GB579671A - Demodulating arrangements for frequency modulated electric waves - Google Patents

Abstract

579,671. Frequency modulation. STANDARD TELEPHONES & CABLES, Ltd. Nov. 12, 1943, No. 18911. Convention date, Nov. 13, 1942. [Class 40 (v)] A frequency-modulated carrier wave 9, Fig. 2, is demodulated by clipping it at the levels 11, 13, differentiating the resulting pulses 10a to produce short pulses 12a alternately positive and negative, and utilizing the pulses 12a to re-constitute square pulses 14a. The re-constitution of the square pulses may be effected in a trigger valve circuit, Fig. 3, having two stable conditions, and being switched from one to the other by the alternate positive and negative pulses 12a. It comprises two pentode valves 22, 24 with anode resistors 50, 52 and control grid resistors 64, 66 and with reciprocal back couplings comprising resistance-shunted condensers 60, 56. The input 12a, Fig. 2, is applied between the cathodes and the suppressor grids 30, 32, and the output pulses 14a are taken from the anode of the valve 24. These pulses are again differentiated to produce the series 16a, Fig. 2, of constant amplitude, and the negative pulses are eliminated by a rectifier to give the series 18a, which is fed to an integrating circuit to produce the modulating signal 21. In a modified arrangement, the series 12a produced by the first differentiation, or the series 18a, are fed to a valve pair (Fig. 5, not shown), the output of which comprises pulses 73 of greater but constant width, which are integrated to produce the signal 21. This latter valve pair is designed with one permanently stable condition to which it returns a determined time interval after being tripped by positive pulses 12a or 18a, thus giving pulses 73 of uniform duration.