in negative potential of the grid Go of T6. Thus, the voltage drop across T6 is increased to maintain a constant voltage across ab when the voltage from the high voltage rectifier increases. If the connection point at I! is broken, the tubes T4, T5 and Te maintain nearly constant voltage across a and b.

The voltage from terminal H being impressed on the grid G4 of tube T4 will vary the voltage across the output terminals a and b and thereby vary the frequency of the generator in a manner determined by its voltage versus frequency characteristic. As explained above, in a Klystron or similar type of oscillator the frequency increases with an increase in plate voltage and decreases with a decrease in plate voltage. For more detailed circuit connections to such an oscillator, reference is made to Figure 1 of my copending application Serial No. 446,457, filed June 10, 1942.

Since the voltage I1 is determined by the amount the generator is off frequency this voltage can be used to correct the frequency. If the drift of the generator is large it would not be possible to correct it entirely by means of the circuits shown in Figure 3 since there is usually an optimum voltage for this type of generator. However, minor corrections could be made with this circuit up to a certain point where a motor operated as illustrated in Figure 1 could take control to make a major correction in frequency. This correction would also bring the supply volt age back to normal. This combination would result in less operation of the relays and, hence, longer life for the relays.

In other words, the frequency controlling systems of Figures 1 and 3 may be used simultaneously for frequency controlling the high frequency generator HFG of Figure 1. The conductor 4F in this case, of Figure 3, would be connected to the terminal T of Figure l and the relay coil H of Figure 1 would be connected in shunt to the resistors l4, 15 of Figure 3. This shunt connection is possible when the relay coil is of suificiently high resistance, in which case it may be connected in place of resistors I4, 15. If the relay coil H is of low resistance it may be connected in series with and between resistors I4 and I5.

The function of the condenser resistance networks |3|4, l3l5 of Figure 3 and the network of capacity and resistance 30?) immediately above I1 is to smooth out the ripple voltage due to the pulses of current through tubes VT1 and VTz. The variations of voltage at I! due to frequency changes are of a comparatively slow rate. The function of condenser 302 between the point D and the tap 316 on the potentiometer is to increase the smoothing of the power supply ripple voltage from the high voltage rectifier. This condenser will result in a higher percentage of the ripple voltage which appears across ab being impressed on the grid of T4 than would result with just the potentiometer [6. Of course, increased regulation of slow voltage variations across ab would result if the tap on the potentiometer could be moved to b, but this would result in excessive bias on T4 unless glow tube regulators with an operating voltage nearly equal to the voltage across a-b were used.

The resistance 3|8 between the grid G4 of T4 and the potentiometer tap is to permit the grid to follow the voltage variation on H. Similarly the resistance 32E] between I! and the grid of T4 is to permit the grid to follow the voltage variations of the tap 3|6. In some cases the regulation of the potentiometer would be high enough so that the resistor 3| 8 adjacent the tap could be omitted.

The balanced detector 10 may also be used to directly operate a reversible motor for frequency control. Such an arrangement is illustrated in Figure 4. It will be noted that the armature of the reversible frequency controlling motor 4RM is connected between terminals GT and 4T1. The field for the reversible motor is connected between the terminals 4T1 and 4T2.

Having thus described my invention, what I claim is:

1. In combination, an adjustable electrical de- Vice carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said device, a source of constant relatively low frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying oscillations derived from said cyclically tuned element, means for rectifying waves from said low frequency source, means for combining the rectified oscillations and waves, and means responsive to the combined waves for changing the adjustment of said device.

2. In combination, a generator of high frequency oscillations having an adjustable frequency controlling device, a low loss resonant element, means for exciting said element with oscillations derived from said generator, a source of relatively low frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying and limiting oscillations derived from said cyclically tuned element, means for rectifying waves from said low frequency source, a balanced detector for combining the rectified oscillations and waves, and means responsive to unbalance of the detector waves for changing the adjustment of said device.

3. In combination, a circuit carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of constant, relatively low frequency waves, means for cyclically changing the tunin of said element under control of waves from said source, means for rectifying oscillations derived from said cyclically tuned element, means for rectifying waves from said low frequency source, means for combining the rectified oscillations and waves, and means responsive to the combined waves for changing the tuning of said circuit. .1

4. In combination, a generator of high frequency oscillations having an adjustable frequency controlling reactor, a low loss resonant element,

aeogscs meansfor exciting said element with oscillations derivedfrom said generator, a source of audio frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying oscillations derived from said cyclically tuned element, means for rectifying waves from said audio frequency source, means for combining the rectified oscillations and waves, and means responsive to the combined waves for changing the adjustment of said reactor.

5. In combination, a circuit carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of relatively low constant frequenc waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rec- .tifying and limiting oscillations derived from said cyclically tuned element, means for full wave rectifying waves from said low frequency source, means for combining the rectified oscillations and waves, and means responsive to the combined waves for changing the effective frequency of operation of said circuit.

