GB671194A - Improvements in or relating to electric frequency modulated oscillators - Google Patents

Abstract

671,194. Frequency mediation; valve generating circuits. STANDARD TELEPHONES & CABLES, Ltd. July 12, 1950, No. 17454/50. Classes 40 (v) and 40 (vi). The feed-back loop of a frequencymodulated valve oscillator includes in series a piezo-electric crystal and a device for shifting the phase of the feed-back voltage in accordance with the signal voltage, the arrangement being such that with zero signal voltage the oscillator frequency is equal to the frequency of series resonance of the crystal. In Fig. 1, the anode of the oscillator valve 1 is connected through a crystal 2 and condenser 3 to a phaseshifting or modulating device 5. This comprises two valves 23, 24 with their grids fed from the condenser 3 through a phase-advancing capacity 33 and a phase-retarding coil 34. Their cathodes are connected by a resistor 29 with a biasing resistor 32 tapped on the resistor 29, so that the valves can be balanced to give equal mutual conductance. The grids are fed in push-pull from the signal source 10 and the anodes are connected in parallel to the grid of the oscillator valve 1. As the phase-shift at the modulator 5 varies in the rhythm of the signal, the oscillation frequency adjusts itself until the phaseangle of the crystal 2 is equal and opposite to that produced by the modulator. The effect of shunting capacities across the crystal is tuned out by a coil 13 so that the crystal operates at series resonance. The anode circuit of the valve 1 is broadly tuned to the crystal frequency, and the modulator anodes are fed through a tuned coil 21 and resistance 25. Output is taken from the tuned anode circuit 19 of an amplifier 17 whose grid is connected to the oscillator anode through condensers 18, 3 and the crystal 2. In Fig. 2, feedback to the valve 1 is effected through an amplifying-valve 39 which has a tuned anode circuit 41 shunted by a thermionic reactance 43. As the value of this reactance is varied by the signal voltage applied at terminals 44, 45, the phase of the anode voltage is varied, and the oscillation frequency changes until the crystal 2 gives a corresponding phase-shift in the opposite direction. In a further modifiedtion, Fig. 3 (not shown), a tuned transformer is inserted between the anode lead from the valve 1 of Fig. 2 and the grid lead to the valve 39. The crystal 2 and the thermionic reactance are connected in series between the transformer primary coil and earth. The invention is particularly applicable to frequency-shift telegraph systems.