GB754185A - Multiplex frequency modulation transmitter - Google Patents

Abstract

754,185. Multiplex radio signalling. ARMSTRONG, E. M., [Representative of ARMSTRONG, E. H.]. Oct. 5, 1954 [Oct. 12, 1953], No. 28661/54. Drawings to Specification. Class 40 (5). In an F.M. multiplex system an oscillation f 1 is frequency modulated by a main channel signal applied to a phase-shift modulator, frequency multiplied to a mean frequency nf 1 and applied to a frequency converter, and the oscillation f 1 , without modulation, multiplied to the same frequency nf 1 , beat with a second oscillation f 2 to form a carrier frequency (nf 1 Œf 2 ), and the latter frequency modulated in a second phase shift modulator by a second signal channel and also applied to the frequency converter so that a carrier of centre frequency f 2 , frequency modulated by both the main and auxiliary channels is obtained. This may be further frequency multiplied before transmission. A plurality of auxiliary channels, each a frequency modulated subcarrier of different centre frequency, may be applied to the second phase shift modulator, or an additional channel, not frequency modulated, may be added to a subcarrier applied to the second phase shift modulator. The phase shift modulators are preferably of " Serrasoid " type, employing a sawtooth wave to generate a pulse time modulated by the channels. Bandpass filters in the main and auxiliary channels may limit the frequency deviation to avoid cross-modulation of channels, and compensa; tion networks with rising frequency characteristics may counteract attenuation of the higher frequency deviations. At the receiver high pass filters separate the auxiliary channels. Specifications 621,228, 668,030 and 752,628 are referred to.