US2456763A - Frequency stabilization - Google Patents

Frequency stabilization Download PDF

Info

Publication number
US2456763A
US2456763A US488582A US48858243A US2456763A US 2456763 A US2456763 A US 2456763A US 488582 A US488582 A US 488582A US 48858243 A US48858243 A US 48858243A US 2456763 A US2456763 A US 2456763A
Authority
US
United States
Prior art keywords
frequency
oscillator
voltage
discriminator
arrangement
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US488582A
Other languages
English (en)
Inventor
Ziegler Marc
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hartford National Bank and Trust Co
Original Assignee
Hartford National Bank and Trust Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to FR957704D priority Critical patent/FR957704A/fr
Priority to NL69105D priority patent/NL69105C/xx
Application filed by Hartford National Bank and Trust Co filed Critical Hartford National Bank and Trust Co
Priority to US488582A priority patent/US2456763A/en
Priority to CH265358D priority patent/CH265358A/de
Application granted granted Critical
Publication of US2456763A publication Critical patent/US2456763A/en
Priority to DEP26533D priority patent/DE809818C/de
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03C—MODULATION
    • H03C3/00—Angle modulation
    • H03C3/02—Details
    • H03C3/09—Modifications of modulator for regulating the mean frequency

Definitions

  • Z2 Z0 is f I I I CONTROLLED REACTANCE OSCILLATOR TUBE MODULATOR VOLTAGE MVIXER V AMPLIFIER mscRmuM/noR 30 I I l J I I2 32 l4 CRYSTAL OSCILLATOR MARC 'ZIE INVENTOR.
  • This invention concerns an improvement in frequency stabilization, especially useful for application in frequency modulation devices.
  • the systems for generating frequency modulated waves can be divided in two general classes.
  • the systems of the first class operate with an oscillator, the frequency of which is very stable, owing to the special physical properties of the oscillator elements used and the circuit chosen, as in the case, for example, of a quartz oscillator.
  • the proper frequency of the oscillator hence cannot be varied, but a signal derived from it can be applied to a phase shifting device so arranged that the output of that device is phaseshifted with regard to the input, the phase shift being a function of a variable controlling magnitude applied to that device.
  • the variable controlling magnitude itself is a function of the intelligence and, by convenient choice and adaption of these functions, the resulting phase modulated signal can be considered as being frequency modulated by the intelligence.
  • the central frequency is determined by the oscillator, which, as stated above, is inherently very stable, and therefore the central frequency is also very stable and constant, which is the principal feature of the system.
  • a phase modulation of about 20 degree amplitude is impressed on a signal of relatively low frequency and high frequency stability, and the resultant phase modulated signal is multiplied several times in frequency and passed, if necessary, through one or more frequency changer stages in order to obtain the modulated signal of desired central frequency and desired frequency excursion.
  • the frequency of the oscillator is made variable by the variation of one of its circuit constants, for example its capacity, proportionately to the in- 6 Claims. (Cl. 179-1715) tellige-nce.
  • the introduction of a variable element in the oscillator circuit reduces generally its inherent stability and constancy, so that it has proved essential to stabilize the central frequency of the oscillation with respect to a very constant frequency derived from a crystal oscillator, by a convenient automatic frequency control (A. F. C.) arrangement.
  • the frequency controlling device of the A. F. C. link, and the frequency modulating arrangement can be combined, and generally some reactance tube circuit is chosen, capable of varying the circuit capacity of the oscillator as a function of the tension applied on the grid of the latter.
  • a first drawback is the relative sensitivity of the system to undesired modulating effects, microphonics and the like, inherent in the modulability of the oscillator frequency.
  • the invention to be described hereinbelow has for its object to eliminate thesecond drawback inherent in frequency modulation systems based on direct modulation of a modulable oscillator and automatic stabilization of the central frequency of the same.
  • This second drawback consists therein that the correct frequency controlling action of the known A. F. C. arrangements, which can subsist only within a certain range of frequency on both sides of the desired frequency, is completely disturbed as soon as a certain frequency limit is momentarily passed.
  • the correct adjustment of the A. F. C. then is lost completely and this adjustment does not re-establish itself, when the cause of such a momentarily ()VellOadihg of the A. F. C. device disappears.
  • Fig. 1 represents the discriminator characteristic and the frequency control characteristic for a stabilizing arrangement using a counter-type frequency discriminator.
  • Fig. 2' is a block diagram of the improved frequency stabilizing arrangement.
  • Fig. 3 is a diagram of a complete frequency modulating; unit incorporating the new invention.
  • Fig. 4 is a graph showing the stabilizing effect of the arrangement of Fig. 3.
