US2481902A - Automatic frequency control circuit for frequency modulation television systems - Google Patents

Description

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o W0 40 1x 120 l k m W. E. BRADLEY AUTOMATIC FREQUENCY CONTROL CIRCUIT FOR FREQUENCY MODULATION TELEVISION SYSTEM Flled Aprll 10 1946 Sept. 13, 1949:

Patented Sept. 13,1949

AUTOMATIC FREQUENCY CONTROL CIR- CUIT FOR FREQUENCY MODULATION TELEVISION SYSTEMS William E. Bradley, Swarthmore, Pa., assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application April 10, 1946, Serial'No. 660,866

9 Claims. 1

This invention relates to frequency-modulation carrier wave systems, and more particularly to unsymmetrlcally modulated frequency-modulation systems wherein the modulated carrier signal includes different modulation components, one of which varies over a predetermined frequency deviation range of the carrier signal while another varies over a deviation range beyond the range of variation of the first component. A specific and important example of such a system is a frequency-modulation television system wherein the carrier wave, during the video intervals; varies within certain predetermined fixed frequency limits, while the carrier wave, during the synchronizing intervals, is shifted to a preassigned frequency beyond the range sf vari ation obtainin during the video intervals. For proper operation, it is essential that either the black level (i. e. the blanking signal level) or the synchronizing signal level of the modulated carrier be maintained substantially constant at its preassigned frequency value in order to preserve the D. C. component of the video signal.

In some instances, it is desirable or expedient to convert the modulated carrier to a different frequency. For example, the modulated carrier may be transmitted through one or more relay stations or transmitters, and the latter ma convert the modulated carrier from one frequency to another. This may be accomplished by a conventional frequency converter arrangement employing a beating oscillator. Such frequency conversion, however, may cause frequency drift of the converted signal due to the inherent tendency of the beating oscillator to drift or to have its frequency varied by some circuit condition.

The principal object of the present invention is to provide an automatic frequency control system adapted to maintain an unsymmetrically modulated frequency-modulated carrier wave within a preassigned wave channel.

A more specific object of the invention is to provide, in a frequency-modulation television system, an arrangement by means of which the black-level frequency may be maintained at a predetermined fixed value.

' 2 bined action of two forces, as hereinafter described.

A further object of the invention is to provide a novel automatic frequency control arrangement in a frequency-modulation television sys-.

trations which will facilitate a clear understand Referring particularly to Fig. 1, there is illustrated in block form an FM television relay sys tem comprising an amplifier I, a mixer stage 2, a' second amplifier 3, and a beating oscillator These elements may be of conventional form, and therefore, it is unnecessary to illustrate or describe them in detail. The amplifier 1 is adapted to receive the incoming frequency-modulated carrier signal, which is converted to a different frequency in the mixer stage 2 by the heterodyne action of the beating oscillator 4, as will 'be well understood. The amplifier 3 is adapted to transmit the converted modulated carrier, which may be transmitted for reception at the new frequency or may be transmitted to another relay station or transmitter. By way of example, it may be assumed that the modulated carrier received by the system in Fig. 1 has a band center frequency of me. and a maximum frequency deviation (from center frequency) of 10 mc. In such case, the amplifier I should be adapted to operate at a frequency of 75 mc. and should have a pass band of 20 me. It may be assumed further that the incoming signal is to be converted from '75 me. to mc., and that the oscillator 4 is adapted to operate at a frequency of mc. to give such frequency conversion. Accordingly the amplifier 3 may be adapted to operate at a frequency of 115 mc. and may have a pass band of 20 me.

' Fig. 2 illustrates the general character of the frequency-modulated carrier signal employed in an FM television system. In this illustration, the ordinate line represents different frequency values. The dot-and-dash line represents the whitelevel frequency Jw of the video component of the modulated carrier; the dashed line represents the preassigned peak frequency f5 of the synchronizing pulses P, and the dotted line represents the preassigned black or blanking level frequency fb- The video component V of the modulated carrier varies between the frequency limits in and fw. The synchronizing component comprises the time-spaced pulses P, the peaks of which should be accurately levelled along the line is. The purpose of the present invention is to effect automatic frequency control in a system employing such a signal, particularly in a system of the character shown in Fig. 1.

