Patent GB-641891-A (GB)
Title: Improvements in and relating to radio-frequency oscillators
Inventor: RATCLIFFE STANLEY
IPC: H03B5/18
Filed: 1947-02-12
Abstract: 641,891. Automatic frequency control systems. MINISTER OF SUPPLY. Feb. 12, 1947, No. 4069. [Class 38 (iv)] [Also in Group XL (c)] In a superheterodyne system of the kind set forth in Specification 641,889 and wherein the frequency of a local oscillator is controlled in dependence on the tuning error and arrangements are made for sweeping the oscillator frequency over a range embracing both the alternative correct settings, the difficulty arises that for one of these settings a positive discriminator output requires a reduction of frequency, and for the other, an increase. This difficulty is overcome by providing switching means which prepare a frequency control circuit in such a way that it responds only to signals of one sign from the discriminator. The system as a whole is described in Specification 641,889, but the present invention includes the valve circuits shown in Fig. 6, which are an embodiment of the parts marked 19-22, and 25-32 in Fig. 3. The trigger circuit 19 comprises reciprocally-coupled valves V2, V3, to the respective control grids of which are fed the discriminator output via the memory circuit 16, and the rapid saw-tooth output from the source _18. The valve with the higher grid voltage is conductive and the other blocked. Their anodes are connected to the contacts 20, 25 of a relay A2, the movable contact 21 of which is connected to the frequency-connecting device of the oscillator, which may be of the thermallytuned type. Thus if the non-essential source 18 is ignored, positive input from the discriminator results in contact 20 being at low voltage, and contact 25 at high voltage. Negative input from the discriminator produces the opposite result. In the rest position the valves revert to the former condition. In order to search for a correct setting, the timing is swept at a slow rate over a range including both correct settings, 12, 13, Fig. 2, by the provision of a transitron oscillator V4, giving a saw-tooth wave of about five seconds duration, at the end of which a pulse is applied to the grid of one or other of two reciprocally-coupled valves V5, V6, one of which has a relay A in its anode circuit. Thus the relay A is alternately operated for five seconds, and cut out for five seconds. The contacts A1 of the relay A distribute the pulses alternately to the two grids, and the contacts A2 sweep the tuning of the oscillator to and fro over the desired range. When during this sweep a positive pulse is received from the discriminator 16, no effect is produced on the trigger pair V2, V3 because V2 is already conducting. The sweep thus proceeds through the correct setting 12 or 13 and a negative discriminator output is produced. This cuts off the valve V2 and opens the valve V3, giving a low voltage on the anode of valve V3 which is communicated to the grid of the transitron V4 to stop its operation. The condition of contacts 20, 25 of the relay A2 is reversed, so that the tuning sweep is reversed in direction; positive discriminator output is again restored, and the tuning gradually locks in at the correct setting 12 or 13. If after the first reversal of the direction of sweep, the I.F. input 15 to the discriminator 10 should fail, the sweep would continue in the reverse direction. This is prevented by a transitron valve VI which is brought into operation by the high potential on the anode of valve V2. The valve V1 runs, down and a negative pulse is fed from its screen grid to the control grid of the valve V3, thus reversing the conditions at the contacts 20, 25 and the direction of the tuning sweep. If the I.F. input 15 to the discriminator 10 is not restored, the high voltage on the anode of valve V3 causes the valve V4 to start up and the five second sweep search to be resumed. If during the search the oscillator frequency is in the region X when transmission starts up, the first discriminator output is of negative sign and causes a reversal of the direction of sweep. Thus if the initial sweep is towards the setting 13, the reverse sweep encounters a negative output in the vicinity of the setting 12. This will produce no effect on the valves V2, V3 and the sweep continues through the setting 12 until a positive output is obtained, and the tuning gradually locks in at the setting 12. The Specification also includes the control arrangement for thermally-tuned oscillators forming the subject-matter of Specification 641,890.