Patent GB-1410251-A (GB)
Title: Superheterodyne receiver particularly for the short wave or  ultrashort wave range
IPC: H03J7/02
Filed: 1973-01-05
Abstract: 1410251 Superheterodyne radio receivers; automatic frequency control H GRUNING 5 Jan 1973 [12 Jan 1972] 782/73 Headings H3A and H3Q In a superheterodyne receiver the bandwidth of a filter which precedes the mixer is automatically adjusted at periodic intervals by means of a control signal dependent upon oscillations generated within the receiver. A receiver operable over the range 144-146 MHz comprises an input stage 2, a highly selective tunable filter 3, mixer 4, 9 MHz quartz crystal filter 5, I.F. amplifier 6, demodulator 7 and L.F. amplifier 8. Local oscillations are generated in a 133-7 MHz oscillator 11 and fed to mixer 4 over buffer stage 10. At periodic intervals, switches S1-S3, which are normally closed, are briefly opened and switches S4-S8 closed, by means of control circuit 30; the input to filter 3 is then derived from mixer 15. Switches S1-S6 are arranged to operate before switches S7, S8. Mixer 15 obtains one input from local oscillator 11 via buffer circuits 10, 16, and a second input from 9 MHz oscillator 12 via modulator 13 in which two side bands are superimposed on the signal by means of 15 kHz oscillator 14. During the periodic intervals, switch S6 being closed, the output from mixer 4 is applied to an I.F. amplifier 17, the output of which is brought to a constant level by means of a.g.c. circuit 23. The output from I.F. amplifier 17 is rectified by diode 18 and the 15 kHz output amplified by L.F. amplifier 19: the output is rectified by diode 20, amplified by amplifier 21, and applied to a storage device, e.g. a capacitor 22. The voltage on storage device 22 is applied to input 3b of filter 3 to control its bandwidth. Automatic frequency control.-The output from I.F. amplifier 17 is also applied over a further amplifier 24 to a mixer 25 the second input of which is at 8À6 MHz, from oscillator 26, the mixer output being at 0À4 MHz and applied to Foster-Seeley phase discriminator 27. The output is applied via amplifier 28 to a storage element 29, the voltage across which constitutes a frequency-control signal applied to input 3c of filter 3. In order to improve the control within a narrow range of the desired frequency, a phase comparator circuit 31 is associated with the Foster-Seeley phase discriminator 27. Frequency search circuit.-Sawtooth generator 34 is started, via relay member 33, by means of a control circuit 32 which generates a control signal whenever the signals at the inputs 32a, 32b, derived from phase discriminator 27, vanish. The output from sawtooth generator 34 is applied through switch S12 to the input 29a of storage element 29. The output from control circuit 32 is also applied via delay circuit 35 to the switch control circuit 30, which is thereby set continuously for "control": it is also applied to a pulse control 36, which at the beginning of each frequency search, causes storage element 29 to be zeroized. To assist the frequency control circuit in finding and locking on to the frequency, the storage element 22 controlling the bandwidth of filter 3 is supplied via switch S10 with a constant voltage, from source 38. Switches S9-S13 are operated by control circuit 32. Output 28b of amplifier 28 is connected via control circuit 37 to the control circuit 30 so as to reduce the time elapsing between the switching periods, whenever a major correction is necessary.