GB673770A - Improvements in or relating to automatic frequency control arrangements for radio communication systems - Google Patents

Abstract

673,770. Radio signaling. STANDARD TELEPHONES & CABLES, Ltd. March 30, 1950, No. 8044/50. Class 40 (v). [Also in Group XXXV] In a radio receiver, transmitter, or repeater, provided with an automatic frequency control motor, a separate automatic frequency search motor is adapted to sweep the frequency of the oscillator to be corrected backwards and forwards over a given frequency range, and is arranged to be set in operation when the frequency to be corrected deviates from the specified value by an excessive amount and to be put out of operation when the frequency returns to the specified value. As shown in Fig. 1, in an F.M. radio repeater the incoming signals at frequency F3 are passed through a filter 2 to a crystal mixer 3 provided with an input at a frequency F1 from a local oscillator 4, which may be of the velocity modulation type, and the I.F. at frequency f 1 is amplified at 5 and 6 and applied to a mixer 7 supplied at a frequency F2 from a local oscillator 8, the resulting output at frequency F4 being amplified at 10, which may be a travelling wave amplifier, and retransmitted. In order to keep the I.F. f 1 constant an automatic frequency control arrangement, 12, 13, 14, of the type described in Specification 663,803 is used, a similar arrangement 22, 23, 24, maintaining the difference of the mean input and output frequencies, f2 constant. The discriminator 12 produces a pair of unidirectional output voltages which are equal when the I.F. has the correct value, the difference of these voltages controlling the A.F.C. motors 14 and the sum controlling the search motor 19, and includes a filter which passes a narrow band of frequencies centred on f 1 so that if the I.F. departs abnormally from f 1 the discriminator provides no output. This allows a relay in the search control circuit 18 to operate and switch on the search motor 19, the relay being normally held unoperated by the search motor control voltage. The motor 14 corrects the frequency by driving a tuning plunger in the resonator of oscillator 4 through a shaft 16 and differential gear 17, Fig. 3 (not shown), and the motor 19 also drives the shaft 16 through the differential gear to sweep the tuning plunger backwards and forwards until the I.F. waves reappear at the output of amplifier 5, the search motor 19 is switched off, and the A.F.C. motor 14 resumes control. A reduction gear is inserted between the shaft of the motor 14 and the shaft 16 to give fine control. To control the oscillator 8 in like manner, the outputs of oscillators 4 and 8 are applied to a crystal mixer 21 and the resulting frequency f2 is applied to an A.F.C. discriminator 22, controlling A.F.C. motor 24 and search motor 29. When the repeater forms part of a television system, additional elements 31, 32, are included. The discriminator 31 recovers the television signal wave from the frequencymodulated I.F. output of the amplifier 5, and the gating pulse generator 32 selects the line synchronizing pulses from the television wave and converts them into gating pulses which unblock the discriminator 12, in this case arranged to be normally blocked, so that it only accepts waves during the period of the line synchronizing pulses. The arrangement may be applied to a transmitter, Fig. 2 (not shown), the output from a crystal-controlled master oscillator being passed through a frequency multiplier and applied to a mixer together with a portion of the output from the transmitting oscillator and the resulting I.F. supplied to a frequency discriminator whose output is applied to the automatic frequency control and the search control. For a television transmitter the line synchronizing pulses are selected and control the discriminator as in the case of the repeater. The arrangement may also be applied to a receiver. The search control circuit 18, Fig. 4 (not shown), is provided with a control potential from the discriminator which is amplified and rectified to provide negative bias to cut off a valve having the search motor relay in its anode circuit. When the discriminator provides no output the relay operates to drive the search motor, switches reversing the motor at each end of its travel. A trigger circuit associated with the valve cuts it off for a short period on the application of an impulse generated on the operation of either reversing switch to momentarily stop the motor before it is reversed. Specifications 581,613, 641,900 and 670,404 also are referred to.