GB726903A - Automatic frequency control circuits - Google Patents

Abstract

726,903. Automatic frequency and phase control circuits. PYE, Ltd. Dec. 24, 1952 [Dec. 24, 1951], No. 30166/51. Class 38 (4). Relates to automatic frequency and phase control circuits of the kind in which waveforms derived from a reference oscillation and from the controlled oscillator are mixed in a phase discriminator to produce a D.C. control voltage which is used to adjust the frequency of the controlled oscillator; and consists in feeding the D.C. voltage not only to the controlled oscillator but also to the discriminator in such a way as to alter the D.C. component of the waveform derived from the reference oscillation. In this way, when a change of frequency occurs, the necessary frequency-correcting D.C. voltage is supplied without requiring a displacement of phase between the controlled oscillation and the reference oscillation. The system is particularly applicable to television apparatus for synchronizing a scanning-source with a reference wave or pulses. Referring to the diagram, the controlled oscillator 3 feeds rectangular pulses P through a phase-splitter 2 to opposite corners of a phase-comparing diode bridge 1. The reference waveform which may be of sawtooth shape, or modified as described in Specification 714,442, is applied to the grid of a valve 6 whose anode output voltage is fed to a third corner of the bridge 1. D.C. voltage proportional to the phase difference of the two bridge inputs is taken off at the fourth corner of the bridge to the grid of a D.C. amplifier 4, and the resulting changes in anode voltage are applied to the controlled oscillator 3 to adjust its frequency. The D.C. voltage is also fed to the grid of the valve 6 and a fraction of the D.C. anode voltage of this valve is added to its A.C. output and applied to the third corner of the bridge 1 over a circuit R3, C1 having a long time constant and D.C. amplifier 6. The amplifier 6 has its gain adjusted so that the D.C. fed back to the bridge 1 is equal in amplitude to the amplitude of the sawtooth wave at the point at which the rectangular pulses are acting. When the relative frequencies of the two bridge inputs vary, the voltage controlling the oscillator 3 is varied as in the curve of Fig. 2 (a), and the relation of the pulses and sawtooth wave as shown in Fig. 2 (c) at (1), (2) and (3). At this stage, the voltage fed back to the discriminator over a long time constant circuit begins to vary, and makes a slight re-adjustment of oscillator 3 to vary the phase relation until the stage 4 is reached. Oppositely-poled diodes 7, 8 limit the amplitude of the sawtooth voltage at the anode of the valve 6. An A.C. negative feedback path from anode to grid of the valve 4 gives greater immunity from the effects of interfering pulses. The frequency of the pulses P may be a sub-multiple of the reference frequency.