Patent GB-641889-A (GB)
Title: Improvements in and relating to automatic frequency control of radio-frequency oscillators
Inventor: RATCLIFFE STANLEY
IPC: H03L7/04
Filed: 1947-02-12
Abstract: 641,889. Automatic frequency control systems. MINISTER OF SUPPLY. Feb. 12, 1947, No. 4068. [Class 38 (iv)] [Also in Group XL (c)] The frequency of a thermally tuned oscillator is controlled in dependence on the tuning error by a heating current operative on a tuning strut and varying between two limiting values, the ratio of the times for which these values are maintained depending on the sense and magnitude of the error. The system is particularly applicable to a superheterodyne receiver, width-modulated pulses being applied to the control grid of the local oscillator for the purposes of tuning it to a frequency removed from that of the incoming signals by the intermediate frequency. Fig. 1 shows the magnetron oscilla. tor 6 of a pulsed radar transmitter and a thermally tuned local oscillator 7 of an associated receiver feeding a mixer 8 supplying an intermediate frequency amplifier 9 and discriminator 10. Control means 14 fed from the discriminator are operated to cause the local oscillator to take up one or other of the frequencies 12, 13, Fig. 2, relatively to the magnetron frequency 11. Fig. 2 shows the discriminator output plotted against the frequency of the local oscillator 7. The control means 14 comprises a " memory " circuit 16, which gives a steady voltage output, Fig. 4, b, representative of the peak amplitude and sense of the pulses derived from the discriminator, at the frequency of the radar repetition frequency. The circuit 16 together with a saw-tooth generator 18 feeds a trigger circuit 19 which produces a square-wave output which is widthmodulated in dependence on the output of circuit 16 as shown in Fig. 4, e. The saw-tooth generator is rendered operative by impulses over circuit 23, 24 only when the transmitter is in operation so that under these conditions the square-wave output of trigger circuit 19 is applied over relay contacts 20, 21 to the control grid of a triode associated with the thermallytuned velocity-modulated oscillator 7 so as to control the heating current in a strut which forms the anode electrode of the triode and operates to distort the cavity resonator of the oscillator. Thus, when the transmitter is in operation, any departure of the local oscillator frequency from its correct value 12 or 13 will be accompanied by compensating heating effects in accordance with the variation of the integrated power supplied to the tuning strut about a mean value corresponding to zero tuning error. Associated with the trigger circuit is a circuit 26 which generates a saw-tooth sweep of about five seconds period to pulse a multivibrator 30 and relay 31. This circuit is inoperative when signals are arriving over 15 but if in the absence of such signals the local oscillator is required to search, the generator 18 is inoperative and the relay 31 is pulsed and power switched on and off to the tuning strut of the oscillator at fivesecond intervals. In this manner the oscillator searches through a range extending beyond the frequencies 12, 13. If, during searching, the transmitter is switched on when the oscillator is beyond the frequencies 12, 13, the oscillator will lock to the discriminator output first encountered: Within region X rising frequencies lock on 13 while falling frequencies lock on 12. Sweep circuit 32 is provided to overcome the possibility that the oscillator frequency is trapped between one limit of its search and positive discriminator output in the event that the transmitter fails momentarily after a frequency change due to a negative output. Circuit 32 reverses the condition of the trigger circuit so as to reverse the direction of frequency sweep. To ensure that the rate of change of frequency is proportional to the tuning error and independent of the exponential heating effects of the tuning strut, a voltage may be taken from the trigger circuits and applied to the grid of a valve whose anode and grid are coupled by a resistance and condenser in parallel (Fig. 5, not shown). The output of this valve is differentiated in a second valve and fed back to the input of the trigger circuit as described in Specification 641,890. Specification 641,891 also is referred to.