Patent GB-498186-A (GB)
Title: Frequency stabilization systems
IPC: H03C3/08
Filed: 1937-10-01
Abstract: 498,186. Automatic control systems; wireless signalling. STANDARD TELEPHONES & CABLES, Ltd., SMITH, J. W., and THAYER, G. N. Oct. 1, 1937, No. 26621. Convention date, Oct. 17, 1936. [Class 38 (iv)] [Also in Group XL] Oscillation frequency.-In frequency - stabilizing means in high - frequency carrier - wave communication systems in which energy is fed back at constant amplitude to control the carrier wave generator, quick-acting automatic volume control means control the amplifiers in the feed-back circuit, and the control potentials for these are obtained from an auxiliary circuit fed with demodulated waves from the same source as the feed-back circuit. This auxiliary circuit may set limits to the frequency range over which the feedback circuit output can be operative. The feed-back circuit, which may be used as a receiving circuit for signals from a distant station, may comprise two amplifying circuits, the attenuations in which vary with frequency in opposite senses from equality. Energy radiated at 9 from an oscillating circuit comprising an oscillator 1 and Lecher wires 2 has its frequency controlled by a variable condenser comprising fixed plates 15, 16, and a movable plate 17 which is mounted on a polarized electromagnetic driving device 14. A microphone 10 can be coupled by switch 12 to the device 14 to produce frequency modulation or to the transformer 13 to give amplitude modulation of the carrier wave. Radiated energy is received at 20 and heterodyned at 21 whereby frequency variations are accentuated. A wide band amplifier 25 with uniform gain over the frequency range, and preferably comprising a large number of stages, feeds into two parallel paths comprising buffer amplifiers 26, 33, frequency selective filters 27, 34, rectifiers 28, 35 and output resistances, respectively similar except that the filter circuits produce attenuations which are equal for the assigned value of mean carrier frequency but vary in opposite senses with variation of that frequency. For operation at high frequencies, these filters may be simple resonant circuits tuned respectively to frequencies above and below that desired. The low-frequency voltages, varying with the frequency variations of the carrier, thus obtained are differentially applied through an equalizer, or phase correcting filter network 30, to push-pull amplifiers 31, 32 which energize the device 14 to correct the frequency. Amplitude variations in the input to filters 27, 34 are corrected by an automatic volume control voltage from a resistance 49 in a third circuit comprising a band-pass filter 47 fed from amplifier 25. This circuit, through amplifier 51, also energizes a relay 38 which may operate an indicator 39 and disconnects the device 14 from the feed-back circuit when the frequency range approaches the limits of resonance of the filters 27, 34. Volume limiters of known types may be used in place of the resistance 49, e.g. the tubes of amplifiers 26, 33 may be operated above plate current saturation. The system is stated to produce compensating adjustments of oscillator frequency when operating with amplitude modulation. By substituting a resistance-capacity filter, transmitting only direct current and very low frequencies, for the phase equalizer 30, the oscillator may be stabilized for slow variations only. The feedback portion of the circuit may be used for reception of frequency or amplitude modulated signals from a remote transmitter, switches 42, 43, 45 being provided. The system is described in connection with more detailed drawings in the Specification. The timing control device is shown in Fig. 2, springs 18 holding the centre tapped operating coil centrally in the air-gap of the magnet 14,