GB955300A - Stabilised frequency synthesisers - Google Patents

Abstract

955,300. Automatic frequency control systems. COMPAGNIE INDUSTRIELLE DES TELEPHONES. Jan. 16, 1962 [March 23, 1961], No. 1594/62. Heading H3A. In a key controlled decade frequency synthesizer with preliminary adjustment by a capacitor of the oscillator frequency to a preselected value and then fine control by an A.F.C. voltage, the oscillator frequency is swept by the sawtooth voltage output of a motor-driven potentiometer applied to a reactance tube 7 until the frequency is approximately correct, whereupon the motor 28 stops and a previously short-circuited output V 1 of the phase discriminator 14 then added to the potentiometer output V 0 and both conjointly applied to the reactance tube 7. The sweep motor stops when conjointly (1) there is output # 3 from a decade filter set 31 receiving frequency reduced signals from the oscillator 7 and (2) absence of lowfrequency output from a filter 36 receiving from a mixer 37 oscillations of same low frequency f 2 as the output of the phase discriminator 14, this arising when # 2 is less than 400 c/s. Expressed logically, the motor stops when #F 2 .F 3 =0, i.e. when #F 2 .F 3 =1, where F 3 is rectified output of f 3 and #F 2 denotes absence of output of rectified f 2 from an inverter circuit 26, the product #F 2 .F 3 being formed in an AND circuit 25. An OR circuit 22 is energized from a capacitor 24 for about 5 sees. on setting up the required frequency, thus ensuring the search motor 28 is through a relay 27 always started. This relay 27 through contact b also disables attenuator 32 so that the search control sensitivity is increased until synchronism is reached, whereupon transients, due to attenuator 32, have little effect in recommencing the search motor 28. Relay 27, after a short time due to network 33, then operates relay 34 which shortcircuits output V 1 of the discriminator 14 (so that only V 0 is applied to the reactance tube 7) and by contact d removes f 0 as input to the phase discriminator. In a modification the search motor 28 is driven from an OR circuit energized by # 2 (rectified) and by rectified f 3 through an inverter circuit, and the motor stops only when F 2 +#F 3 =0, i.e. absence of # 2 and presence of f 3 at the same time. The system may be applied to single sideband receivers. Specification 912,607 is referred to.