GB1098964A - Frequency synchronising system - Google Patents

Abstract

1,098,964. Automatic frequency control systems; frequency discriminators. INTERNATIONAL STANDARD ELECTRIC CORPORATION. July 29, 1966 [Aug. 5, 1965; Aug. 14, 1966], No. 34200/66. Heading H3A. The frequency of a variable frequency oscillator 2 is swept by a sawtooth generator 3 over a range, including several harmonics of the frequency f 1 of a reference pulse generator 1 with which the oscillator output is mixed in a modulator 8. The output of the modulator has a constant amplitude if the frequency f 0 of the oscillator 2 is equal to a harmonic of f 1 ; otherwise the amplitude varies, and the amplitude modulation is detected by a rectifier 10 and passed through a low-pass filter 12 to provide a control signal to be applied to a capacitance diode controlling the frequency of oscillator 2. The output of rectifier 10 (or a succeeding amplifier 11) is coupled to a pulse former 13 which responds to each case of synchronization with a harmonic and advances a counter 6 which was started by comparator 4 when the frequency f 0 exceeded the frequency of a crystal oscillator 5 which is equal to the first multiple of f 1 within the swept range of oscillator 2. When the counter 6 reaches a desired count it stops the sawtooth generator 3 and opens gate 15 to allow the frequency of oscillator 2 to be controlled by the control signal from the filter 12. In an alternative mode of operation which can be used when the A.F.C. system has a wide hold range, switches 16-18 are put in their other positions. The oscillator 2 is then held at one harmonic whilst the sawtooth generator 3 continues its sweep, and jumps to the neighbourhood of the next harmonic when the sweep exceeds the hold-in range. Frequency discriminator.-The variable frequency f 0 is fed to one diagonal of a diode bridge (19, Fig. 2, not shown), and the pulse signal f 1 to the other diagonal which is coupled by a transformer (21) to the base of a transistor (22) feeding a coil (23). The energy stored in the coil during each pulse is discharged at the end of the pulse into a rectifier circuit (10) providing an output via a low-pass filter (12). In a modification (Fig. 3, not shown), the two signals f 0 and f 1 are supplied to the bases of respective transistors (32, 34) via transformers (33, 35), the emitter-collector paths of the transistors being connected in series between a power supply (U B ) and the base of the output transistor (22) operating in the same manner as in Fig. 2. In either case the output is constant if f 0 = nf 1 and otherwise varies at the difference frequency.