Patent GB-1298125-A (GB)
Title: A system of automatic tuning of a controllable-frequency oscillatory circuit to a given frequency
IPC: H03J7/28
Filed: 1970-05-29
Abstract: 1298125 Radio receiving circuits INTERNATIONAL STANDARD ELECTRIC CORP 29 May 1970 [29 May 1969] 25933/70 Heading H3Q A method of automatically tuning a resonant circuit e.g. the input circuit of a radio receiver to a given frequency comprises the steps of firstly, causing the natural frequency of the circuit to vary by successive steps toward the given frequency, while comparing the output voltage from the circuit with a reference voltage. Secondly, as soon as the output voltage exceeds the reference, variation of the frequency of the circuit is continued by successive steps each double the previous steps. Thirdly, as soon as the output voltage returns to the reference value, the generation of frequency steps is stopped and the steps of double value replaced by steps of the original value, thus tuning the circuit of the centre of its response curve. A pulse 41, Fig. 1, applied to line 14, e.g. by momentary closure of switch 16, restores digital counter 11 to its zero condition, and reverses bi-stable circuit 18 to the "1" condition whereby it energizes clock pulse generator 13. Repetitive pulses from clock pulse generator 13 are then applied to counter 11 the output of which is converted in digital to analogue converter 1À2 to a stepped voltage which is applied over amplifier 74 to tune the resonant circuit or circuits e.g. by means of varactor diodes (Figs. 7, not shown) or by means of magnetically varied inductors (Fig. 8, not shown). The signal voltage developed across the resonant circuit or circuits is received at a terminal 30, and after amplification in amplifier 73 is rectified by detector 29 and applied to compariaion circuit 19, to the second input of which is applied a reference potential from terminal 28. As soon as the output from detector 29 exceeds the reference voltage from terminal 28 an input is applied over line 25 to AND gate 23 whereby clock pulses arriving from generator 13 over line 24 are passed to a second digital counter 20. From this point onwards counter 20 applies an output to digital to analogue converter 21 the output of which comprises a stepped voltage which is added to the output from converter 12 at the input of amplifier 74. The resonant circuit is then turned by double frequency steps (Fig. 2, not shown). When the output from detector 29 again falls below the reference voltage on terminal 28 an output is applied from comparision circuit 19 to bi-stable circuit 18 to cause it to revert to the "O" condition: an output from the bi-stable circuit 18 then stops the clock pulse generator 13, causing counter 11 to cease counting, and in addition an output over line 75 resets the counter 20 and so removes the output from converter 21 from the tuning means. Generating reference voltage for incoming signal.-In a second embodiment, the second input for comparision circuit 19, Fig. 4 is derived from the resonant circuit over a second detector 42 with a long-time-constant. Initiating pulse 41, as in the embodiment of Fig. 1, sets bi-stable circuit 18 in the "1" condition, starting clock pulse generator 13 which causes digital counter 11 and digital to analogue counter 12 to apply a stepped voltage to terminal 22 to line the resonant circuit. Pulse 41 in addition sets a second bi-stable circuit 45 to its "01" condition and this blocks amplifier 46 and enables amplifier 44. The resonant circuit is then turned over its pass-band, detector 42 generating a voltage which is proportional to the height of the centre of the passband and which decays slowly (53, Fig. 5, not shown). At the end of the tuning operation, counter 11 reverts to its zero condition and this causes a sharp pulse to be passed over line 50 to bi-stable circuit 50 causing it to revert to the "O" condition Bi-stable circuit 50 then blocks amplifier 44 and enables amplifier 46 whereupon the resonant circuit is again tuned in a stepwise fashion, but in the same manner as the arrangement of Fig. 1, the reference voltage for the comparator 19 comprising the output from the detector 42 which decays only slowly. Instead of using a continuously variable reactances, the digital counters may be arranged to switch fixed capacitors across the resonant circuit (Fig. 9, not shown), or switched inductors may be used.