Patent GB-544502-A (GB)
Title: Improvements in or relating to automatic frequency control in frequency or phase modulated wireless transmission
IPC: H03C3/09
Filed: 1940-12-23
Abstract: 544,502. Automatic control systems for high-frequency oscillations ; frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 23, 1940, No. 18063. Convention date, Dec. 22, 1939. [Class 38 (iv)] [Also in Groups XXXVI and XL] The automatic frequency control in a wireless transmitter using a frequency or phase modulated carrier is rendered inoperative in the presence of signals. Valves 118, 120 effecting frequency control are biased to cut off by a signal of the lowest value desired for transmission. Valves 26, 28 generating oscillations at carrier frequency have anodes regeneratively cross-coupled by condensers 30, 32 to a tuned grid circuit comprising a coil 22 the tuning of which is variable according to leading or lagging current drawn by reactance valves 14, 16, the grid circuits of which receive impulses from a signal pick-up 2. The tank circuit 34 feeds the antenna 48 through a power amplifier and/or limiter 44 and frequency multiplier 46. To prevent drift in the mean carrier frequency a coil 50 is coupled to coil 22 or to the tank coil 36 and energy is thereby fed to suppressor grids of valves 56, 58 wherein beats are set up with oscillations controlled by a piezo-electric crystal 68 and suitably higher or lower than the carrier frequency. These beats are resonated in a tuned plate circuit 80 and fed to a resonant line 84 symmetrically about a point 90 to which are applied additive e.m.f.s from coils 94; 96 coupled to the tuned circuit ; alternatively a large condenser may be connected between points 90, 106. The grids of detectors 118, 120 are excited from symmetrical variable points on the resonant line which is one half-wave length long at the beat frequency. The rectified output of valve 118 or of 120 is the greater depending on the sense of any drift in the carrier frequency and the voltage drop in an output resistance 130 is added to or subtracted from the voltage applied to suppressor grids of the reactance valves 14, 16 to correct the drift. A second pair of reactance valves may be connected to the plate circuit 34 of the oscillation generator to prevent the frequency controlling system from opposing frequency modulation by the signal, part of the signal voltage is amplified and rectified, is applied to a time constant circuit 186 so that in the presence of the lowest value of signal desired to be transmitted the voltage impressed in series with a voltage from potentiometer 124 is such as to bias valves 118, 120 to cut off ; where a signal is transmitted frequency drift control is therefore suspended. A switch 180 may be used to permit frequency control at all times. To limit amplitude modulation of the oscillators 26, 28 in addition or alternatively to limiters 44, 46, a rectifier 190 with its input coupled to coil 22 or 36 or to an output circuit of limiter 44 or 46 feeds through a time constant circuit 194 into a resistor 198 in the grid bias lead to the valves 26, 28. The condenser of tuned circuit 80 may be omitted or replaced by a resistance to broaden the pass band fed into coil 82. Taps 890, 892 provide for compensation of any asymmetry in the modulation due to the asymmetric characteristic of the amplitude time characteristic of speech waves ; a reversing switch 894 may facilitate this adjustment or polarity reversal of the signal input may be automatic. Specification 507,826, [Group XXXIX], is referred to. The Specification as open to inspection under Sect. 91 describes also the subject-matter of Specification 546,011, [Group XL]. This subject-matter does not appear in the Specification as accepted.