Patent GB-936473-A (GB)
Title: Improvements in or relating to electric variable frequency oscillators
Inventor: LAWSON ANTHONY NEWTON
IPC: H03L7/22
Filed: 1960-01-07
Abstract: 936,473. Automatic frequency control systems. STANDARD TELEPHONES & CABLES Ltd. Jan. 7, 1960, No. 575/60. Class 40 (5). Relates to an oscillation generator of the type which can be set to any one of a plurality of discrete frequency values within a predetermined frequency range, and including a source of a standard frequency wave by which the selected frequency value is maintained constant. An alarm is operated if the automatic frequency control fails. As described, the generator provides outputs within the range 3 to 6.999875 mc/s. in steps of 125 c/s. and is controlled by five switches (not shown). Switch I selects frequencies in the range 3 to 6 mc/s. in 1 mc/s. steps, switch II selects frequencies in the range 0 to 900 kc/s. in 100 kc/s. steps, switch III selects frequencies in the range 0 to 90 kc/s. in 10 kc/s. steps, switch IV selects frequencies in the range 0 to 9 kc/s. in 1 kc/s. steps, and switch V selects frequencies in the range 0 to 875 c/s. in 125 c/s. steps. As shown, an oscilator 1 is set to a value F1 within the frequency range by the switches I, II, III, to the nearest 10 kc/s., and supplies output line 2 via amplifier 3, the finer adjustment being carried out by a frequency control device 4. A standard fixed frequency oscillator 5 supplies a standard frequency wave of 100 kc/s. from which a frequency generator 6 provides reference frequencies of 1 mc/s, 100, 10 and 1 kc/s., and 125 c/s. The output of the oscillator 1 is supplied to a circuit 7 comprising three mixing stages and corresponding local oscillators and filters, Fig. 2 (not shown), the local oscillators being phase-locked by the frequencies 1 mc/s., 100 kc/s., and 10 kc/s. from the generator 6 and the oscillator frequencies being set by frequencies I and II so that the frequency F2 from the mixing circuit is within the range 110.125 to 210 kc/s. The frequencies supplied by all the local oscillators are each an integral multiple of one or other of the reference frequencies. The output of the mixer 7 is supplied to a phase discriminator 8 which also receives a frequency F3 lying between 110.125 and 210 kc/s. from an oscillator 14 in a comparison oscillator circuit 10 which is controlled by switches III, IV and V. The output of the phase discriminator 8 is supplied via a low-pass filter 8A to control the frequency of the oscillator 1 through the device 4 to provide a fine adjustment to the nearest 125 c/s. as set by the switches. If the automatic frequency control is lost, a lowfrequency scanning oscillator 12 comes into operation, and as soon as control is regained the impedance presented to the oscillator 12 falls to a low value and stops the oscillator. The circuit 10, Fig. 3 (not shown), comprises an oscillator 13 which is phase-locked to the reference frequency 1 kc/s. and by frequency division produces an output frequency F5 having one of eight values in the range 17.125 to 18.0 kc/s. in steps of 125 c/s. selectable by switch V. The oscillator 14 is provided with a frequency control device 15 and supplies the waves F3 also to a three-stage mixing circuit 16 similar to mixer 7 and controlled by the waves of frequencies 1 and 10 kc/s. from the generator 6. In circuit 16, gating circuits are used instead of the phase discriminators of circuit 7. The output F6 of the circuit 16, lies within the range 17.125 to 18 kc/s. and is set by switches III and IV. The waves F5 and F6 are supplied to a phase discriminator 17 to control the oscillator 14 so that the frequency F6 is made equal to the frequency F5. Since F2 has been made equal to F3 the output frequency F1 at 2 corresponds to the setting of the five switches. A scanning oscillator 19, similar to 12, re-establishes the automatic frequency control of the oscillator 14 if it should be lost. An alarm circuit 20, Fig. 2 (not shown), is controlled by the 125 c/s. reference frequency operating three gating circuits respectively connected to the output of the two automatic frequency control circuits included in the circuit 7 so that if, due to failure of a frequency control circuit, the frequencies at these connecting points are not an exact multiple of 125 c/s., an alarm is given. Arrangements may also be provided so that the scanning oscillator 12 operates an alarm to indicate failure of the main frequency control loop. In an alternative frequency generator circuit, Fig. 4 (not shown), the final output frequency is built up in successive mixer stages by an addition process. All the oscillators are phase-locked to the reference waves and there are no main frequency control loops 11 and 18 as in Fig. 1.