GB1125916A - Improvements in or relating to frequency synthesisers - Google Patents

Abstract

1,125,916. Automatic frequency control systems. MINISTER OF TECHNOLOGY. 19 April, 1967 [22 April, 1966; 25 Nov., 1966], Nos. 17677/66 and 52893/66. Heading H3A. A voltage-controlled oscillator 4 is locked to a harmonic of a crystal oscillator 1 by means of a mixer 3, an I.F. amplifier 5 which also acts as a band-pass filter, a phase comparator 6, and an oscillator 7 operating at the centre frequency of the amplifier 5. The amplifier 5 also feeds a detector 10 and threshold circuit 11 which responds when the oscillator frequency is near a possible controlled frequency. When the threshold circuit 11 operates it opens a gate 9, thereby completing the phase locking loop, steps on a digital counter 14 controlling a visual frequency display 15, and reverses a commutator 8 because the locking frequencies are alternately below and above harmonics of the oscillator 1. The commutator 8 may reverse the phase of the input from oscillator 7 to the phase comparator 6. The output frequency of oscillator 4 is determined by an input voltage I which is applied through an inertial network 16 which prevents too rapid an alteration of frequency, so that the oscillator 4 passes through all possible locking frequencies before reaching the selected one, and the counter 14 registers the correct frequency. A connection from the network 16 to counter 14 determines the direction of counting. The gate 9 may be omitted, and the phase comparator 6 and oscillator 7 may be replaced by a frequency discriminator. The oscillator 4 may be tuned by a varactor diode. A more detailed circuit, Fig. 2 (not shown), shows how the commutator and counter are controlled by the detector 10 and by the input I. This input may be derived from a manually adjustable potentiometer (25, Fig. 2), or from a potentiometer (41, Fig. 3, not shown) adjusted automatically by an electric motor (M) until a number on the frequency display is equal to a desired number fed into the display unit. The oscillator 4 may form the first local oscillator of a radio receiver, a second local oscillator being used to interpolate between the possible frequencies of the oscillator 4.