Patent GB-587544-A (GB)
Title: Automatic frequency controlling systems
IPC: H03C3/09
Filed: 1944-06-30
Abstract: 587,544. Automatic control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 30, 1944, No. 12518. Convention date, June 30, 1943. [Class 38 (iv)] In an angle-modulated transmitter stabilisation of the mean carrier frequency is effected by (1) heterodyning the modulated transmitter output with oscillations characteristic of the transmitter output and derived therefrom by means of a degenerative loop whereby the phase variations of the resultant energy, corresponding to the phase or frequency variations of the transmitter output, are reduced within small limits, e.g. one radian, (II) detecting the reduced phase variations and (III) applying these detected variations to the transmitter to vary the mean frequency thereof in a sense to nullify drifts therein. A portion of the modulated output of the transmitter 10, Fig. 1, which with the reactance type modulator 14, may be as described in Specification 504,766, is heterodyned in converter 16 with energy derived from a stable crystal controlled oscillator 18. The difference-frequency modulated output is then heterodyned in a converter 20 with modulated oscillatory energy from a selfexcited generator 28. The output of converter 20 is passed to a frequency modulation detector 24 the output of which is filtered at 25 to remove direct current and the low frequency drift component, amplified if necessary, and passed to a reactance type modulator 26, similar to 14, to modulate the carrier generated at 28. The output from oscillator 28 is thus modulated similarly to the desired signal modulation of the transmitter (10) output and the modulation is arranged to be degenerative whereby the phase variations of the output of converter 20 are reduced to within one radian. This output is applied to a phase detector 38 wherein the anodes of diodes 42, 44 are energized through transformer 40, in anti-phase, by the output from converter 20 and, in phase, through transformer 45 with energy of a frequency equal to the mean frequency of the output from converter 20 by the oscillator 18. Since the phases of the potentials applied to the diode anodes through transformer 40 vary with the phase variations of the output of converter 20, a D.C. potential is built up across-the resistors 46, 48, which, among other components, represents the relatively slow frequency drift of...the carrier generated by oscillator 10. High frequency components of the demodulated output from detector 38 corresponding to the signal modulation are removed by byepass condensers shunting the load resistors 46, 48. Audio-frequency components are filtered out by a network 50. The remaining low-frequency and D.C. components of the output potential from detector 38 are applied to the reactance modulator 14 in known manner in a sense to oppose drifts in the mean frequency of oscillator 10. Since in the arrangement of Fig. 1 the output of detector 38 contains audio frequencies corresponding to the modulating signals applied at 14, the F.M. detector 24 may be dispensed with and the output of detector 38 applied to the filter 25. In a modification, Fig. 2 (not shown), the phase detector 38<1>, Fig. 3, replaces the converter 20, detector 24 and phase detector 38 of Fig. 1.. The modulated output from generator 28 is applied through a transformer 40<1>, the output from converter 16 through transformer 45<1> and that from oscillator 18 through transformer 45<11>. The diodes 42, 44 form all possible beats between the various signals impressed on them. Capacitors 46<1>, 48<1> byepass all frequencies greater than audio so that the demodulated output 4 contains only the audio corresponding to the difference between the frequency modulation components from the outputs of converter 16 and oscillator 28 plus a low-frequency and a direct current component corresponding to the frequency drift in generator 10 relative to generator 18. This output is applied to the filter 25 in the degenerative loop and to the audio filter 50 which passes only the low frequency and D.C. components to the modulator 14 as indicated above. Specifications 489,094 and U.S.A. Specification 2,229,640 also are referred to.