GB641916A - Improvements in or relating to electric frequency discriminators - Google Patents

Abstract

641,916. Frequency and phase modulation. STANDARD TELEPHONES & CABLES, Ltd. Feb. 24, 1948, No. 5407. [Class 40 (v)] A frequency discriminator for demodulating frequency or phase-modulated waves comprises two frequency-dependent networks to which the waves are applied separately, preferably through constant-current pentode valves. The networks have different impedance-frequency characteristic curves which intersect at the mean carrier frequency and have equal and opposite slope and zero curvature at the point of intersection. This results in a linear characteristic over a wide band of frequency deviation and renders the discriminator suitable for wideband multiplex signalling. As shown, the F.M. wave is applied in parallel to the control grids of pentode valves 31, 32, the anodes of which are connected through condensers 41 to dissimilar networks comprising resistors 42, 56 shunted respectively by sections of transmission line 43, 55 open or shorted at their outer ends. The networks are shunted by separate rectifiers 44 feeding load resistors 46 through low-pass filters 45. The resistors 46 are connected in the grid circuits of amplifiers 47, 51 and the demodulated signal is taken off at terminals 29, 30 across the two anodes. The principles involved in the design of the discriminator are explained in Specification 641,915. The coaxial lines 43, 55 may be replaced by parallelresonant circuits or other resonating devices. Trimming adjustments may be provided; for example, the heater current of one of the diode rectifiers 44 may be adjustable. The invention may also be applied to the known discriminators in which the F.M. wave is passed through a quarter-period delay network, and the voltages of the delayed and undelayed waves are added and subtracted, and the resultants applied to the two rectifiers.