6. In combination, a circuit carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said element, a source of relatively low frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying oscillations derived from said cyclically tuned element, means for combining waves derived from the rectified oscillations and waves derived from said low frequency source, and means responsive to the combined Waves for changing the effective frequency of operation of said circuit.

7. In combination, a circuit carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of relatively low frequency Waves, means for cyclicall changing the tuning of said element under control of waves from said source, means for rectifying and limiting oscillations derived from said cyclically tuned element, means for full wave rectifying waves from said low frequency source, means for combining the rectified oscillations and waves, and means responsive to the combined waves for changing th tuning of said circuit.

8. In combination, a circuit carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of relatively low frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying oscillations derived from said cyclically tuned element, means for limiting the rectified oscillations, means for rectifying waves from said low frequency source, means for combining the rectified, limited oscillations and Waves, and means responsive to the combined waves for changing the effective tuning of said circuit.

9. In combination, a circuit carrying high frequenc oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of relatively low frequency Waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying and limiting oscillations derived from said cyclically tuned element, means for full Wave rectifying waves from said low frequency source, means for combining the rectified oscillations and waves,

and means responsive to the combined waves for changing the effective tuning of said circuit.

10. In combination, a circuit carrying high frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of relatively low frequency Waves, means for cyclically changing the tuning of said element under control of waves from said source, means for limiting and rectifying oscillations derived from said cyclically tuned element, means for rectifying waves from said low frequenc source, means for combining the rectified oscillations and waves, and means responsive to the combined waves for changing the effective tuning of said circuit.

11. In combination, a circuit carrying high'frequency oscillations, a low loss resonant element, means for exciting said element with oscillations derived from said circuit, a source of relatively low frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying and limiting oscillations derived from said cyclically tuned element, means for rectifying waves from said low frequency source, means for combining the rectified oscillations and Waves, and means responsive to the combined waves for changing the tuning of said circuit.

12. In combination, a generator of high frequency oscillations, a low loss resonator element, means for exciting said element with oscillations derived from said generator, a source of relatively low frequency waves, means for cyclically changing the tuning of said element under control of waves from said source, means for rectifying oscillations derived from said cyclically tuned element, a balanced detecting arrangement for combinmg the rectified oscillations with waves derived from said source of relativel low frequency waves, and means responsive to the output of said balanced detecting arrangement for changing the frequency of operation of said generator.

13, In combination, a generator of high frequency waves; a stable low loss concentric conductor tank circuit tuned to a desired frequency of operation, said tank circuit having its outer conductor perforated; a circuit coupling said generator to said tank circuit so as to excite said tank circuit with Waves derived from said generator; a physically movable tuning device mounted for movement in said perforation so as to vary the tuning of said tank circuit; a source of 60 cycle low frequency waves; apparatus responsive to said low frequenc waves for cyclically moving said device so as to vary, at a synchronous rate with respect to the frequency of said source of low frequency waves, the tuning of said stable concentric line tank about a desired frequency of operations; a detector coupled to said cyclically tuned tank circuit; a balanced vacuum tube detecting system coupled to said detector and to said source of 60 cycle Waves, said detecting system producing a variable voltage as the frequency of said high frequency generator varies with respect to the mean frequency of operation of said variable tuned tank circuit, and means utilizing the variable voltage so produced for con trolling the frequency of operation of said high frequency generator.

14. In combination, a generator of high frequency Waves, a stable low loss concentric conductor tank circuit tuned to a desired frequency of operation, said tank circuit having its outer conductor perforated, a circuit coupling said generator to said tank circuit so as to excite said tank circuit with waves derived from said generator, an electromechanically driven tuning device mounted for physical movement in said perforated conductor so a's'to Vary the tuning of said tank circuit, a source of low frequency waves,-apparatus responsive to said low frequency waves for cyclically moving said device so as to vary the tuning of said concentric line tank about a desired frequency of operation, a'detector coupled to said cyclically tuned tank circuit, a balanced detecting system coupled to said detector and to said source of low frequency waves, said balanced detecting system producing a voltage which varies as the frequency of said high frequency generator varies with respect to the mean frequency of operation of said tank circuit, and means utilizing the variable voltage produced by said balanced detecting system for controlling the frequency of operation of said high frequency generator.

In combination, a generator of oscillations, a stable circuit tuned to a desired frequency of oscillations generated by said generator, said stable circuit being coupled to said generator so as to be excited by oscillations generated by said generator, a source of relatively low frequency waves, means responsive to waves from said low frequency source for cyclically Varying the tuning of said stable circuit about a desired frequency of operation, a rectifier rectifying waves derived from said variably tuned stable circuit, a balanced detecting system responsive to and comparing waves derived from said rectifier and from said relatively low frequency source so as to produce a voltage which varies in polarity depending upon departures in frequency of said oscillation generator away from a desired frequency of operation, and instrumentalities responsive to the voltage produced by said balanced detectin system to reduce the departure in frequency of operation of said oscillation generator away from th desired frequency of operation.

