US2558100A

US2558100A - Frequency stabilizer

Info

Publication number: US2558100A
Application number: US57966A
Authority: US
Inventors: Sheldon I Rambo
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1948-11-02
Filing date: 1948-11-02
Publication date: 1951-06-26
Anticipated expiration: 1968-06-26

Description

5. A BO FREQUENCY STABILIZER June. 26, 1951 3 Sheets-Sheet 1 Filed Nov. 2, 1948 Fig.1.

Fig.2.

Frequency Frequency 7 INVENTOR Sheldon I. Ro mbo,

ATTORNEY Frequency Frequency WITNESSES:

June 26, 1951 v v s, RAMBO 2,558,100

FREQUENCY STABILIZER 'Filed Nov. 2, 1948 s Sheets-Sheet 2 Fig.5. Oscillator H.56MC

zMciAf Low Puss Mixer v Filter Brood-Bond ,l? {l9 Discriminutpr Low Pass Detector M'xer Filter llo l8 RFGenerotor 0sc|llotor l3.6MC 556W:

+Af 2| i u ,w R Mixer M 00:35:32? Dlscrqmmmor Unit rl2 Frequency Standard l3.56MG

l3.6MC-Af aemmm Fig.7.

WINESSES: INVENTOR BYSheldon l. Rambo.

ATTORNEY 3 Sheets-Sheet 5 S. l. RAMBO FREQUENCY STABILIZER B BEom June 26, 1951 Filed Nov. 2, 1948 Ill-ll Illilrw m m. H Q

m mm m ucum uuem Sheldon I. Rqmbo. fizfiwuA ATTORNEY WITNESSES:

Patented June 26, 1951 STATS T OFFICE FREQUENCY STABILIZER Application November 2, 1948, Serial No. 57,966

12 Claims.

This invention relates to frequency stabilizers for radio frequency generators, and it relates more particularly to automatically operated, frequency stabilizers for radio frequency generators used in high frequency induction and dielectric heating systems.

A material heated by energy from a radio frequency generator in conventional circuits, has an impedance which is reflected back into the load circuit of the generator, and as the impedance of the material changes with the changes in temperature, the generator is detuned from its assigned frequency. Other conditions such as changes in the characteristics of circuit components, and fluctuations in Voltage of the power source, may act to change the frequency of the generator.

It is desirable for reasons of efficiency, and for compliance with the requirements of the Federal Communications Commission, that the assigned frequency of a radio frequency generator used for heating, be maintained within close limits. Accordingly, it is the practice to provide automatically operated, frequency adjusting systems which retune a radio frequency generator to its assigned frequency when it departs therefrom. Many such systems use discriminators of considerably narrower band widths than the total tunable ranges of the generators, requiring means for automatically bringing the generator frequency within the discriminators bandwidth in order to correct for large abrupt load changes during operation, or for marked changes in conditions between the end of one heat cycle and the beginning of another. A so-called search system may be used to accomplish this purpose. Such a system is disclosed in the copending application Serial No. 750,970, new Patent No. 2,542,837, in which I am a joint applicant.

A disadvantage of such a search system is that it has no sense. That is to say, when the discriminator relinquishes control to the search circuit near the discriminator band edge, the search circuit may drive the correcting variable reactance in the generator tank circuit in the wrong direction for frequency correction instead of in the right direction, since search circuit operation depends merely upon the magnitude of voltage developed across a parallel circuit resonant at the desired, mean generator frequency. If the control circuit happens to be in the condition which causes searching in the wrong direction for frequency correction, the variable reactance operating element must run to its limit to trip a reversing switch, and then run all the way back in the proper direction to bring the frequency back within the discriminator band, and allow it to make the final correction. Obviously, such a system allows the generator to sweep almost completely across its entire tunable band width before locking in, if the frequency changes abruptly beyond the range of the disscriminator.

If the descriminator band width were Wider than the total tunable range of generator, then no search system would be necessary since the discriminator would maintain sense even for sudden changes of frequency. A discriminator must, however, have a sharp characteristic at cross-over, and it is not practical to provide a single discriminator having such a characteristic, and at the same time covering such a wide band that a search system would not be needed.