Automatic frequency control (A. F. 0.) systems adapted accurately to fix the mean frequency of frequency modulation transmitters of the type used in sound broadcasting are, of course, well known in the art. These known systems employ circuits responsive to the mean transmission frequency and are adapted to maintain the said mean frequency at a preassigned frequency value. These prior devices, are, however, not adapted for use in conjunction with frequency modulated television transmitters or receivers for the reason that there is no mean carrier frequency in any valid sense of the word. What would otherwise pass for the mean frequency is, in a television system, a function of the video signal and varies with the average tone of the picture being transmitted. Thus if the picture were very dark in tone the mean frequency would be very near the black level is, while if the picture were very light in tone the mean frequency would be found very near the white level ,fw. Thus in a frequency modulated television system the term mean frequency is meaningless and prior A. F. C. systems which rely for their operation upon the existence of a mean frequency are inapplicable.

In accordance with the present invention frequency control means are provided which rely for their operation not upon the existence of a mean frequency but rather upon the presence of a fixed, unvarying reference frequency occurring cyclically at a relatively high frequency rate. In practice this reference frequency preferably comprises either the blanking level frequency is or the synchronizing peak frequency is.

As previously stated, when a frequency modulated television signal is subjected to a frequency conversion process in a system such as illustrated in Fig. 1, the inherent tendency of the beating oscillator to drift may cause drift of the converted signal. In accordance with the present invention, this undesirable frequency drift of the converted signal is prevented by means of the arrangement now to be described. The arrangement will also, of course, compensate for frequency drift occurring at an earlier point in the system.

A deviator 5 is provided in conjunction with the oscillator 4 to vary the frequency thereof in. re-

sponse to a control voltage supplied by way ofconnection 6. The deviator 5 maytake the form of any conventional device which is adapted to vary the frequency of an oscillator in response to a control voltage. Such devices are well known, particularly in automatic frequency control arrangements. In accordance withthe present invention the deviator 5 is actuated by a control voltage supplied by the frequency-selective detector, or discriminator, arrangement designated as a. whole by the dotted line rectangle 1.

. diode load resistors.

Referring specifically to the detector arrangement, there is provided a resonant circuit 8, preferably of relatively low Q, tuned approximately to a frequency midway between the frequencies is and fw of the converted carrier signal. The circuit 8 is arranged toreceive a portion of the signal energy from the amplifier 3 to which it may be coupled in any suitable manner. For example, the said circuit may be connected to the anode of the last tub'e, represented at I2, through a suitable coupling condenser I3, the load impedance of the tube being represented at M. The resonant circuit 8 comprises a capacitance element 9, an inductance element l0, and a resistance element ll, all connected in shunt relation.

It will be understood that the resistance element I imparts the desired low Q characteristic to the circuit. A diode detector |5 has its cathode connected to the high potential side of the resonant circuit 8, while the diode anode is connected to one side of the resistance-capacitance load circuit [6. The latter is connected to the low potential side of resonant circuit 8 through the indicated ground connections.

A second resonant circuit preferably of relatively high Q, is also arranged to receieve a portion of the signal energy. This is conveniently accomplished by loose reactive coupling with the low Q resonant circuit 8, as indicated by the bracket and letter M. The resonant circuit comprises shunt-connected capacitance and inductance elements It and I9, respectively, and this circuit is tuned to a frequency just beyond the peak frequency of the synchronizing pulses, the purpose of which will appear presently. A second diode detector 20 has its anode connected to the high potential side of the resonant circuit l1, while the diode cathode is connected to a resistance-capacitance load circuit 2| which, in turn, is connected to the low potential side of the resonant circuit I! through the illustrated ground connections.

