Patent GB-931190-A (GB)
Title: Signal generator system
IPC: H03L7/04
Filed: 1960-08-24
Abstract: 931,190. Automatic frequency-control systems. AVCO CORPORATION. Aug. 24, 1960 [Aug. 25, 1959], No. 29313/60. Class 40 (5). In a microwave signal generator a tunable microwave oscillator 30 is stabilized at a frequency # = F(m F 1 n ##) where F is that of a stable master oscillator 33, m F 1  (m being an odd number) are selectable reference frequencies which are a multiple of ##, and n ## (n an odd number) are the centre frequencies of a plurality of selectable frequency discriminators connected in parallel with common output applied through a switchable polarity reversing stage, to control the tunable microwave oscillator. As shown, a selector/ programmer 20, using a staircase voltage source 70, initially tunes the oscillator 30 approximately to the frequency f required. The tunable microwave oscillator 30 of frequency f is beat with a stable master oscillator of frequency F in a mixer 32 to derive a difference frequency (##F) which after passing through a broad frequency band amplifier 34 is again mixed with a selected one of reference frequencies mF 1  in a mixer 35 again to derive a difference frequency mF 1 -(##F). This, again amplified, is passed through a selected gated amplifier/limiter to one of four parallel connected dual slope frequency discriminators 43, 53, 54 or 55 having a common output connected through a reversible polarity gate 58 to an A.F.C. line stabilizing the oscillator 30. Each discriminator comprises a broad band and a narrow band discriminator in cascade, giving a broad control range, yet finally precise control. One or more frequencies in succession may be produced by the selector and programmer control 20. The latter may also, by a staircase voltage, adjust the frequency F of master oscillator 33 by short steps of e.g. 0.1 Mc/s. To this end the frequency F is applied to a pair of rectifier mixers 74, 75 connected to a cavity resonator off-tuned from the frequency F by, e.g. 30 Mc/s., the second mixer 75 feeding an I.F. amplifier 79 tuned to this difference frequency and with output fed back to the first mixer 74. Oscillations are therefore generated by amplifier 79 of phase shift, and therefore frequency, sensitive to any deviations of F, and this I.F. oscillation after frequency reduction in a mixer 87 is detected in a discriminator 88 and used to stabilize the frequency F of master oscillator 33. The frequency reduction is obtained from a tunable oscillator 82 normally stabilized by feedback to a reactance stage 85 from a comparator 84 which compares two counters 83, 81, respectively producing a negative going output from the frequency of tunable oscillator 82 and a positive going output from a stable crystal oscillator 80. The selector 20 may however apply a staircase wave to either of the counters 83, 81 to alter stepwise the frequency of the tunable oscillator 82 and hence the input to discriminator 88 and frequency of master oscillator 33, and therefore the frequency f of the tunable microwave oscillator 30. The signal generator may be used for producing a programme of frequencies, and for driving a steerable antenna array in a track-while-scan system.