  • the automatic frequency stabilizing arrangement to be considered is similar to that shown in Fig. 2, except that in the prior arrangement the discriminator id is connected directly to the reactance tube 20 through the point of injection 39 of the modulating voltage.
  • the beat frequency signal obtained by mixing a voltage derived from the local oscillator 22 to be stabilized (frequency fosc) and a voltage from a crystal oscillator (frequency fcrys) is applied, if desired after passing through an amplifier 32, to the input of a frequency discriminator ill.
  • the output of the frequency discriminator M is zero when the beat frequency has the desired value fint and the correct operation consists therein that any deviation from the desired frequency produces a positive or negative potential at the output of the discriminatcr M which, when applied to the frequency 4 controlling arrangement of the local oscillator 22, produces a controlling action in the opposite sense, which tends to bring the frequency of that local oscillator to the right value again.
  • the graphs of Fig. 1 show both how the output voltage V of the frequency discriminator varies as a function of the oscillator frequency f, and how the frequency of the oscillator varies as a function of the input voltage of the reactance tube.
  • the first mentioned curve will be called the discriminator characteristic and the second one the frequency control characteristic.
  • the graph would have the form of two straight lines, passing through zero both the times that the beat frequency fosc-fcrys is equal to the intermediate frequency fint, that is, in points C and E, and meeting for fosc jcrys in the point D at a value VB of the voltage V equal to the compensating potential applied to the frequency counter.
  • a frequency counter saturates for high firequencies, a deviation from the straight line as indicated by the broken curves CH and EI would be obtained.
  • the transmission of the beat frequency signal from mixer to frequency discriminator is frequency dependent.
  • the shape of the curve is quite normal: the slope is maximum at the working point and the frequency variation obtainable is limited at both ends of the characteristics of the reactance tube.
  • the reactance tube and oscillator arrangement are so designed that the sensibility in kc. of oscillator frequency per volt of input voltage is large, e. g. of the order of hundreds of kc./volt. That means that the total range may easily be a thousand kc. or more, which explains why the limits of the frequency control characteristic are so far removed as compared with the central part of the figure.
  • the two characteristic curves have five intersection points, K, L, C, M, N. In each of these points the condition is satisfied that the output of the r iscriminator for the existing oscillator frequency is equal to the voltage required at the input of the reactance tube for obtaining that frequency.
  • the working point C is a point of stable equilibrium: any small deviation of the frequency from the value at that point produces a counteracting effect which tends to keep the frequency at that value. It can be observed that point N is also .a point of stable equilibrium. Points L and M on the other hand are points of unstable equilibrium: if the oscillator is adjusted at one of these points any small frequency variation will create a voltage variation of the discriminator output which causesthat frequency to be moved away still more.
  • point K happens to be a point of stable equilibrium, due to the returning slope of the frequency control characteristic.
  • the oscillator frequency can jump from the desired value of point C to either point N or point K and be kept there automatically, even when the perturbing cause has disappeared.
  • the frequency has increased momentarily to a value larger than that corresponding to point M the controlling voltage produced starts to shift it further until the stable adjustment N is obtained.
  • the frequency decreases below the frequency of point L it shifts automatically towards point K where it is stabilized. In both cases the desired adjustment is lost and cannot re-esta'olish itself automatically.
  • the modulating means for example the microphone circuit
  • the modulating means may produce abnormal peak voltages (switching-on or switchingoif voltages) many times larger than the modulating voltage required for maximum excursion. Any other reason for which the voltage applied to the reactance tube may momentarily become abnormally large (as for instance, transitory phenomena after switching on) may similarly cause a frequency deviation leading to a shift to operation to the points K or L.
  • the oscillator frequency also may happen to alter by reason of some change in the oscillator circuit as, for example, capacity or inductance change by hand effect, mechanical shock and the like. Any of these causes may, therefore, have as a consequence that the correct adjustment of the frequency stabilization arrangement is lost if no special measures are taken.
  • a manual readjustment is
  • the object of the present invention is to eliminate all possibilities of adjustments other than the desired one, by conveniently limiting the range of the frequency control, so that the desired point becomes the sole intersection of the frequency control and frequency discrimination curves.
  • the oscillator frequency control is limited to one point of stable operation without loss of sensitivity by interposing a limiter stage between the reactance tube and the source of voltage for controlling the reactance tube.
  • An arrangement is accordance with the invention is shown schematically in the blocked diagram of Fig. 2.