The time constants of circuits l6 and 2| are both large in comparison to the time interval between the synchronizing pulses, and unidirectional voltages of negative and positive polarity, respectively, are established thereacross. When the system is properly balanced, these voltages tend to be of the same order of magnitude, one of the voltages normally predominating slightly.

The opposite-polarity voltages established across circuits l6 and 2| are-applied to conductor 6 by way of resistors 22 and 23, which may have values substantially equal to or larger. than the The resistors 22 and 23 form a high resistance voltagedivider extending between the output terminals of the diode load circuits I6 and 2|. Since this voltage divider is center-tapped by the conductor 6, it will be apparent that an average of the output voltages of the two detectors is derived by way of conductor 6. Thus if the voltages were equal, the average voltage on conductor 6 would be zero. In a welldesigned system, the voltage on conductor 6 will not vary widely from such a balanced condition.

The operation of the system may be clearly understood with the aid of the illustration of Fig. 3. In that figure, there is shown the output or response characteristic of the frequency selective detector arrangement 1. Portion 24 of theillustrated curve represents the response characteristic of the low Q- circuit 8 and its associateddetector, while the sharply peaked portion" 25 represents the response characteristic of thehigh Q circuit IT and its associated detector.

As previously mentioned, the low Q circuit 8 is tuned to a frequency intermediate the frequencies flu and fw, while the high Q circuit I1 is tuned to a frequency ,fx just beyond the peak frequency is of the synchronizing pulses. To further facilitate an understanding of the operation, a portion of the modulated carrier signal is represented in association with the response curve.

In operation, assume that the negative voltage across the detector load circuit I6 is slightly greater than the positive voltage across circuit 2|, so that a small negative voltage is applied to the deviator '5. This small votlage tendsto change the frequency of the oscillator 4 by a small amount in a direction to shift the synchronizing pulses 1? toward the frequency value is. The force exerted by the small bias voltage may be likened to a nudging action. As a result of such action, the synchronizing pulse tips enter the region of substantial response of the high Q circuit I1.

This results in an increased positive output from the detector system 2ll-2l and a consequent decrease in the negative voltage applied to the deviator. Thus the action of the arrangement provided by the invention involves two opposing forces, one of which acts to nudge the synchronizing pulses in a direction to bring the other force intoplay. By the conjoint action of the two forces, the preassigned frequency limits in and jw of the video carrier signal are prevented from shifting to any appreciable extent, by effectively leveling the frequency modulated composite signal along the peaks of the synchronizing pulses. The very steep side slope of the response characteristic of the high Q circuit I! greatly enhances this leveling action.

In the design of the discriminator I, it is important that while the Q of the resonant circuit I! should be high compared to the Q of resonant circuit 8, it must not be so high as to result in unduly sustained ringing when shock excited @y the synchronizing signal or by sidebands of the video components of the carrier signal. In general the Q of the circuit I! is preferably held within such limits as will ensure a ringing duration of no more than a fraction (e. g. A; or less) of the horizontal synchronizing signal duration. In other words, the A.-C. time constant of the high Q circuit should be short compared to the duration of the synchronizing pulse.

The use of a differentially-connected frequency discriminator in the present invention is of great practical importance, in that through its use there is provided. in the A. F. C. system, a fixed reference frequency or zero crossover point (see Fig. 3) at which the discriminator output is substantially independent of the amplitude of the signal applied to the discriminator circuit 1. By so adjusting the discriminator components that this crossover point occurs near, and preferably between, the frequencies is and fb great accuracy of frequency levelling is provided, independently of signal strength.

In the system illustrated, only one frequency conversion is involved, but it will be apparent that such a system may involve more than one frequency conversion, and the invention may be utilized to prevent frequency drift at each point of conversion. It will also be apparent that the invention is not limited to the specific apparatus shown but is capable of various modifications within the scope of the appended claims.

