Patent GB-883687-A (GB)
Title: Drive pulse generator for providing different selectable frequencies
IPC: H03B5/30
Filed: 1959-04-03
Abstract: 883,687. Valve circuits for pulse generating; limiting; frequency selective amplifying; oscillating; frequency dividing. CREED & CO. Ltd. April 3, 1959 [April 9, 1958], No. 11416/59. Class 40(6). A drive pulse generator comprises a tuning fork coupled to an oscillatory circuit connected to an amplifier, and means for squaring the amplifier output and producing from the square wave sharp driving pulses which are applied to the driving coil of the tuning fork. Oscillator circuit. The oscillator comprises a tuning fork 12 driven by a coil L1 and coupled to a transformer T1, the secondary of which is tuned to the resonant frequency of the fork (1800 c./s.) by C2, C3. The sine wave output from this tuned circuit is connected to the grid of V1A, and the output from VIA is fed to V1B. A feedback path from V1B to the cathode of VIA contains rectifiers CR1, CR2 which are biased to conduct when the signal of the anode of V1B exceeds a predetermined value, and so provide negative feedback to V1A. The resulting waveform produced at the anode of V1B is a square wave w, which is fed through the differentiating circuit C1R1L1 to provide sharp pulses p to drive the tuning fork. An output is also taken from the anode of V1B and fed via a cathode coupled clipper through a differentiating circuit to the divider stages. Frequency dividers. The dividers comprise an asymmetrical multivibrator V3A, V3B which divides by three, two binary dividers V4 and V5 (producing a 150 c./s. output), a cathode follower V6, a differentiating output transformer and two silicon or germanium diodes CR3 and CR4. Valve V3A is held saturated by the positive bias on its grid so that it only responds to negative going input pulses, and the time constant of discharge path of capacitor C9 is set by R20 to allow the stage to be triggered by every third negative input pulse. The primary of transformer T2 is broadly tuned to a centre of the desired band of frequencies (1200 c./s. to 1500 c./s.) and the output which contains odd harmonics is fed over conductor 18 to a contact on one section S3 of the frequency selector switch (Fig. 2). The transformer output is also rectified by diodes CR3, CR4 to produce a signal containing even harmonics which is fed over a lead 17 to a second contact on S3. An 1800 c./s. output is also fed direct from V2B via lead 16 to a third contact on S3. Filter and output amplifier circuits. The signal selected by S3 is connected via the cathode follower V7A to a cathode driven amplifier stage V7B and a further two stage amplifier V8. Negative feedback is applied from V8B to the grid of V7B through one of the twin-T null filters 20-24 each of which is tuned to reject a required output frequency. For example with the switch in the position shown, negative feedback is applied to V7B at all frequencies except 1350 c./s., so that this frequency alone receives full amplification through V7 and V8, all other frequencies being cancelled by the negative feedback. This desired output signal is fed through a limiter circuit CR5, CR6 which produces a two volt peak to peak square wave at the grid of V9A. The square wave output at the anode of V9A is thus differentiated by C28, R66, and the cathode follower V9B is biased to cut off so that only the positive pulses (12 volts peak) are present at the output terminal 20. Only 5 output frequencies (1200, 1350, 1500, 1650, 1800 p.p.s.) are provided as illustrated, but more may be obtained by the use of additional filters tuned to other frequencies and connected to the other contacts on switch sections S1, S2, corresponding further connections being made on S3.