  • the arrangement shown in Fig. 2 consists of a crystal oscillator I ll, mixer l2, discriminator I l, limiter l8, reactance tube 2t, and a controlled oscillator 22 serving as the source of frequency modulated waves and .to which a transmitting antenna is coupled with or without the interposition of a power amplifier and/or a frequency multiplying stage in accordance with the known practice (see Fig. 3)
  • the injection of a modulating voltage such as a voltage representing intelligence, is indicated at 30 between the discriminator and the limiter.
  • an amplifier 32 may be included between mixer l2 and discriminator i i.
  • a frequency control characteristic such as curve PCQ of Fig. 1 may be obtained.
  • the limited characteristic does not prevent the frequency of the oscillator from being moved somewhat faraway from the desired value by one of the causes mentioned hereinabove, but as soon as such an extraordinary disturbing action disappears, the frequenc of the oscillator automatically retur to the desired value.
  • Fig. 3 the complete circuit of a frequency modulation arrangement is given wherein a limitation of the frequency control is. realized, adapted to the case discussed, for which, as has been seen, limitation at both sides or the working point is required.
  • the circuit comprises a quartz controlled oscillator i'zla corresponding to the crystal oscillator of Fig. .2, a mixing arrangement lZa corresponding to the mixer iii of Fig. 2, an inductance type frequency counter [4a corresponding to the discriminator i l followed .by a filter I6, a limiter device indicated by the general reference number Hid shown as it in Fig. 2, a reactance tube 2PM corresponding to the reactance tube 26 of Fig. 2, second.
  • the oscillator 22a corresponding to the controlled oscillator 22, doubler and power output stages 2% and 26 respectively, antenna 28 and intelligence im pression means shown as a microphone circuit 30a shownas the block 38 in Fig. 2.
  • the intel- 7 ligence impression means is connected between the filler I6 and limiter [8a.
  • the reactance tube circuit 20a, the modulated oscillator 22a, the mixer lZa, the quartz controlled pilot oscillator [9a, and the R. F. amplifier 32a are entirely conventional and, if desired, may be replaced by other circuits well-known to the art.
  • the discriminator I ia is a counter type discriminator as disclosed in copending patent applications, Ser. No. 464,380 filed November 3, 1942, now U. S.
  • the discriminator is so arranged that pulses of like sign vary the current through an inductance to the same extent for each pulse and the output current developed by integrating the voltage impulses is proportional to the number of periods and hence is proportional to the frequency.
  • the output of the discriminator Ma is coupled to the limiter [8a and the modulator source 30a by means of the filter circuit Hi.
  • This filter circuit it may consist of a low pass filter such as is commonly used in discriminator circuits whereby the radio frequency and the audio frequency components are substantially completely attenuated to provide at the output of the filter a direct current potential whose value is a function of the variation in frequency from the desired predetermined value.
  • the constants of filter it are so adjusted that the radio frequency component is completely attenuated and the audio frequency component is passed through unattenuated so that the output of the filter comprises a direct current potential proportional to the frequency variations and a superimposed audio frequency component.
  • the limiting action is based on the cut-off of two triodes 34, 36, connected in series and shown as contained in one valve structure 33.
  • the limiter Mat is a two stage direct coupled amplifier comprising two triodes 34 and 36 which may be enclosed in one envelope 38.
  • the triode 34 which is directly coupled to the output filter I is so biased b the cathode resistor 39 that negative excursions of the applied voltage beyond a predetermined value are limited by reason of the grid cut-off characteristics of the tube.
  • the triode 36 is so biased by the voltage applied of the voltage applied to the control grid thereof beyond a predetermined value are similarly limited by reason of the grid cut-off characteristics of the tube. Since the voltage applied to the grid of triode 38 is in phase opposition to the voltage applied to the grid of tube 34, it will be seen that the triodes 34 and 39 produce limiting of the negative and positive peaks of the voltage obtained from the filter having amplitude values greater than a predetermined desired value. The limiting level is established by the bias voltages applied to the tubes 34 and 36.
  • the output of the limiter is obtained between ground and a point on the voltage divider comprising resistors 4!, 42 and 43 in series.
  • the range of a limiter stage cannot be smaller than a few volts and the sensitivity of the reactance tube itself may have to be reduced in order to get the frequency range required.
  • the decrease of reactance tube sensitivity is obtained by biasing the grid in the well-known manner as shown in Fig. 3 by interposing a resistor between ground and the cathode; the cathode in this way becomes positive which permits direct connection of the positive output of the limiter stage to the grid of the reactance tube.
  • the amplification obtained from the limiter more than compensates the loss in sensitivity of the reactance tube.
