Patent GB-741867-A (GB)
Title: Improvements in or relating to frequency stabilisers and monitors
Inventor: LEA NORMAN
IPC: H03B5/34
Filed: 1952-10-23
Abstract: 741,867. Automatic frequency control systems. MARCONI'S WIRELESS TELE. GRAPH CO., Ltd. Oct. 19, 1953 [Oct. 23, 1952], No. 26671/52. Class 38 (4). [Also in Group XL (c)1 An oscillator 1. is stabilized or monitored in frequency with reference to the frequency of a standard resonant circuit 2 by including the latter and a resistance 3 in series in one limb of a bridge to which the oscillator frequency is applied, the other limb 4 also being a resonant circuit with series resistance tuned to the same standard frequency, the phase at the output of whose junction 8 is wobbled symmetrically with respect to the applied phase and hence to the phase of the other junction 7 when the oscillator frequency is exactly at its controlled frequency. As shown, the phase wobbling may be effected by a multi-lobed conducting element 13111 rotated at constant speed, from a motor M, adjacent equal inductances 11a, 11b in opposite arms of the limb 4, to vary differentially the inductances. The output at 8 therefore varies in phase between 8b and 8a (Fig, 5) relative to the applied phase 6, 5, and as the oscillator deviates in frequency the phase at terminal 7 may vary between 7 and 71. The difference in amplitude between the junction 7, 8 therefore varies between the vector 7, 8b and vector 7, 8a, at the phase wobbling frequency and its relative amplitude 7, 8b or 7, 8a during any particular half-cycle of the wobbling frequency depends on the sense of the frequency deviation. Preferably, for maximum sensitivity, R/R 3  = R10a/R10b so that the point 7 lies approximately on the line joining 8a, 8b. The sinusoidally amplitude-modulated difference wave may be applied at 21a to a radio frequency amplifier and through a cathode follower circuit 21b, an amplitude modulation detector 22 and low-frequency amplifier 23 to one input phase of a two-phase motor M<SP>1</SP>. The other phase of the motor is derived from a similar but angularlyadjusted lobe 27, also driven from the motor M, acting on an electromagnet 28 so that after amplification at 2a it is in phase quadrature at the low-frequency with the first input to the motor M<SP>1</SP> and acts as a reference source to the said first input variable amplitude. The motor M<SP>1</SP> is therefore driven in one or other direction and acts through a gear-box 31 to apply a correcting adjustment to the tuning condenser 32 of the oscillator 1. In modifications the lobes 13<SP>111</SP>, 27 are not employed but the junction 8 connected to a motor-driven differential capacitor in parallel with the arms of the circuit 4, or through slip-rings with a motordriven inductance coupled with the inductances 11a, 11b. Or a driven double pole switch may alternately open and close a short-circuit across both an inductance 11a and capacitor 12b, the inductances 11a, 11b each having twice the reactances of the capacitors 12a, 12b, so that the series resonance is unaffected, or a single-pole driven switch 13<SP>11</SP> (Fig. 8) may be connected to the junction 8 and alternately connect a series tuning capacitor ( - 2X) in the upper or lower arm of the circuit 4. Junctions 7 and 8 in the latter cases are connected to an amplitude difference detector with output connected to a phase sense gate, synchronously driven with the switch. The output is then passed through a low-pass filter to the device controlling the oscillator frequency, e.g. a reactance valve.