GB1482089A - Automatic frequency control - Google Patents

Abstract

1482089 Automatic frequency control; receiver tuning; transistor oscillator circuits POST OFFICE 11 April 1975 [11 March 1974] 10677/74 Headings H3A H3Q and H3T An AFC arrangement comprises means 19 to compare a reference frequency 17 with a frequency 18 to be controlled, a digital counter 21 arranged to store a count representative of the frequency difference, and means 23, 9 foradjusting the frequency to be controlled which is responsive to the stored count. As described, the AFC arrangement forms part of a quadruple conversion superheat receiver, the IF 12 after the third conversion (2À6 MHz) being mixed with a nominal 2À7 MHz signal from a local oscillator 9 to provide a nominal 100 kHz IF13 which is demodulated to produce an audio output by means of a 100 kHz reference 16. Signals 13, 16 are mixed with a 11 kHz signal 15 to give 10 kHz reference and nominal signals 17, 18 which are compared at 19 to produce an error signal 20 which controls the direction of counting of reversible counter 21. Pulses from a clock 22 are counted until comparator 19 is reset, the count being converted to analog form at 23 and used to control oscillator 9. Counter 21 may receive command signals 24 for manual/automatic searching and AFC on/off modes. Comparator 19 may comprise two identical counters connected respectively to the two frequencies, and the frequency comparison may be based on the time taken to fill the counters (Figs. 3 and 5, not shown). Oscillator 9 (Fig. 4, not shown) may include a varactor diode for adjusting its frequency.