This invention overcomes the difficulties of the prior systems by combining in a single system a sharp discriminator and a broad-band discriminator. In one embodiment of the invention for maintaining the frequency of a radio frequency generator at 13.56 megacycles (me) within plus or minus 6780 cycles, a sharp discriminator which crosses over from one maximum to the other in 10 kilocycles (he) is provided. The generator has a variable, frequency adjusting reactance, the range of which is about 250 kc. on each side of 13.56 me. The broad-band discriminator has an effective width of 500 kc, and has its cross-over point coincident with that of the sharp discriminator. The peaks of the broad-band discriminator are within approximately 50 kc. of crossover so as to overlap the peaks of the sharp discriminator.

A feature of this invention is that the broadband discriminator is provided by using a first band-pass filter to provide one-half of the desired broad-band characteristic on one side of crossover, and by using a second similar band-pass filter to provide the other half of the desired broad-band characteristic on the other side of cross-over. Energy from the generator, the frequency of which is to be controlled, is mixed with energy from a fixed frequency generator, operating at a frequency which may be 2 mo. above the mean frequency to be maintained, and supplied into the input of one of the band-pass filters. Energy from the generator is mixed with energy from a fixed frequency generator, operating at a frequency which may be 2 mo. below the frequency to be maintained, and supplied into the input of the other band-pass filter. The outputs of the two filters are fed apart in I) phase, into a common detector for providing the desired wide band discriminator characteristic. An object of the invention is to maintain the frequency of a radio frequency generator constant within narrow limits.

Another object of the invention is to combine in a frequency stabilizer circuit for a radio frequency generator, sharp and broad-band discriminators.

Another object of the invention is to provide the two halves of a broad-band discriminator on the opposite sides of cross-over, with the outputs of two band-pass filters.

Another object of the invention is to combine the outputs of two, similar, band-pass filters, 130

apart in phase, to provide a broad-band discriminator.

The invention will now be described with reference to the drawing of which:

Figure l is a graph illustrating the output characteristics of a sharp discriminator which may be used in this invention;

Fig. 2 is a graph illustrating the output char acteristic of one of the band-pass filters used in the broad-band discriminator of this invention;

Fig. 3 is a graph illustrating the output characteristic of the other of the band-pass filters used in the broad-band discriminator of this invention;

Fig. 4 is a graph illustrating the output characteristics of the sharp and broad-band discriminators as combined in this invention;

Fig. 5 is a simplified block diagram illustratin a circuit embodying this invention;

Fig. 6 is a circuit schematic illustrating components which may be used in the blocks of Fig. 4; and

Fig. '7 is a vector diagram illustrating the phase relations of the voltages at the input at the sharp discriminator detector.

Figure 1 is an output-frequency characteristic curve of a sharp discriminator, and shows that in order to have a desired sharp characteristic at cross-over, one which for example, crosses over from one maximum to the other in 10 kc. cannot ordinarily have a band width greater than 166 kc.

Fig. 2 illustrates the output characteristics of one band-pass filter of the broad-band discriminator of this invention, its total band width extending from 1.6 mc. to 2 mc., and Fig. 3 illustrates the output characteristic of the other band-pass filter before it is inverted, its band width extending from 2 mc. to 2.4 Inc.

Fig. 4 illustrates the output characteristics of the sharp and broad-band discriminator and shows that the effective band width is at least 500 kc. wide, and that there is a cross-over of 10 kc. from peak-to-peak.

Referring now to Fig. 5, it is assumed by way of example, that the radio frequency (RF) generator H] is designed to operate at an assigned frequency of 13.56 mc., plus or minus a tolerance frequency of M which may be 6780 cycles. It is desired that the discriminator used in the frequency control system have a sharp output characteristic at cross-over of 10 kc. peak-topeak, and at the same time to have a wide-band charactertistic of 500 kc. To accomplish this, energy from the generator H] at 13.56 mc. i M is mixed in the mixer H with energy from the frequency standard |2 at 13.56 mc. The mixer 1| supplies energy at the difference frequency M to the sharp discriminator l3.

