US2463685A - Automatic frequency control system - Google Patents

Description

March 8, 1949. G. L; FRE'DENDALL ET AL 2,463,685

AUTOMATIC FREQUENCY CONTROL SYSTEM 3 Sheets-Sheet 1 Filed July 31., 1944 GORDON L.FREDENDALL ALFRED QSCHROEDER ATTORNEY March 8, 1949. G. FREDENDALL ETAL 2,463,

AUTOMATIC FREQUENCY CONTROL SYSTEM I Filed July 51, 1944 5 Sheets-Sheet 2 as K 66 F1 5.1 T1 VIDEO 69 5+ AMPLIFIER 6 TO OUTPUT OF SEGGND DETECTOR o.c. RESTORER 3 AND SYNC 551:5 T T SEPARATOR llllll I'vIII" m HORlZQNTAL DEFLECTIO AAAAAA InvvIIv- 7 HORIZONTAL qq i i-151 109 T I HORIZONTAL =3:-

- PHASE.

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1 81 1 141. vzrmom. DEFLECTI'ON s asm CONTROL .L VERTlCAL I 5 SAWTOOTH PHASE 141 T GENERATOR CONTROL 2 i l NVENTORS GORDON L, FREDENDALL ALFRED C- SCHROEDER ATTORNEY March 8, 1949. G. 1.. FREDENDALL ET AL 2,463,

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed July 51, 1.944 5 Sheets-Sheet 3 lllllll IIIIIII INVENTORS I GJORDON FREDENDALL ALFRED C. SCH ROEDER ATTORNEY Patented Mar. 8, 1949 AUTOMATIC FREQUENCY CONTROL SYSTEM Gordon L. Fredendall and Alfred C. Schroeder,

Feasterville, Pa., assignors to Radio Corporation of America, a corporation of Delaware Application July 31, 1944, Serial No. 547,355

2 Claims. (01. 315-29) The present invention relates to automatic frequency control or phase control circuits and particularly to novel apparatus for the synchronization of cathode ray deflecting circuits in television receivers or the like.

In some locations, generally near the edges of the service area of a television station, local interference or weak signals, or perhaps both, impair the accuracy of synchronization in a television receiver to such an extent that the horizontal resolution is appreciably reduced. There may also be failure of interlace and even sporadic failure of synchronization. This invention aims to provide an improvement in synchronizing action that will improve performance in these marginal locations without introducing compromising factors. For signal conditions which are at all satisfactory for reception, substantially the maximum horizontal resolution permitted by the television channel may be realized in a receiver embodying the present invention when allowance is made for unavoidable noise effects seen in the image as beam modulation. Also, noise effects do not destroy interlacing or cause tearing of the picture in horizontal strips. Appreciable improvements in vertical synchronization is also obtained.

In accordance with this invention, these and other desirable results are attained by providing improvements in previously prepared television receiving equipment in which synchronization of an oscillator is to be obtained by employing an automatic frequency control circuit responsive to any change in the time relationship of the synchronizing pulses with respect to saw-tooth wave used to control or provide deflection.

A further object of the invention is to provide an improved method of and means for synchronizing the deflecting circuit or circuits of an image producing device.

A further object of the invention is to provide an improved television synchronizing circuit that is less affected by noise in the incoming signal than previously known devices.

A still further object of the invention is to provide an improved television receiver.

In accordance with the invention, a phase detector compares the relative phase of the incoming synchronizing pulses and the saw-tooth deflection wave and generates a signal which varies in accordance with the phase.

Other and more specific objects of the invention will become apparent and suggest themselves to those skilled in the art to which the invention is directed upon reading the following specification and claims in connection with the accompanying drawings in which:

Fig. 1 is a schematic representation of a portion of a television receiver embodying the invention;

Figs. 2a to 2d are curves illustrating the operation of the schematic circuit of Fig. 1;

Fig. 3 is a circuit diagram of substantially the entire deflecting arrangement of a television receiver embodying a development of Fig. 1; and

Fig. 4 is a modification of the receiver circuits of Fig. 3.

