GB562315A - Improvements in or relating to frequency and phase modulation receivers - Google Patents

Abstract

562,315. Frequency and phase modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 22, 1942, No. 18222. Convention date, Dec. 22, 1941. [Class 40 (v)] A balanced demodulator for frequency- or phase-modulated waves is arranged to be non-responsive to amplitude modulation, without the use of a preceding limiter, by suitable choice of the input amplitude and of the load resistances of the balanced rectifiers. The rectifiers may also be arranged to de-emphasise the higher modulation notes. As shown, the receiver comprises a pre-amplifier 31, local oscillator 33, mixer 32, I.F. amplifier 35 and frequency - discriminator 71, 91 of the phasing type feeding balanced grid-leak detectors 1, 2 with their anode circuits coupled differentially to the LF stages. The Specification includes diagrams, Figs. 2 and 3 (not shown), which indicate that, particularly for high values of the grid leaks 3, 5, the output to the LF stage is substantially independent of carrier amplitude between considerable limits, so that the receiver is non- responsive to amplitude modulation. The time constants of the grid leaks and condensers may be such that loss of high notes occurs, and may be designed to counterbalance pre-emphasis of the high notes at the transmitter, or to give correct reception of a phase-modulated transmission. AFC voltage may be obtained across condensers interposed between the inner ends of the primary'coils 50, 51 of the output transformer 52 and may be applied to a reactance tube 70 for correcting the frequency of the local oscillator 33. The carrier level at the input to the demodulator may be varied manually in any suitable way, or an AVC rectifier 37 and load resistance 38 may be used to regulate the amplification of the stages 31, 35. The discriminator may be of the type described in Specifications 296,678, [Class 40 (v)], and 426,228, using circuits tuned respectively above and below the intermediate frequency, Figs. 1 and 5 (not shown) ; and the output transformer 52 may be replaced by resistance coupling, Fig. 4 (not shown).