Patent GB-583117-A (GB)
Title: Improvements relating to selection and sychronising arrangements for electrical pulses
Filed: 1943-11-26
Abstract: 583,117. Pulse selecting, synchronising and demodulating systems. STANDARD TELEPHONES & CABLES, Ltd., and EARP, C. W. Nov. 26, 1943, No. 19826. [Class 40 (v)] Regularly repeated pulse signals are selected by a gate circuit under the control of pulses derived from a sinusoidal wave generated by a source under the control of pulses which have passed the gate. The arrangement may be used for avoiding interference or as a frequency divider or for selecting particular pulses in a multi-channel system, or as a means of synchronising at a pulse frequency, or at a submultiple thereof, one or more oscillation generators by pulses sent over a line or radio link. The output may be taken from the gating valve or from the generator of the gating pulses. Arrangements using selective filter as sine wave generator : Fig. 2 shows a gating valve V, the output from which is used to drive a tuning fork F. The fork output is applied to a limiter L the output of which is differentiated in a circuit C1, R1 to obtain the gating pulse. The rectifier or diode X provides a level control and ensures that between the gating pulses the valve is blocked, but is open in the absence of pulses, and on the positive peaks of the gating pulses. Instead of the steel tuning fork type of filter shown, a piezo crystal or a conventional wave filter, or other resonant system may be used. The tuning fork ensures in addition that variation in amplitude produces the phase change necessary to adjust the timing of the gating pulses for which otherwise some form of non-linear network is necessary. Similar non-linearity may be obtained by utilizing variations in the saturation of a magnetic core or by overbiasing the first stage of the limiter. In a modification, Fig. 3 (not shown), early and late gating valves, are used so that when the gating pulses are correctly timed the incoming pulses pass through both channels. With this arrangement the sine wave generator is driven or controlled by the valves, the energy from the one being 90 degrees phase displaced with respect to the other. The limiting valves are appropriately biassed so that the output and gating pulses may be derived. Figure 4 shows an arrangement with an initially open gating valve VC and early and late opening gating valves VA VB. The gating valves are coupled to the generator SWG with phase differences of 90 degrees so that at synchronism the drives from the valves VA, VB are in opposition. The Specification shows the waveforms produced by the limiting valves LVC, LVA, LVB, which are arranged in cascade and from which appropriately timed gating and ..output pulses are obtained. The gating pulses fed to the valves VA, VB are unsymmetrical in consequence of being derived in part from the anode of the limiter VC, and in part from either the limiter LVA or the limiter LVB so as to ensure appropriate timing. Arrangements using a valve oscillator as the sine wave generator : Synchronization ion this case is effected by coupling the tuning inductance of the oscillator to the ouput of the gating valve, Fig. 6 (not shown). An arrangement is shown in Fig. 7 in which early and late gating valves VA, VB provide opposite compensating effects on the tuning of the oscillator VO so that the incoming pulses are meshed between the two series of gating pulses, preferably a crystal or fork controlled oscillator is used to enhance the selection. A wave filter may be inserted between the gating valves and the oscillator to prevent transient phase control of the oscillator while amplifiers may be inserted before or following the oscillating or other sine wave source. In arrangements using self-running oscillators, the gating valves are initially shut so that it is unnecessary to provide a level control such as the element, Fig. 2. Arrangements avoiding hunting : Fig. 9 shows a sine wave generator SWG driven through the amplifiers A1, A2 from early and late gating valves VA, VB to which gating pulses are applied, as described with reference to Fig. 3. The gating valves VA, VB have a differential output load R11A, R11B feeding differential rectifiers QA, QB to produce voltages which control the gain of the ampliers A1, A2. The gating valves VA, VB also feed a common load transformer TA from which the amplifiers A1, A2 are fed in quadrature so that unbalance will vary the relative gain in the amplifiers and thus the phase fed to the generator SWG. Over-correction or hunting is prevented by the time constant circuit associated with the rectifiers, and a nori-linear element to convert amplitude into phase variations is unnecessary. In one modification, Fig. 8 (not shown) a single gating valve drives the generator through a pair of amplifiers, while in another, Fig. 10 (not shown), the amplifiers are fed in phase opposition from a transformer in the common circuit of a pair of gating valves. Since a very highly selective filter or stable oscillator is necessary for separating closely adjacent pulse trains, and since a low decrement resonating system is a pre-disposing cause of hunting, a small aperiodic leak across the resonant element may be required completely to eliminate hunting. A bridge connection may be used and be connected in series or in parallel with the input and output circuits, Figs. 14 and 15 (not shown). In such an arrangement the bridge is unbalanced by the necessary amount to produce the required leakage. Multi-channel and frequency dividing arrangements : An identifying tone, for example, a sub-audible frequency of 50 p.s. for a telephone channel, may be sent to enable a particular channel to be selected or one channel may be reserved for identification of phase. Alternatively, frequency division of the incoming pulse train may be obtained by tuning the filter or the oscillator to a sub-harmonic of the incoming pulse train. In this way, the gate or gates will be opened only to admit every nth pulse of the train. Fig. 11 shows a modification in which the gating valve V feeds through a transformer tuned to an identifying signal. In this arrangement rectified pulses bearing the identifying tone, control the gain of the amplifier A2 fed in quadrature with the oscillator VO through the condenser C16, and feeding the oscillator tank circuit LO. In this way drift in the gating pulses varies the gain in the amplifier A2 and thus adjusts the phase in the circuit LO. Both the transformer T and the coil L3 are tuned to the frequency of the identifying tone frequency. The modulation may be of amplitude, duration or time phase, or a combination thereof. With time phase modulation, the pulses occurring in varying positions up and down the trailing edge of the gating pulses will produce amplitude modulation in the output of the gating valve. In a modification similar to Fig. 9, pairs of gating valves and amplifiers associated by a differential rectifier are used together with a fork controlled oscillator, Fig. 12 (not shown). In a further arrangement, Fig. 13 (not shown), using two gating valves and two amplifiers associated with an oscillator, the output is taken through a differential transformer associated with the gating valves in the case of time phase modulation, but by a transformer common to the two gating valves with duration or amplitude modulation. The range over which synchronization can be maintained depends on the range of the phase control which can be extended if variation in phase through the resonating element is eliminated. Fig. 16 shows an amplifying selective element in which the valves A1, A2 are driven in quadrature and gain is controlled by rectifiers QA, QB. These rectifiers are fed in push-pull by the transformer TK and in parallel by the transformer TJ tuned to the pulse frequency so that the output at the terminals OUT is maintained in phase with the input at terminals IN despite phase changes introduced by the fork system. In a modification a piezo-electric crystal, Fig. 17 (not shown), is substituted for the tuning fork.