Energy is also fed from the generator ii] at 13.56 mc. i A! to the mixer I4 where it is combined with energy from the oscillator l5 operating at 11.56 mc. Energy at the difference frequency of 2 mc. i M is passed through the bandpass filter !6, the output of which has the characteristic shown by the curve of Fig. 2.

Energy is also fed from the generator ill at 13.56 i M to the mixer H where it is combined with energy from the oscillator is operating at 15.56 mc. Energy at the different frequency of 2 mo. F A) is passed through the band-pass filter IS), the output of which has the characteristic shown by the curve of Fig. 3.

The outputs of the band-pass filters l6 and i9 are fed into the broad-band discriminator detector 25 in which the output of the filter H3 is inverted and mixed with the output of the filter |'B, the combined rectified energy being supplied together with the output of the sharp discriminator 13, into the reactance correcting unit 21, which acts to retune the generator it when, due to load or other changes, it departs from the tolerance limits of its assigned frequency.

The output of the broad-band discriminator detector 2!], combined with the output of the sharp discriminator i3, provides the output characteristic curve shown by Fig. 4.

The total band Width of the combined discriminators extends over a frequency band of 2.4 mc.-1.6 mc. which equals 0.8 mc. or 800 kc. There is seen to be sufficient gain at 500 kc. to cause the control system which will be described in the following, to operate satisfactorily. The combined discriminators have substantially the same cross-over characteristic as that of the sharp discriminator alone.

Referring now to Fig. 6, the RF generator It is a conventional vacuum tube oscillator using the two, push-pull vacuum tubes 35, the plates of which are connected through the tank inductance 3| to a positive terminal (B+) of a plate voltage supply source which is not illustrated. The tank inductance is inductively coupled to the load 32 which may be a dielectric, inductive, or other load. The tank circuit is tuned by the variable capacitors 35, the rotors of which are adapted to be rotated by the electric motor 35 when the oscillator departs from its assigned frequency of 13.56 me. The tuning range provided by the capacitors 34 extends about 250 kc. each side of 13.56 mc.

The pick-up coil 36 is inductively coupled to the inductance 3|, and is connected at one end to ground, and at the other end through the Wire 31 and the coupling capacitor 38, to the grids 4| and 42 of the two similar pentagrid converter tubes 39 and A0.

The resistor 43, connected between the grid 4| of the tube 39 and ground, and the resistor 44 connected between the grid 42 of the tube 40 and ground, are bias resistors for the tubes. The tube 39 performs the functions of the oscillator I5 and the mixer M of Fig. 5, and the tube 40 performs the function of the oscillator l8 and the mixer ll of Fig. 5.

The grid 45 of the tube 39 is connected through the piezo-electric crystal 46 to ground, the grid resistor 41 being shunted across the crystal. The cathode of the tube 39 is connected through the bias resistor 48 to ground, the bypass capacitor 49 being shunted across the resistor 48. The screen grid 50 of the tube 46 is connected through the inductance 5| to 3+.

The oscillator portion of the tube 39 employs a tuned-grid tuned-anode circuit. The crystal 46 is in the tuned grid circuit and determines the frequency of oscillation. The screen grid 58 serves as an anode in the oscillator circuit in which the inductance 5I is tuned to the frequency of the crystal 46, which in this case is 11.56 mc. The feed-back is through the inter-electrode capacity.

The plate of the tube 39 is connected through the inductance 52 to 3+, the capacitor 53 being shunted across the inductance 52 and forming therewith a parallel-resonant circuit which is tuned slightly higher than 2 mc., the difference frequency between the 11.56 mc. frequency of the oscillator portion of the tube 39, and the mean frequency 13.56 mo. of the generator I8.