  • Frequency modulation arrangements operating with other typesof discriminators, may have a discriminator characteristic such that limitation of the frequency control is required on one side only of the working point. In such a case, the limiter stage is of course, of simpler design.
  • Fig. 4 is shown for the arrangement disclosed in Fig. 3, to what value any oscillator frequency deviations are reduced by the stabilizing arrangement. Horizontally is plotted out the frequency of the oscillator without stabilization,-
  • Fig. 3 shows a remarkable stability. This is partly because of the inclusion of circuit constants bringing about a negative feedback action obtained by choosing the cut-off frequency of the low pass filter so that all audio frequencies pass unattenuated, as disclosed in my copending U. S. Patent application Serial No. 482,439 filed April 9, 1943.
  • peak voltages obtained by connecting and disconnecting the microphone or by shaking the same, or violent shocks on the chassis are practically imperceptible at the receiving end of a communication system comprising a transmitter constructed according to the invention. Touching the oscillator grid stops the signal but the same comes in correctly as soon as the hand is removed.
  • the object of the invention is apparently full attained. I
  • Apparatus for generating a 'wave having a stable frequency comprising a main oscillator, a pilot oscillator, frequency controlling means connected to said main oscillator for maintaining the frequency of said main oscillator at a given frequency with respect to the frequency of the pilot oscillator, said frequency controlling means being responsive to a control potential and having a plurality of possible operating conditions of stable equilibrium, frequency mixing means coupled to said main and said pilot oscil- Large modulating lator, means to produce a control voltage substantially proportional to the frequency difference between said main and said pilot osci1- lators comprising a frequency responsive detector energized b said mixing means, and amplitude limiting means for said control voltage inserted between said frequency responsive detector and said frequency controlling means to restrict the operation of said frequency controlling means to only one of said conditions of stable equilibrium.
  • a frequency modulation system comprising a main oscillator for generating an adjustable frequency oscillation, means to control the frequency of said oscillator, modulation means connected to said frequency control means, a pilot oscillator, frequency mixing means connected to said main and said pilot oscillators, a frequency responsive detector connected after said frequency mixing means and comprising means for generating electrical impulses of substantially constant area for each of the half-periods of like sign of the difference oscillation between said main and said pilot oscillation, means to integrate said electrical impulses to produce a control voltage having a zero amplitude for a given mean value of said difference oscillation, means to apply said control voltage to said frequency controlling means to maintain the mean frequency of said main oscillation at a given frequency difference with respect to said pilot oscillation, and amplitude limiting means inserted between said frequenc responsive detector and frequency controlling means and connected after said modulation means to simultaneously restrict the frequency control range of said modulation means and said frequency responsive detector.
  • the steps comprising generating a control voltage having amplitude variations proportional to frequency variations of the difference frequency between said adjustable oscillation and a pilot oscillation, controlling the frequency of said source proportional to the amplitude of said generated Voltage, and limiting the generated voltage to amplitude variations within a given range of amplitude values at which said source is limited to frequency variations about only one condition of stable equilibrium.
  • the steps comprising generating a control voltage having amplitude variations proportional to frequency variations of the difference frequency between said adjustable oscillation and a pilot oscillation, controlling the frequency adjusting element by means of said generated potential to produce variations of the frequency of said source proportional to the amplitude variations of the generated potential, and limiting the generated potential to amplitude variations within a given range of amplitude values at which the frequency adjusting element is limited to operation at only one of said conditions of stable equilibrium.
  • the steps comprising mixing said adjustable oscillation and a pilot oscillation to produce an oscillation having a frequency equal to the frequency difference between said adjustable oscillation and the pilot oscillation, generating electrical impulses for each half-period of like sign of said difference frequency oscillation, integrating said electrical impulses to produce a control voltage having amplitude variations proportional to frequency variations of said difference frequency, controlling the frequency adjusting element to produce variations of the frequency of said source proportional to the amplitude variations of the control voltage, and limiting the control voltage to amplitude variations within a given range of amplitude values at which the frequency adjusting element is limited to operation at only one of said conditions of stable equilibrium.