16. In combination, a generator of high frequency waves of a desired frequency, a stable low loss tank circuit tuned to said desired frequency of operation, a circuit coupling said generator to said tank circuit whereby said tank circuit is excited with waves derived from said generator, a source of low frequency waves, apparatus operated by said source of low frequency waves for cyclically and continuously changing the tuning of said tank circuit about said desired frequency of operation, a rectifier fed with waves derived from said variably tuned tank circuit, a detecting system supplied with waves derived from said rectifier and said low frequency source, said detecting system operating to combine the waves supplied to it from said rectifier and from said low frequency source to produce a voltage which changes in polarity when the frequency of said high frequency generator passes through the mean frequency of operation of said variably tuned circuit, and means responsive to said produced voltage acting to restore the frequency of operation of said high frequency generator back to a desired operating frequency.

17. In combination, a controllable system carrying high frequency oscillations, a stabl tank circuit tuned to a'desired frequency of oscillations generated by said generator, said stable circuit being coupled to said system so as to be excited by oscillations derived from said system, a source of relatively low frequency waves, means responsive to Waves from said low frequency source for synchronously and electromechanically varying the tuning of said stable circuit about a desired frequency of operation, a rectifier rectifying waves derivedfrom said variably tuned stable circuit,a detecting system supplied with and re sponsive to waves derived from said rectifier and from said relatively low frequency source to produce a voltage which varies in polarity depending upon changes in frequency of said controllable system above and below a desired frequency of operation, and instrumentalities responsive to the voltageproduced by said detecting system to change the frequency of operation of said controll'able system in a direction towards the desired frequency of operation.

'18. In combination, a generator of high frequency waves, a stable low loss concentric conductcr tank'circuit tuned to a desired frequency of operation, said tank circuit having its outer -conductor perforated, a circuit coupling said generator to said tank circuit soas to excite said tankcircuit With- Waves derived from said generator, a mechanically movable conductor mounted for movement in said perforation so as to vary the tuning of said tank circuit, a source of low frequency Waves, apparatus responsive to said low frequency waves for electromechanically moving said movable conductor in the space between said concentric conductors so as to vary, in synchronism with said low frequency waves, the tuning of said stable tank circuit about a desired frequency of operation, a detector coupled to said cyclically tuned tank circuit, a balanced detecting system coupled to said detector and to said source of low frequency waves, said balanced detecting system producing a variable voltage as the frequency of said high frequency generator varies with respect to the mean frequency of operation of said stable tunable tank circuit, and means utilizing the variable voltage so produced for controlling the frequency of operation of said high frequency generator.

19. In combination, a generator of high frequency waves of a, desired frequency, a stable low loss tank circuit having an outer radiation confining conductor tuned to said desired frequency of operation, a circuit coupling said generator to said tank circuit whereby said tank circuit is excited with waves derived from said generator, a source of low frequency waves, rotatable tuning apparatus mounted for movement in an opening in said outer conductor operated by said source of low frequency waves for periodically and continuously changing the tuning of said tank circuit about said desired frequency of operation, 2, rectifier fed with waves derived from said variably tuned tank circuit, a balanced detecting system supplied with waves derived from said rectifier and said low frequency source, said balanced detecting system operating to produce a control voltage which varies when said high frequency generator varies in frequency away from the mean frequency of operation of said variably tuned circuit, and means responsive to said produced control voltage to modify the frequency of operation of said high frequency generator.

20. In combination, a generator of high frequency waves of a desired frequency, a stable low loss two-conductor concentric line tank circuit tuned to said desired frequency of operation, a circuit coupling said generator to said tank circuit whereby said tank circuit is excited with waves derived from said generator, a source of low frequency waves, a device mounted for movement in the space between the conductors of said concentric line, apparatus operated by said source 1 l of low frequency waves for synchronously and continuously moving said device thereby periodically changing the tuning of said, concentric line tank circuit about said desired frequency of operation, a rectifier fed with .waves derived from said variably tuned tank circuit, a balanced detecting system supplied with waves derived from said rectifier and said low frequency source, said balanced detecting system operating to produce a Voltage which changes in polarity when said high frequency generator varies in frequency through the mid-frequency of operation of said variably tuned circuit, and means responsive to said voltage produced by said balanced detector to alter the frequency of operation of, said high frequency generator.

21. In combination, a generator of high frequency oscillations, a stable concentric line circuit tunedto a desired frequency of oscillations generated by said generator, saidstable, circuitbeing coupled to said generator so as to be excited by 12 oscillations generated, by said generator, 9. source of relatively low frequency waves, means responsive to waves from said low" frequency source for cyclically varying the tuning of said stable circuit about a desired frequency of, operation, said means including a physically movable conductor mounted for movement. in an opening in said outer conductor, a rectifier rectifying waves derived from said variably tuned stable circuit, a balanced vacuum tube. detecting system responsive to waves derived'from said rectifier andifrom said relatively low frequency source to produce a voltage which varies. in polarity depending, upon changes in frequency of said oscillation generator above and below a desired frequency of operation, and instrumentali-ties responsive tothe voltage produced by said balanced vacuum tube detecting system to change the frequency of operation of said oscillation generator in a. direction towards the desired frequency of operation.

ORVILLE E.. DOW.

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