I claim:

1. In a frequency modulation television sys- 6. tem, a carrier wave source, the carrier wav'e supplied by'said source being frequency modulated with both video and synchronizin intelligence, an oscillator of controllable frequency, means responsive to frequency variations within the deviation limits of the video intelligence components of said carrier wave for producing a first control voltage, said responsive means having a substantially flat frequency-response characteristic at frequencies within said video-intelligence deviation limits, means responsive to a frequency near the synchronizing-signal deviation limits of said carrier wave for producing a second control voltage, said last-mentioned responsive means having a steep frequency-response characteristic at frequencies near said synchronizing-signal deviation limits, and means for utilizing said control voltages differentially to control the frequency of said oscillator.

2. A frequency modulation television system as claimed in claim 1, characterized in that said first and second control voltage producing means, in combination, possess an output-versus-frequency characteristic having a reference frequency, within the deviation limits of said synchronizing signal, at which the differentially-combined control voltage output is substantially zero irrespective of the amplitude of said carrier Wave.

3. Ina frequency-modulation receiver; an automatic-frequency-control system comprising a source of 'asymmetrically-frequency-modulated carrier signal including different modulation components, one of said components varying within a preassigned carrier frequency range and another component extending outside said preassigned range; means including a beating oscillator for converting the carrier signal to a different frequency band; means for varying the frequency of said oscillator; means responsive to frequency variations of said first-named component of said converted carrier for so actuating said frequency-varying means as to urge the oscillator frequency in one direction, said responsive means having a substantially fiat frequencyresponse characteristic throughout the range of frequencies of said first-named component as converted; and means responsive to said other component of said converted carrier for so actuating said frequency-varying means as to urge the oscillator frequency in the other direction, said last-named responsive means having a steep frequency-response characteristic at frequencies in the region of the converted frequency of said other component.

4. In a frequency-modulation television system employing a carrier wave which is frequency modulated with signals including video signals and synchronizing pulses, the wave frequency of said synchronizing pulses being outside the wave band of said video signals, the location of said modulated wave in the frequency spectrum being a function of the operating frequency of a controllable oscillator; means responsive to frequency variations of the video component of the carrier wave for producing a control voltage, said responsive means having a substantially flat frequency response characteristic throughout the range of said video component of said carrier wave; means responsive to a frequency in the neighborhood of the frequency of the synchronizingcomponent of the carrier wave for producing another control voltage, said last-named responsive means having a steep frequency-response characteristic at frequencies in said neighborhood; and means for utilizing said control voltages differentially to control the frequency of saidoscillator so as to maintain the black-level frequency of the carrier wave substantially constant. l

5. In a frequency-modulation television system employing a carrier wave which is frequency modulated with'signals including video signals and synchronizing pulses, the wave frequency of said synchronizing pulses being outside the wave band of said video signals, the location of said modulated wave in the frequency spectrum being a function of the operating frequency of a controllable oscillator; means responsive to frequency variations of the video component of the carrier wave for producing a control voltage, said responsive means having a substantially flat frequency response characteristic throughout the range of said video component of said carrier wave; means responsive to a frequency just beyond the frequency of the synchronizing comfrequency of the synchronizing component of the carrier wave; and means for utilizing both of said control voltages to control the frequency of said oscillator so as to maintain the black-level frequency of the carrier wave substantially constant.

6. 'In a frequency-modulation television system employing a carrier wave which is frequency modulated with signals including video signals and synchronizing pulses, the wave frequency of said synchronizing pulses being outside the wave band. ofsaid video signals, the location of said modulated wave in the frequency spectrum being a function of the operating frequency of a controllable oscillator; means for varying the frequency of said oscillator; means responsive to the video components of the carrier wave for actuating said frequency-varying means so as to urge the oscillator frequency in one direction, said responsive means having a substantially fiat frequency response to said video components; and means responsive to synchronizing components of the carrier wave for actuating said frequencyvarying means so as to urge the oscillator frequency in the other direction, said last-named responsive means having a. steep frequency-response characteristic at frequencies near to said synchronizing components.