Landscapes

  • Stabilization Of Oscillater, Synchronisation, Frequency Synthesizers (AREA)
  • Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)
US488582A 1943-05-26 1943-05-26 Frequency stabilization Expired - Lifetime US2456763A (en)

Priority Applications (5)

Application Number Priority Date Filing Date Title
FR957704D FR957704A (es) 1943-05-26
NL69105D NL69105C (es) 1943-05-26
US488582A US2456763A (en) 1943-05-26 1943-05-26 Frequency stabilization
CH265358D CH265358A (de) 1943-05-26 1947-08-07 Schaltung zum Konstanthalten der mittleren Frequenz eines frequenzmodulierten Hochfrequenzoszillators.
DEP26533D DE809818C (de) 1943-05-26 1948-12-24 Schaltung zum Konstanthalten der zentralen Frequenz eines frequenzmodulierten Oszillators

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US488582A US2456763A (en) 1943-05-26 1943-05-26 Frequency stabilization

Publications (1)

Publication Number Publication Date
US2456763A true US2456763A (en) 1948-12-21

Family

ID=23940249

Family Applications (1)

Application Number Title Priority Date Filing Date
US488582A Expired - Lifetime US2456763A (en) 1943-05-26 1943-05-26 Frequency stabilization

Country Status (5)

Country Link
US (1) US2456763A (es)
CH (1) CH265358A (es)
DE (1) DE809818C (es)
FR (1) FR957704A (es)
NL (1) NL69105C (es)

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2584780A (en) * 1948-10-28 1952-02-05 Rca Corp Frequency control circuit
US2593395A (en) * 1950-03-17 1952-04-15 Rca Corp High-frequency amplitude modulated transmitter
US2653243A (en) * 1948-08-17 1953-09-22 Westinghouse Electric Corp Automatic tuning of resonant circuits
US2717359A (en) * 1947-01-10 1955-09-06 Honeywell Regulator Co Measuring apparatus
US2768293A (en) * 1950-04-11 1956-10-23 Hartford Nat Bank & Trust Co Frequency-modulated oscillator with automatic frequency correction
US2778999A (en) * 1952-11-15 1957-01-22 Gen Precision Lab Inc Automatic frequency control for frequency modulated generators
DE1023797B (de) * 1956-03-21 1958-02-06 Siemens Ag Schaltung zur elektrischen Einrastung eines abstimmbaren Oszillators
US2848610A (en) * 1953-05-25 1958-08-19 Vitro Corp Of America Oscillator frequency control apparatus
US3078752A (en) * 1951-12-26 1963-02-26 Rca Corp Circuit for simulating vibrato effect by amplitude modulation of tone by sawtooth waveform
US3289096A (en) * 1964-09-21 1966-11-29 Jr Robert Noel Longuemare Crystal oscillator frequency stabilization system

