GB305703A - Improvements in or relating to radio and the like high frequency receiving systems - Google Patents

Abstract

305,703. Mathieu, G. A. Nov. 9, 1927. Multiplex signalling; fading, preventing; measuring and testing apparatus.-A multiplex heterodyne system suitable for receiving a number of separate signals modulated on a common carrier-wave is characterized by the feature that an identical intermediate frequency is obtained at different points along the chain of receiving currents, whereby tuning is simplified and the correct setting can be verified by utilizing a zero beat effect between the said intermediate frequer.cies. The Figure illustrates a receiving network for receiving one telephonic and two telegraphic messages simultaneously. The incoming wave is received on a sharply-tuned aerial T and is stepped down in frequency by a first heterodyne H1, M1 and is then amplified at A1. The telephone frequency-band, which lies at the centre of the whole amplified band, is separated out by a coupling valve CV1 and an associated filter network, and is further filtered at F1, rectified at D1, and passed through an automatic gain-regulator AGC, A2 of the type described in Specification 288,371. The remainder of the amplified band passes through CV2, and is heterodyned at H2, M2 to produce a carrier frequency of 30000 cycles. This is suppressed by a filter F2 but the upper and lower sidebands of the telegraphic signals pass respectively through parallel filters F3, F4. This separation is utilized as a means of reducing fading effects. After amplification at A4, A5 the original 30,000 cycle carrier is re-introduced at constant amplitude by modulators R1, R2 fed from a generator G through coupling-valves CV3, OV4. The separated telegraphic signals after passing through parallel band filters F5, F6, F<1>5, F'6 are rectified at D2 - - D5 and go through limiting-valves L1V1, L2V2 to line in the usual way. The local geneiator G also feeds a detector controlling circuit M3 branched off from the filter F2. Accordingly, if there is any difference in frequency between the carrier from the first and second heterodynes and then from the generator G, this will be heard as a beat-note in a loud-speaker connected to the amplifier A6. As in practice, H2 and G can be anchored together, turning adjustment is limited to setting the first heterodyne H1 to give a zero beat-note. In a modified arrangement an harmonic from the generator G can be used as the second heterodyne H2. Neon tube wavemeters W1, W2 are used to control the frequency of the generators H2 and G, and flashlamp indicators, operated by relays tuned say to 250, 350, and 450 cycles may be inserted in the output circuit of the amplifier A6.