Referring for the present to Fig. 1 of the drawings, which illustrates the invention in simplified form, a phase detector III is fed with sync pulses applied to the primary l2 of a transformer M by way of the terminals l6. These sync pulses are separated in any known manner from the video signal in the sync separating circuits.

of a television receiver of which the apparatus of Fig. 1 forms a part. that in the discussion which is to follow, these sync signals will be either horizontal or verti-.

cal sync pulses, the disclosure of Fig. 1 being applicable to either. The phase detector is also fed.

with a saw-tooth wave of voltage from a scanning oscillator l1 over a connection I8, this saw-tooth voltage wave being the same as or similar to a wave fed to the deflection coils or deflection plates of the image producing device (not shown) over a connection I9. The sawtooth wave is applied at the midpoint of the secondary 2| of the transformer 14 so that it is applied in phase to condensers 24 and 26. The condenser 24 serves to couple the end 28 of the secondary to the anode of a diode 29, the other end of the secondary 29 being coupled through the transformer 26 to the cathode of a diode 3|. The cathode of the diode 29 is connected to the anode of the diode 3!, this connection serving as the output terminal, indicated by reference character 32, of the phase detector ID. The D. C. return paths for the diodes 29 and 3| are provided by resistors 34 and 36 respectively, having a common terminal which is grounded as shown, thereby providing a balanced arrangement for the phase detector I0.

The diodes 29 and 3| function as peak detectors by selecting appropriate values for the resistors 34 and 36. The discharge paths of the condensers 24 and 26 have a relatively long time constant.

The point 32 is physically connected by a conductor 39 to the grid of a D. C. amplifier 4| com- It will be understoodprising the tube ii. A filter 53, having an impedance characteristic such that inappreciable saw-tooth voltage appears at the grid of the amplifier fit, is connected between the point 32 and ground. The filter is indicated as comprising a pair of branched circuits, but inasmuch as design methods for providing a suitable filter are well known to those skilled in the art, no detailed description of the design or operation of the filter will be given. The output of the amplifier tube All is D. C. coupled to the scanning oscillator in accordance with the signal impressed across the amplifier 42. An analogous operation with respect to a blocking oscillator is described in Patent No. 2,344,810, granted totheseapplicants on March 21, 1944.

The operation of the arrangement of Fig. '1 with synchronizing pulses derived, for example, from the sync separatingpircuits of an associated television receiver and impressed on the primary [2' of the transformer 14 will be described. In the secondary 2i, sync signals of opposite polarity with respect to ground are super-imposed on a saw-tooth wave derived from a source such as either the horizontal or vertical deflection circuits, for example, of the associated television receiver. It may be assumed for purposes of description that the horizontal deflection. circuit is representedin'Fig. 1 by the device designated previously by the reference character IT. The signalson condensers 24 and'26, shown in Figs. 2a and 2?), correspond to one assumed condition of phasing of the saw-tooth voltage and the sync pulses. The saw-tooth voltages ear and en; at these points are ;of the same polarity, Whereas the voltage pulses es and 351 are of opposite polarity. Any-change in the phase relationship of the saw-tooth wave and sync pulses will cause an increase in the peak amplitude of one composite-signal and a decrease in the peak amplitude of the other. In order-to simplify the drawing, the voltage pulses es and esl are not shown as differentiated pulses, only the portion of "the pulses that is effective being shown.

Across the resistors 3,4 and 36, that is to say on the diode side of the condensers, the voltages are those shown in Fig. 20. These voltages include the D. C. voltage inserted bythe peak rectifienaction of the diodes 2,9 and 3|. The peaks of the sync pulses are held at the same reference level, this level being indicated as zero for the given example.

Thesignal appearing at the point ,32 is shown in Fig. 2d, and it will be seen that the sync pulses have been balanced out, leaving a signal containing only the saw-tooth wave components. The saw-tooth m o ent of th s enals own n 2d is filtered out ,bythe filter .43, and a D. C. control potential is obtained whichis not a fluctuating saw-tooth w-ave but is a very slowly varying signal when noise is present.