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1847160A (en) * 1932-03-01 Frequency control
US2279659A (en) * 1937-04-13 1942-04-14 Rca Corp Frequency modulator
US2285044A (en) * 1941-03-11 1942-06-02 Rca Corp Wave length modulation system
US2315050A (en) * 1940-09-26 1943-03-30 Rca Corp Frequency modulation system
US2354827A (en) * 1941-05-23 1944-08-01 Rca Corp Frequency control
USRE22587E (en) * 1940-11-20 1945-01-02 Fixed frequency difference
US2377327A (en) * 1942-09-29 1945-06-05 Rca Corp Automatic frequency control system

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1847160A (en) * 1932-03-01 Frequency control
US2279659A (en) * 1937-04-13 1942-04-14 Rca Corp Frequency modulator
US2315050A (en) * 1940-09-26 1943-03-30 Rca Corp Frequency modulation system
USRE22587E (en) * 1940-11-20 1945-01-02 Fixed frequency difference
US2285044A (en) * 1941-03-11 1942-06-02 Rca Corp Wave length modulation system
US2354827A (en) * 1941-05-23 1944-08-01 Rca Corp Frequency control
US2377327A (en) * 1942-09-29 1945-06-05 Rca Corp Automatic frequency control system

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2717359A (en) * 1947-01-10 1955-09-06 Honeywell Regulator Co Measuring apparatus
US2653243A (en) * 1948-08-17 1953-09-22 Westinghouse Electric Corp Automatic tuning of resonant circuits
US2584780A (en) * 1948-10-28 1952-02-05 Rca Corp Frequency control circuit
US2593395A (en) * 1950-03-17 1952-04-15 Rca Corp High-frequency amplitude modulated transmitter
US2768293A (en) * 1950-04-11 1956-10-23 Hartford Nat Bank & Trust Co Frequency-modulated oscillator with automatic frequency correction
US3078752A (en) * 1951-12-26 1963-02-26 Rca Corp Circuit for simulating vibrato effect by amplitude modulation of tone by sawtooth waveform
US2778999A (en) * 1952-11-15 1957-01-22 Gen Precision Lab Inc Automatic frequency control for frequency modulated generators
US2848610A (en) * 1953-05-25 1958-08-19 Vitro Corp Of America Oscillator frequency control apparatus
DE1023797B (de) * 1956-03-21 1958-02-06 Siemens Ag Schaltung zur elektrischen Einrastung eines abstimmbaren Oszillators
US3289096A (en) * 1964-09-21 1966-11-29 Jr Robert Noel Longuemare Crystal oscillator frequency stabilization system

Also Published As

Publication number Publication date
FR957704A (es) 1950-02-24
CH265358A (de) 1949-11-30
DE809818C (de) 1951-08-02
NL69105C (es)

Similar Documents

Publication Publication Date Title
US2279659A (en) Frequency modulator
US2279660A (en) Wave length modulation system
US2338395A (en) Signal transmission system
US3393380A (en) Phase locked phase modulator including a voltage controlled oscillator
US2406309A (en) Frequency stabilization
US2456763A (en) Frequency stabilization
US2494795A (en) Frequency-detector and frequency-control circuits
US2441567A (en) Variable frequency oscillator
US2284266A (en) System for signaling by electromagnetic waves
US2470573A (en) Oscillator modulating system
US2410489A (en) Nonlinear frequency modulation signaling system
US1993395A (en) Signal generator
US2682640A (en) Arrangement for modulating electric carrier wave oscillations
US3831094A (en) Means to prevent coincidental phase modulation in an amplitude modulation transmitter
US2764681A (en) Frequency stabilized and controlled oscillatory system
US2486265A (en) Variable frequency oscillator
US2923893A (en) Phase
US2813977A (en) Frequency modulated oscillation generator
US3737809A (en) Modulated carrier frequency sources
US2315050A (en) Frequency modulation system
US2705321A (en) Percentage modulation system for proportional control
US3137816A (en) Transmitter automatic frequency control network including modulation cancelling means
US2509280A (en) Cathode-driven oscillator
US3075157A (en) Automatic rest frequency control for pulsed frequency modulated oscillator
US2489948A (en) Radio receiving system