This difference frequency voltage from the plate circuit of the tube 39 is supplied to the input of the band-pass filter I6, through the capacitor 54 to the parallel circuit consisting of the capacitor 55 shunted across the inductor 56, and which is tuned to a frequency slightly lower than that to which the plate circuit of the tube 39 is tuned. The inductor 56 is connected at one end to the control grid of the tube amplifier 51, and at the other end to ground through the grid leak 58 and its bypass capacitor 59. The voltage developed across the inductor 56 is applied to the control grid of the tube 51.

The cathode of the tube 51 is connected to ground through the bias resistor 68 across which is shunted the bypass capacitor 6|. The screen grid of the tube 51 is connected through the resistor 64 to 3+, and is connected through the bypass capacitor 65 to ground. The plate of the tube 51 is connected through the inductor 66 to 13+, the capacitor 91 being shunted across the inductor 66. The parallel circuit formed by the inductor 66 and the capacitor 61 is resonant at approximately the same frequency as the plate circuit of the tube 39.

The coupling capacitor 69 is connected to the plate of the tube 51, to the diode plate 13 of the broad-band discriminator diode tube 28, and to one side of the inductor 18, the other side of which is grounded. The capacitor H is shunted across the inductor 18. The load resistor 14 is connected to the diode cathode 88 of the tube 28 and to ground, the bypass capacitor 15 being shunted across the resistor 14. A direct-current voltage is developed across the load resistor 14 which is proportional to the radio frequency voltage appearing across the inductor 18.

The grid 63 of the tube 48 is connected through the piezc-electric crystal 16 to ground, the grid resistor 11 being shunted across the crystal. The cathode of the tube 48 is connected through the bias resistor 18 to ground, the bypass capacitor 19 being shunted across the resistor 18. The screen grid 88 of the tube 48 is connected through the inductance 8| to 3+.

The oscillator portion of the tube 48 employs a tuned-grid tuned-anode circuit. The crystal 16 is in the tuned grid circuit and determines the frequency of oscillation. The screen grid 88 serves as an anode in the oscillator circuit in which the inductor BI is tuned to the frequency of the crystal 16, which in this case is 15.56 mc. The feed-back is through the inter-electrode capacity.

The plate of the tube 48 is connected through the inductor 82' to 3+, the capacitor 83 being shunted across the inductor 82 and forming therewith a parallel-resonant circuit which is tuned slightly higher than 2 mc., the different frequency between the 15.56 mc. frequency of the oscillator portion of the tube 48, and the mean frequency 13.56 mc. of the generator I8.

This difference frequency voltage from the plate circuit of the tube 48 is supplied to the input of the band-pass filter I9, through the capacitor 84 to the parallel circuit consisting of the capacitor 85 shunted across the inductor 86, and which is tuned to a frequency slightly lower than that to which the plate circuit of the tube 48 is tuned. The inductor 86 is connected at one end to the control grid of the amplifier tube 81, and at the other end to ground through the grid leak 88 and its bypass capacitor 89. The voltage developed across the inductor 86 is applied to the control grid of the tube 81. i

The cathode of the tube 81 is connected to ground through the bias resistor 98 across which is shunted the bypass capacitor 9|. The screen grid of the tube 81 is connected through the resistor 94 to 3+, and is connected through bypass capacitor 95 to ground. The plate of the tube 81 is connected through the inductor 96 to 3+, the capacitor 91 being shunted across the inductor 96. The parallel circuit formed by the inductor 96 and the capacitor 91 is resonant at approximately the same frequency as the plate circuit of the tube 48.

The coupling capacitor 99 is connected to the plate of the tube 81, to the diode plate 12 of the broad-band discriminator diode tube 28, and to one side of the inductor I88, the other side of which is grounded. The capacitor I8I is shunted across the inductor I88. The load resistor I84 is connected to the diode cathode I83 of the tube 28 and ground, the bypass capacitor I85 being shunted across the resistor I84. A direct-current voltage is developed across the load resistor I64 which is proportional to the radio frequency voltage appearing across the inductor I88.

The output voltages of the two band-pass filters are supplied apart in phase into the detector tube 28 so that the characteristic curve of Fig. 3 is inverted and combined with the characteristic curve of Fig. 2 so as to form a curve below the frequency base line of the upper curve which is similar to the upper characteristic curve but inverted, the two curves merging together.

