GB794908A - Improvements in or relating to frequency divider circuits - Google Patents

Abstract

794,908. Automatic frequency control. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Aug. 19, 1955 [Aug. 19, 1954], No. 24122/54. Class 38 (4). [Also in Groups XL (b) and XL (c)] Fig. 1 shows the application of a pulse frequency-divider to a television wave-form generator in which it is required to produce pulses of the field repetition frequencies from input pulses of double the line repetition frequency. The master oscillator 1 generates pulses of double line frequency, Fig. 2A, which are fed with positive polarity to a two stable state trigger circuit 2 thus producing output pulses at line repetition rate, Fig. 2B. These are fed to a shaping circuit 3 which generates a short pulse coincident with the beginning of input pulses fed to it. The output from shaping circuit 3 is fed to a coincidence gate 4 the output from the gate being used to trigger a flip-flop 5 the duration of the unstable state of which is about 100 microseconds, i.e. approximately a line period. Thus output pulses of duration 100 microseconds are produced from flip-flop 5 and are applied via a further shaping circuit 7 which generates a short output pulse coincident with the trailing edge of each input pulse. The output from shaper 7 is applied to a further flip-flop 8 the unstable state of which has a duration of 20 milliseconds, i.e. approximately one field period, Fig. 2D. Only during the coincidence of a positive pulse from flip-flop 8 and from shaper 3 does the gate transmit an output pulse. In every field cycle one additional pulse is fed to the trigger circuit 2 via the delay device 6 so that the circuit 2 is caused effectively to divide the master pulse frequency from oscillator 1 by an odd number in any field period. The value of this odd number is set according to the duration of the unstable state of flip-flop circuit 8. In order to lock the output frequency to a reference frequency, such as for example the mains frequency, a control signal may be applied to flip-flop 8 to vary the duration of its unstable state in dependence upon the difference between the output frequency of the divider and the reference frequency.