.7. In a frequency-modulation television sys tem employing a carrier Wave which is frequency. modulated with signals including video signals and synchronizing pulses, the Wave frequency of said synchronizing pulses being outside the wave band of said video signals, the location of said modulated wave in the frequency spectrum being a function of the operating frequency of a con trollable oscillator; means for varying the frequency of said oscillator; a frequency-variation response network having a substantially fiat frequency response to video frequency components of the carrier wave; a detector coupled to said network and adapted to produce a unidirectional control Voltage of certain polarity; a frequencyvariation response network having a steep frequency-response characteristic at frequencies just beyond the frequency of the synchronizing component of the carrier wave, said last-mentioned network being reactively coupled to said first-mentioned network; a detector coupled to said last-mentioned network and adapted to produce aunidirectional control voltage having .a'

polarity opposite that of the first-mentioned control voltage; and means for applying said control voltages to said frequency-varying means, the'polarity of said first-mentioned control voltage being suchas to urge the frequency of the synchronizing component toward the frequency to which said last-mentioned network is most responsive, while the second control voltage acts in opposition to such change.

8. In a frequency-modulation television system employing a carrier Wave which is frequency modulated with signals including video signals and synchronizing pulses, the wave frequency of said synchronizing pulses being outside the wave band 1' of said video signals, the location of said modulated wave in the frequency spectrum being a function of the operating frequency of a controllable oscillator; means for varying the frequency of said oscillator; a frequency-variation response network having a substantially flat frequency response to Video components of the carrier wave and arranged to receive a portion of the signal energy; a detector coupled to said network and adapted to produce a unidirectional control voltage of certain polarity; a frequency-variation response network having a steep frequencyresponse characteristic at frequencies just beyond the peak frequency of the synchronizing pulses of the carrier wave, said last-mentioned network also being arranged to receive a portion of the signal energy; a detector coupled to said last-mentioned network and adapted to produce a unidirectional control voltage having a polarity opposite that of said first-mentioned control voltage; and means for applying said control voltages to said frequency-varying means, the polarity of said first-mentioned control voltage being such as to urge the peaks of said synchronizin pulses toward the frequency to which said last-mentioned network is most responsive, while the second control voltage acts to prevent any substantial shift of said peaks.

9. In a frequency-modulation television system employing a carrier wave which is frequency modulated with signals including video signals and synchronizingpulses, the wave frequency of said synchronizing pulses being outside the wave band of said video signals, the location of said modulated wave in the frequency spectrum being a function of the Operating frequency of a controllable oscillator; a frequency discriminator responsive to at least a portion of th frequency range of said frequency-modulated carrier wave, said discriminator comprising elements including a frequency variation response network having a substantially fiat frequency response to frequencies within the video-signal range of said Wave, and a frequency-variation response network having a steep frequency-response characteristic at frequencies within the synchronizingpulse range of said wave, the time constant of said last-mentioned network bein small compared to the duration of a synchronizing pulse, the output-versus-frequency characteristic of said discriminator having a zero-crossover point in the range of frequencies occupied by said synchronizing pulses; and means responsive to an output signal of said frequenc discriminator for controlling the frequency of said oscillator to maintain the synchronizing-level frequency of said wave substantially constant.

WILLIAM E. BRADLEY,

(References on following page)- REFERENCES CITED Number Name Date The following references are of record in the g ig g is file of this parbent: Q mson 2,290,517 W11son July 21, 1942 UNITED STATES PATENTS 5 2,296,919 Goldstine Sept. 29, 1942 2 341 649 Peterson Feb. 15 1944 Number Name Date 2 44 2 5 Travis Jan 17 19 9 2:354827 Peterson Aug. 1944 2,413,913 Duke Jan. 7, 1947 Disclaimer 2,481,9O2.-Wz'll2'am E. Bmdley, Swarthmore, Pa. AUTOMATIC FREQUENCY CONTROL CIRCUIT FOR FREQUENCY MODULATION TELEVISION SYSTEMS. Patent dated Sept. 13, 1949. Disclaimer filed Jan. 17, 1952, by the assignee, Phz'lco Corporation. Hereby enters this disclaimer to claims 1, 3, 4, 5, 6, and 8, of said patent.

[Oyficial Gazette February 19, 1952.]

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