Radio frequency from the generator I8 is also applied from the pick-up coil 36 through the wire 31 and the coupling capacitor H8 to the grids III and H2 of the pentagrid converter tubes H3 and H4 respectively, in the mixer II, and is combined in the mixer with the oscillations supplied by the crystal controlled frequency standard I2.

The frequency standard I2 includes a triode vacuum tube H5, with the piezo-electric crystal H6 connected between its grid and cathode, and providing a tuned grid circuit. The grid resistor I81 is connected between the grid and cathode of the tube H5, the cathode being grounded. The plate of the tube I I5 is connected to 13+ through the plate inductor H1 across which the capacitor I I8 is shunted, the inductor I I1 and the capacitor I I8 providing an output circuit tuned to the frequency of the crystal I I6 which in this case is 13.56 mc., the mean assigned frequency of the RF generator I8. Feed-back is through the interelectrode capacitance.

The output of the oscillator tube H5 is connected through the series-connected coupling capacitors I82 and H9, to the grid I28 of the tube I I3, and through the coupling capacitor I 82 and the resistor I2I to the grid I22 of the tube H4. The midpoint connection of the capacitors I02 and H9 is connected through the resistor I09 to ground. The grid I29 of the. tube H3 is connected through the grid resistor I08 to ground. The capacitor IIS} and the resistor I58 produce a voltage at grid I leading by 45 that appearing across resistor I99. The resistor I2I and input capacitance (not shown) of tube II produce a voltage at grid I22 lagging by 45 that appearing across resistor I69. Hence, a phase shift of 90 results between the voltage provided to the grid I20 of the mixer tube H3 and that applied to the grid I22 of the mixer tube I I4.

The output circuits of the mixer tubes I I3 and I I4 have RF currents flowing therein which have a frequency equal to the difference between the 13.56 mc. voltage from the oscillator H5 and the 13.56 main) voltage from the generator iii, the RF current in the output circuit of the tube I I3 being displaced 90 from that in the output circuit of the tube H5.

The plate of the mixer tube II3 is connected through the resistor I25 to 3+, and its screen grid I26 is connected through the resistor I2'i to 3+, is connected through the bypass capacitor I29 to ground, and through the resistor I28 to the cathode of the tube H3. The cathode of the tube I I3 is connected through the bias resistor I23 to ground. The capacitor I29 bypasses the screen I 26 to ground.

The plate of the mixer tube H3 is also connected to the input of the sharp discriminator I3, through the coupling capacitor I36 which is connected to the control grid of the amplifier tube I3I, the plate of which is connected through the resistor I32 to 3+, and the screen grid of which is connected through the resistor I33 to B+, the bypass capacitor I35 being connected between the screen grid and ground. The grid resistor I34 is connected between the control grid of the tube I3I and its cathode which is grounded.

The plate of the amplifier tube ISI is connected through the coupling capacitor I36 to the control grid of the phase-splitter tube I31, the plate of which is connected through the resistor I38 to 3+. The cathode of the tube I3? is connected through the series-connected resistors I39 and I39 to ground, the junction-point connection of the resistors I35 and IE9 being connected to the grid resistor Mil, and through the capacitor IM to one end of the resistor I42 nd to one diode plate M oi the sharp discriminator diode tube I 25. The resistor I22 is connected in series with the similar resistor I43 which is connected to the diode plate M9 of the diode I 15. The junction point of the resistors 32 and I43 is connected to the junction point of the seriesconnected, load resistors I4? and I138 which are connected to the two cathodes of the diode I455.

The plate of the phase-splitter tube I37 i connected to the diode plate M9 of the tube I45 through the coupling capacitor I59 which has the same value as the capacitor It I.

The voltages at the plate of the phase-splitter tube l3? which are supplied to the plate I49 of the diode I45, are 183 out-of-phase with the corresponding voltages at its cathode which are supplied to the diode plate Id? of the diode I 55, so that the voltages at the two diode plates are 180 :apart-in-phase.

