GB1145706A - An automatic frequency control system - Google Patents

Abstract

1,145,706. Automatic frequency control; transistor circuits. THORN ELECTRONICS Ltd. 2 Nov., 1966 [3 Nov., 1965], No. 46624/65. Headings H3A and H3T. In an automatic frequency control system in which an oscillator is compared with a reference signal to produce an output at point X which is fed through a filter R1, R2, C2 to control the oscillator frequency, an amplifier is connected in parallel with the filter to reduce the time constant of the filter circuit when slip occurs between the two frequencies. The amplifier comprises transistors VT1 . . . VT4. When the two signals are at the same frequency, the base collector junctions of transistors VT1, VT2 are forward biased holding their emitters to near base potential so that these transistors and the two following transistors VT3, VT4 are cut-off. Under this condition the output from the phase comparator 4 is fed via the low pass filter R1, R2, C2 to control the frequency of the oscillator 1 so as to correct the phase difference. When slip occurs, an A.C. output at the slip frequency is produced by the phase comparator and this results in the capacitors C3, C4 charging until the collectors, of VT1, VT2 are at about 0 volts and - V volts respectively. Under these conditions the transistors all become conductive and the signal is fed from point X to point Y via the amplifier, thus reducing the effective time constant of the filter. The R circuit is applied to a television line synchronization system in which a circuit 3 produces trapezoidal pulses from the oscillator output, and circuit 5 produces sampling pulses from the reference signal.