Patent GB-628253-A (GB)
Title: Improvements relating to apparatus for controlling the frequency of an oscillator
IPC: H04N5/12
Filed: 1946-02-26
Abstract: 628,253. Automatic frequency-control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 26, 1946, No. 5931. Convention date, July 29, 1944. [Class 38 (iv)] Apparatus for controlling the frequency of an oscillator or oscillators, as in the deflection circuits of television receivers, comprises means for generating a pair of saw-tooth waves having a definite time relation with respect to the frequency of the oscillator and having sloping portions which slope simultaneously in the same direction, a phase-inverting tube to which synchronizing pulses are fed to produce pulses of opposite polarity which are applied simultaneously to the saw-tooth voltages to produce a pair of resultant pulses such that a change in the phase relation of one saw-tooth wave and its superimposed synchronizing pulse causes the peak amplitude of the resultant pulse to increase, whilst a similar change in phase of the other saw-tooth and its superimposed synchronizing pulse effects a decrease in the peak amplitude of the resultant pulse, means for producing a control voltage having a value which changes in response to a change in the relative peak amplitudes of the resultant pulses, and means for controlling the frequency of the oscillator in accordance with the control voltage. In an example the input circuit comprise;' video amplifier 53 with anode connected to a peaking circuit 55 of known arrangement, and to an output point 58 to the control grid of a cathode-ray tube. A duplex type tube 59 functions as a D.C. restorer and synchronizing separator and amplifier, and feeds a synchronizing pulse through a differentiating circuit including a coupling condenser 134 to the grid of a phase-inverter tube 133 in the line timebase circuit, while the grid of a phase-inverter tube 141 in the frame time-base circuit is fed through a condenser 143 and a resistor 144 associated with a condenser 146 in an integrating circuit. Pulses appearing on the control grid 138 of tube 133 are applied in opposite phase by way of an anode connected condenser 124 and cathode connected condenser 126 to a phase detector comprising a duplex diode 159, or a pair of diodes. Coupling resistances 161 and 162 of the tube 133 are such that positive pulses provided on the plate and negative pulses on the cathode are substantially of equal amplitude. These synchronizing pulses . of opposite polarity are superimposed on a pair of saw-tooth waves derived from pulses fed over feed-back line 119 from an output tube 89 which has its anode connected to the primary winding of a transformer 86, the control grid of tube 89 being connected to a horizontal sawtooth generator circuit comprising a blocking oscillator portion of a double triode 88, and a saw-tooth generator portion with an anode 108 connected to a condenser 109. The frequencycontrol of the system is dependent on the degree of bias applied to the grid 183 of the blocking oscillator from a point 171 and over an amplifier 182. The pulses fed over the line 119 from tube 89 are received by a filter comprising a resistor 121 and condenser 122, thereby producing sawtooth waves which are applied to the anode 169 of one portion, and cathode 164 of the other portion of the duplex diode 159, these saw-tooth waves being in phase, and upon these waves are superimposed the synchronizing pulses, the synchronizing pulse applied to anode 169 being in the positive sense while that applied to cathode 164 is in the negative sense. The time relation is such that the synchronizing pulses are positioned on the return or re-trace portion of the saw-tooth, and any change in the phaserelation of the saw-tooth wave and synchronizing pulses causes an increase in the peak amplitude of one composite signal and a decrease in the peak amplitude of the other composite signal. The duplex diode 159 functions as a peak rectifier, and in the signal appearing at the point 171 the synchronizing pulses are effectively balanced out, leaving only the saw-tooth component, which is filtered out by a circuit comprising a resistor 130 and condensers 131, 132, leaving a D.C. control potential which is applied to the grid of amplifier 182. Specifications 562,513 and 589,327, [Group XL], are referred to.