The plate and the screen grid I79 of the mixer tube II are connected through the resistors I and I853 respectively, to 13+. The resistor I82 connects the screen grid of the tube H4 to its cathode. The bias resistor 33 connects the 8 cathode to ground. The grid resistor I connects the grid H2 of the tube I I4 to ground. The capacitor I85 bypasses the screen I'IQ to ground.

The plate of the mixer tube H4 is connected to the input of the sharp discriminator I3, through the coupling capacitor I52 which is connected to the control grid of the amplifier tube I53. The plate of the tube H4 is connected through the resistor I55 to 3+, and its screen grid are is connected through the resistor I80 to B+, the bypass capacitor I8I being connected between the screen grid and ground. The resistor I32 is connected between the cathode of the tube EM and its screen grid. The bias resistor I83 is connected to the cathode of the tube did and to ground.

The grid resistor I56 connects the control grid of the tube amplifier I53 to its cathode and to ground. The plate and the screen grid of the tube I53 are connected throu h the resistors I57 and I58 respectively, to 3+. The screen grid is connected through the bypass capacitor I5 3 to ground. The plate of the tube I53 is also connected through the coupling capacitor I59 to the junction points of the resistors Hi2 and I43 and of the resistors I41 and M3.

The two outputs of the phase-splitter tube I31 are supplied 180 apart in phase to the diode plates of the tube I 35, and the output of the tube 553 is supplied to the diode plates apart in phase from the outputs of the phase-splitter tube.

The voltages at the input of the diode tube i 15 may be best understood by reference to Fig. '7. The solid line, voltage vector E1 represents the voltage from the plate of the phase-splitter tube I31. The equal and opposite, solid line, voltage vector E2 represents the voltage from the cathode of the phase-splitter tube. These equal voltages are seen to be apart in phase. The solid line vector E3 represents the voltage from the plate of the amplifier tube I53 and is seen to be 90 apart in phase from the vectors E1 and E2.

When the diiierence between the frequency of the RF generator I3 is within its tolerance limits, the voltages of opposite phase from the phasesplitter tube, and the voltage from the amplifier tube I53 are in phase quadrature, as illustrated by the solid line vectors of Fig. 7, so that the voltage drop across the load resistors I41 and M8 connected between the cathodes of the diode tube 4:35 is substantially zero.

The capacitors IlI and I55, and the resistors I52 and I43 in the output circuit of the phasesplitter tube I3? have such values that when the frequency of the RF generator exceeds the tolerance limits, the opposite phase voltages E1 and E2 shift in phase with respect to the quadrature voltage Ea, an amount proportional to the difiference frequency between the RF generator re and the frequency standard I2, as illustrated by the dashed line vectors of Fig. 7. The output of one or the other of the diode sections of the tube then predominates, and a voltage of one polarity or the other is impressed between the resistors It! and H38 interconnecting its cathodes.

The cathodes of the diode I45 are connected to ground through the bypass capacitors I60. The cathodes of the broad-hand discriminator diode 25 are connected to the corresponding cathodes of the sharp discriminator diode I45 so hat the outputs of the two diodes are superimposed to provide the overall characteristic curve shown by Fig. 4.

9 The cathodes of the diodes 26 and I45 are connected to the control grids of the push-pull amplifier tubes I6I and I62, the grid resistors I63 being connected between the control grids of the tubes I6I and I62 and ground, the screen grids of the tubes I6I and I62 being connected through the resistor I64 to B+. The plates of the tubes.

I6I and I62 are connected to the series-connected, relay energizing windings I66 and I65 respectively, of the relay I67, the junction connection of the windings being connected to B+. 1

The armature I68 of the relay I6? is connected to one side of the battery ITI, the other side of which is connected to the reversible electric motor 35. The relay armature I68 is movable between the contacts I69 and Ill], and the field winding of the motor 35 is so connected to its armature and to the contacts I69 and I10 in a conventional motor reversing circuit, that when the relay armature touches the contact I69, the battery I'II rotates the motor in one direction, and when the relay armature touches the contact I16, the battery is reversely applied to the motor field, causing the motor to rotate in the opposite direction.