Thiscontrol potential is amplified by theamplifier tube M, which ha in its plate circuit the usual plateload resistor and such other equipment as may incidentallybe included in the scanning oscillator or deflection source Hi. In the case illustrated, the average potential existing between the waves in Fig. 20 at the instants when the sync pulses oc ur is zero. If a small phase displacement occursbetween syn pulses and the saw-too h Wave, the peaks of sync continue to be maintained at zero potential, but; the average pont a atpoint .3 islow red o raised. dep nd n on whether .the. .Dulsemoves toan advanced, or to a retarded position in time relative to the saw- '4 tooth wave. The feedback action is such that the change in speed of the oscillator tends to reduce the magnitude of the phase discrepancy between a saw-tooth cycle and a pulse that gave rise to the increment in the control valtage.

The equilibrium position of the pulses is a function of the settings of the speed control but is constrained to lie somewhere on the return time of the saw-tooth. A speed control is usually to-be found in apparatus of the type designated byreferencecharacter 56. When the amplitude of noise is sufiiciently great, some noise peaks tend to be held at zero potential by the action of -,the diodes .ZSand 3|. The potential at the point 32 tends to fluctuate about zero, if the distribution of the noise is random. Under such circumstances, the D. 1C. component is zero. It is importanthere to note that the wave forms of noise and saw-tooth do not appear at the junction of the diodes as disturbing components of the control voltage. Variations in the difference between the instantaneous A. C. axes of the two sawtooth waves would appear at-point 32 in the absence of the filter 43. The function of the filter is the removal of the higher frequency components of this control voltage and the transmission of only the very low frequency components. In the absence of noise, the latter are a true indication that a readjustment in the phase of the oscillator is necessary for the maintenance of accurate synchronism. Low frequency components are indications of a drift in circuit constants or small changes in the frequency of the transmitted sync pulses.

Referring to Fig. 3 of the drawings, certain parts of a typical television receiver modified to incorporate features of this invention are shown diagrammatically. The viedo amplifier, D. C. restorer, synchronizing separator, and the deflection generators are shown.

A tube 5| serves as a viedo amplifier, the video signals from the second detector of the receiver (notshown) being impressed on the control grid 53 of this tube by way of a connection 54. The output circuit of the tube 51 includes a peaking arrangement indicated generally by reference character 55. Inasmuch as this arrangement has been employed in previously known television receivers, its functions and operation will not be described in detail. .The output to the image producing device, such as a Kinescope, is indicated at 58. Usually, this connection will effectively terminate at the control grid of the Kinescope. A tube 59 of the duplex type serves as a D. C. restorer and also as a sync pulse separator amplifier. The cathode GI and the grid 62 together with the cathode resistor 63 serve as a diode rectifier with its load resistor to perform the function of a D. C. inserter since the control grid circuit of the image producing device is coupled through theresistor 53. The control grid '62 also causes the signals applied over a conductor 66 from the peaking circuits 55 to be repeated by way of a condenser 68 and applied to the control grid .69 of the tube 59. The grid and cathode elements 6i and 62 rectify the peaks of the synchronizing pulses which are applied to the oathode with negative polarity, and only the sync signals, therefore, are passed by the condenser 83 to the grid 69.

The pulses appearing on the grid 69 are present in the plate circuit of the plate 'H' which includes a transformer 14 for passing horizontal sync pulses only and a transformer 16 having its primary 18 and its secondary 19 shunted by condensers BI and 82 respectively for passing the vertical sync pulses only.

The horizontal deflection circuit of the image producing device is fed in the usual manner from the secondary 84 of a transformer 86. A horizontal saw-tooth generator including a tube 88 applies a saw-tooth voltage to a high impedance horizontal output tube 89. The plate circuit of this tube 89 is coupled to the horizontal deflecting means of the image producin device through the previously mentioned transformer 86.

The details of the horizontal saw-tooth generator 81 and the function and operation of the horizontal phase control 92 will now be described. The latter is similar to the phase detector and amplifier shown in simplified form in Fig. 1 of the drawings, and the broad features of the control function, exercised by the horizontal phase control, have been described previously in connection with Fig. 1. The cathode 94 and anode 96 of the duplex type tube 88 are elements of the usual blocking oscillator tube. The blocking oscillator includes, in addition, a feedback transformer 91, a condenser 89, which is charged and discharged, and a speed control resistor I82 which is in series with a resistor I83.