As a result of the actions in their input circuits as described in the foregoing, corresponding cathodes of the diode tubes 26 and I45, those cathodes connected to the control grids of the amplifier tube I6I will provide voltages on the grids of the tubes I6I and I62 which will cause the tube I6I to draw more plate current than the tube I62, and to thereby energize the Winding I66 of the relay I67 more strongly than the relay winding I65 is energized, when the frequency of the RF generator I increases above its tolerance levels. This will cause the relay armature I68 to touch the contact I69, causing the field and armature of the motor 35 to be connected together and to the battery I'II in such a way that the motor will revolve to rotate the rotor of the capacitor 34 in a direction to increase the capacity in the tank circuit of the RF generator, until its frequency is decreased to its mean assigned value.

When the frequency of the RF generator decreases below the tolerance limits, the diode cathodes will provide voltages at the grids of the amplifier tubes I6I and I62 which will cause the tube I62 to draw more plate current than the tube I 6I and to thereby energize the relay winding I65 more strongly than the relay Winding I66 is energized. This will cause the relay armature I68 to strike the contact I causing-the field and armature of the motor 35 to be connected to the battery I?! in a direction to cause the motor 35 to rotate the rotor of the capacitor 34 in a direction to decrease the capacity in the tank circuit of the RF generator, until its frequency is increased to it mean assigned frequency.

When the RF generator is operating within the tolerance limits of its assigned frequency, both of the amplifier tubes I6I and I62 will be conductive to the same extent so that their plate currents will be balanced with both relay windings energized the same, causing the relay armature to be spaced from both contacts I68 and I69 so the motor 35 will be deenergized, and the rotors of the tank capacitor 34 will remain stationary.

I claim as my invention:

1. A frequency stabilizing system for a radio frequency generator, comprising a first oscillator operating at the mean assigned frequency of said generator, a first mixer connected for receiving energy from said generator and from said first oscillator, a sharp discriminator connected to said first mixer, a second oscillator operating at a frequency above said assigned frequency, a second mixer connected for receiving energy from said generator and from said second oscillator, a first band-pass filter connected to said second mixer, a third oscillator operating at a frequency below said assigned frequency, a third mixer connected for receiving energy from said generator and from said third oscillator, a second band-pass filter connected to said third mixer, means for inverting the output of one of said filters and for combining the inverted output with the output of the other filter, reactance adjusting means for tuning said generator, and means for comiecting the outputs of said sharp discriminator and the combined outputs of said filters together and to said reactance adjusting means so as to cause same to retune said generator to said assigned frequency when it departs therefrom.

2. A frequency stabilizing system for a radio frequency generator, comprising a first oscillator operating at the mean assigned frequency of said generator, a first mixer connected for receiving energy from said generator and from said first oscillator, a sharp discriminator connected to the output of said first mixer, a second oscillator operating at a frequency on one side of said assigned frequency, a second mixer connected for receiving energy from said generator and from said second oscillator, a third oscillator operating at a frequency on the other side of said assigned frequency, a third mixer connected for receiving energy from said generator and from said third oscillator, means for inverting the output of said third mixer and for combining the inverted output with the output of said second mixer, reactance adjusting means for turning said generator, and means for connecting the output of said sharp discriminator and the combined outputs of said second and third mixers to said reactance adjusting means so a to cause same to retune said generator to said assigned frequency when it departs therefrom.

3. A frequency stabilizing system for a radio frequency generator, comprising a first oscillator operating at the mean assigned frequency of said generator, a first mixer connected for receiving energy from said generator and from said first oscillator, a sharp discriminator having its input connected to the output of said first mixer, said sharp discriminator including means for rectifying its output, a second oscillator operating at a frequency at one side of said assigned frequency, a second mixer connected for receiving energy from said generator and from said second oscillator, a band-pass filter having its input connected to the output of said second mixer, a third oscillator operating at a frequency on the other side of said assigned frequency, a third mixer connected to receive energy from said generator and from said third oscillator, a second band-pass filter having its input connected to the output of said third mixer, means for in verting the output of one of said filters and for combining the inverted output with the output of the other of the filters and for rectifying the combined outputs, reactance adjusting means for tuning said generator, and means for combining the rectified outputs of said filters and of said sharp discriminator and for applying same to said reactance adjusting means for retuning said generator to said assigned frequency when it departs therefrom.

4. A frequency stabilizing system as claimed in claim 3 in which the second and third oscillators operate at frequencies which are spaced the same number of cycles from the assigned frequency of the generator.

5. A frequency stabilizing system as claimed in claim 4 in which the band-pass filters are similar.

6. A frequency stabilizing system as claimed in claim 3 in which the band-pass filters are similar.

'7. A broad-band discriminator for use with a radio frequency generator having an assigned frequency, comprising a first oscillator operating at a frequency on one side of said assigned frequency, a first mixer connected to receive energy from said generator and from said first oscillator, a band-pass filter having its input connected to the output of said first mixer, a second oscillator operating at a frequency on the other ide of said assigned frequency, a second mixer connected to receive energy from said generator and from said second oscillator, a second band-pass filter having its input connected to the output or" said second mixer, and means for inverting the output of one of said filters and for combining it with the output of the other filter.

8. A broad-band discriminator for use with a radio frequency generator having an assigned frequency, comprising a first oscillator operating at a fixed frequency higher than said assigned frequency, a first mixer connected to receive energy from said generator and from said first oscillator, a first band-pass filter having its input connected to the output of said first mixer, a second oscillator operating at a fixed frequency which is below said assigned frequency by the same number of cycles the operating frequency of said first oscillator is above said assigned frequency, a second mixer connected to receive energy from said generator and from said second oscillator, a second band-pass filter having its input connected to the output of said second mixer, and means for inverting the output of one of said filters and for combining its inverted output with the output of the other filter.

9. A broad-band discriminator as claimed in claim 8 in which the filters are similar.

10. A frequency stabilizing system for a radio frequency generator, comprising a source of fixed frequency, a first mixer connected for receiving energy from said generator and from said source, a sharp discriminator having its input connected to the output of said first mixer,

a first, fixed frequency oscillator operating at a energy from said generator and from said oscillator, a second, fixed frequency first fixed frequency oscillator operating at a frequency spaced said predetermined number of cycles below said assigned frequency, a third mixer connected to receive energy from said generator and from said second fixed frequency oscillator, means for inerting the output of one of said second and third mixers and for combining the inverted output with the output of the other thereof, reactance adjusting means for tuning said generator, and means for connecting the output of said sharp discriminator and the combined output of said second and third mixers to said adjusting means for retuning said generator to its assigned frequency when it departs therefrom.

11. A frequency stabilizing system for a radio frequency generator, comprising a source of fixed frequency, a first mixer having its input connected for receiving energy from said generator and from said source, a sharp discriminator having its input connected to the output of said mixer, a first, fixed frequency oscillator operating at a frequency spaced a predetermined number of cycles above the mean assigned frequency of said generator, a second mixer having its input connected for receiving energy from said generator and from said first fixed frequency oscillator, a first band-pass filter having its input connected to the output of said second mixer, a second fixed frequency oscillator operating at a frequency spaced said predetermined number of cycles below said assigned frequency, a third mixer having its input connected to receive energy from said generator and from said second fixed frequency oscillator, a second band-pass filter having its input connected to the output of said third mixer, reactance adjusting means for tuning said generator, means for inverting the output of one of said filters and for combining the inverted output with the output of the other filter, and means for connecting the output of said sharp discriminator and the combined output of said filters to said adjusting means for retuning said generator to its assigned frequency when it departs therefrom.

12. A frequency stabilizing system as claimed in claim 11 in which the band-pass filters have similar characteristics.

SHELDON I. RAMBO.

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

UNITED STATES PATENTS Number Name Date 2,425,981 Bard et al Aug. 19, 1947