The blocking oscillator section of the tube 88 operates to produce pulses and is controlled in a manner to be described from the phase control 92. The elements I86 and I08 of the tube 88 serve as a discharge tube to generate a saw-tooth of voltage by charging and discharging a condenser I89. The resistor III serves as a width control. The saw-tooth voltage across the condenser I89 is applied through a condenser II2 to the grid N4 of the high impedance horizontal output tube 89. The plate circuit of this tube includes the primary II6 of the transformer 86 which also serves as a coupling impedance for applying the horizontal saw-tooth voltage to the secondary I I8 of the previously mentioned transformer 14 by way of a conductor I I9. A low pass filter composed of a resistor I2I and a condenser I22 produces a wave which is more nearly of saw-tooth form than the wave form of the output of the tube 89, which is of such character that saw-tooth current is produced in the horizontal deflecting coil of the Kinescope. The condensers I24 and I26 correspond to the condensers 24 and 26 respectively of 1. The duplex diode I23 corresponds to the diodes 29 and 3I of Fig. 1. Resistors I21 and I28 correspond to the resistors 34 and 36 respectively of Fig. 1. The filter constituted by the components I3I', I32, and I33, corresponds to the filter 39 of Fig. 1, and I34 is a D. C. amplifier tube similar to the amplifier tube 42 of Fig. 1.

The filter-resistor I33 has a critical value, for each receiver of a given design in which it is used. When this resistor is too large, the sides of the image become scalloped; and when it is too small, the image as a whole vibrates in the horizontal direction. There is an intermediate range of values for which the image is correct.

The output of the D. C. amplifier I34 is applied to the grid I36 of the blocking oscillator trlode section of the tube 88. This output is in the form of a signal which slowly varies when noise is present. The variation in this signal is added to or subtracted from the potential normally applied to the grid I36, this normal potential being variable at will within chosen limits by the speed control resistor I92. The manner in which the signal passed along by the amplifier I34 is varied has III,

been discussed above in connection with Fig. 1 of the drawings.

The vertical deflection circuits and vertical defiection control on Fig. 3 of the drawings are similar to and operate in the manner already discussed in connection with the horizontal deflection circuits. The adjustable resistor I39 serves as a height control and corresponds to the width control III. The adjustable resistor I4I serves as a speed control and corresponds in function to the adjustable resistor I82. The adjustable resistor I42 serves as a vertical linearity control. A condenser I44 is charged and discharged to provide the voltage wave which, in conjunction with the input circuit constants of a tube I46 and a transformer I48, serves to drive a saw-tooth wave of current through the vertical deflecting coil (not shown).

It is desirable in the operation of an automatic frequency controlled television receiver to maintain a reasonably constant plate voltage supply for both sections of this tube I84, and the tube 88 and also its counterpart in the vertical deflection circuit when the gain control of the I. F. amplifier (not shown) is varied. An effective arrangement for insuring a constant plate voltage is disclosed in an application of G. L. Fredendall, Serial No. 526,281, filed March 13, 1944, now Patent Number 2,454,150, issued November 16, 1948.

Fig. 4 of the drawings shows a modification of Figs. 1 and 3 in which a transitron oscillator is substituted for the blocking oscillator and discharge tube shown by way of example in Fig. 3 of the drawings. Only a portion of a vertical r deflecting circuit is shown by Fig. 4; and now with reference thereto, I68 corresponds to transformer I4 of Fig, 1 of the drawings. The separated vertical sync pulses are sync pulses which may further be separated by the aid of the transformer I68 and appear in the primary I62. The secondary I63 of the transformer may be center tapped or may be shunted by a resistor I64, as shown, which provides a point of connection for feedback over a conductor I69 of a saw-tooth wave originating in the oscillator represented in its entirety by reference character I68. The transitron oscillator is described in the Proceedings of the Institute of Radio Engineers, October 1935, by E. W. Horold. The output of the AFC phase detector I18, which includes the diodes HI and I12, appears at a point I14 which corresponds electrically to the point 32 of Fig. 1. The output of the phase detector I18 is applied to the No. 1 grid I16 of the tube I18 which is of the pentode type commonly used as a transitron oscillator.

The plate circuit of this tube, which is connected to a source of positive potential, includes a resistor I8I. A condenser I82 is connected to the plate of the tube I18 and to the cathode return circuit by a connection to the cathode I83 of a sync output tube I84. The condenser I82 is charged through the resistors I8I and I94 in operation of the oscillator. The No. 2 grid I86 of the tube I18 is connected to a source of positive potential through a resistor I88, and the No. 3 grid I89 is connected to ground through an adjustable resistor I9I. A cathode resistor I98 provides a negative bias for the No, 3 grid I89. The grids I86 and I89 are coupled by a condenser I92. The duration of the saw-tooth pulse is controlled by the value of the resistor I9I and the resistor I8I in conjunction witharesistor I94. Assuming that the No. 3 grid I89 is positive and the anode is drawing current, the anode current is then considerably larger than the charging current through the resist-or NH. The condenser I82 is discharged rapidly. When the anode current cut oil, the grid I89 is driven negative, and this condition remains while the condenser I82 charges, the anode potential increases, and a point is reached at which the anode commences to draw current which normally would pass to the No. 2 grid I86. The potential on the No. 3 grid I89 again rises, since the grid I89 and the grid I86 are coupled, until the anode'suddenly takes a large current, thus discharging the condenser I82. The charge and discharge characteristic of the condenser I82 is in the form of a saw-tooth of voltage which is coupled to the grid I98 of the tube I84 through a coupling condenser I99.

A portion of the output of the tube I84 is applied to the phase detector I10 by way of the previously mentioned feedback paths I66. A filter 203 serves to smooth the wave, which appears in the output of the tube I84, before it is applied to the phase detector I10. Thus, small deviations of the saw-tooth frequency and phase relative to the incoming sync pulses are detected by the diodes Ill and I12, and the result of this detection is passed on as a slowly varying control voltage to the oscillator I18. The action of this control voltage tends to restore the synchroism of the oscillator.

The cathode circuit of the tube I84 is connected illustratively in the manner shown for the cathode of the tube I46 of Fig. 3. The illustrative example in Fig. 4 of the drawings, therefore, is especially adapted for use as a vertical deflection circuit. However, it will be understood that the principles of operation explained above in connection with Fig. 4 of the drawings may be applied in a like manner to a horizontal deflection circuit.

Various alterations may be made in the present invention without departing from the spirit and scope thereof, and it is desired that any and all such alterations be considered within the purview of the present invention as defined by the hereinafter appended claims.

Having now described the invention, what is claimed and desired to be secured by Letters Patent is the following:

1. In an automatic frequency control arrangement for a television receiver, a transitron oscillator having a connection for the control of generated frequency, a peak detector, a direct connection from said detector to said oscillator control connection, a deflecting circuit, an output connection from said oscillator to said deflecting circuit, a connection from said deflecting circuit to said detector to provide awave to said detector having a beam deflecting characteristic, and means for applying synchronizing pulses to said detector, said pulses being divided and applied with opposite polarity to said detector and said wave being divided and being applied with like polarity to said detector.

2. In an automatic frequency control arrangement for a television receiver, a transitron oscillator comprising an electron tube having a control grid, a peak detector, a control connection from said detector to said transitron oscillator control grid, 2. deflecting circuit, an output connection from said oscillator to said deflecting circuit, a connection from said deflecting circuit to said detector to provide a Wave to said detector having a deflecting characteristic, and means for applying sync pulses to said detector, said pulses being divided and applied with opposite polarity to said detector, and said wave being divided and being applied with like polarity to said detector.

GORDON L. FREDENDALL. ALFRED C. SCI-IROEDER.

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

UNITED STATES PATENTS Number Name Date 2,066,528 Harper Jan. 5, 1937 2,096,881 Butler Oct. 26, 1937 2,250,284 Wendt July 22, 1941 2,332,681 Wendt Oct. 26, 1943 2,339,536 Wendt Jan. 18, 1944 2,344,810 Fredendall et al. Mar. 21, 1944 2,352,541 Harper June 27, 1944 2,358,545 Wendt Sept. 19, 1944 OTHER REFERENCES The Translation Oscillator, Cledo Brunetti. Proc, of I. R. E., vol. 27, No. 2, Feb. 1929.

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