Patent GB-195838-A (GB)
Title: Improvements in high frequency signalling systems
IPC: H04H20/67
Filed: 1922-02-27
Abstract: 195,838. Western Electric Co., Ltd., (Western Electric Co., Inc.). Feb. 27, 1922. Relaying or repeating systems; thermionic oscillation generators, amplifiers, and modulating systems; harmonics, producing and utilizing; multiplex signalling; homodyne receiving systems. -In order to minimize interference between signalling stations, the wave, frequencies employed by the several stations, both for transmission and for homodyne reception, are compounded cf a number of frequencies radiated from a central control station. Thus if the control station radiates frequencies f1, f2, a a signalling station may pick up any or all of these frequencies and compound them to produce a frequency lf1+mf2+nf3, where l, m, n are any integers. The multiplication of frequency may be effected by producing harmonics and filtering out the desired multiple, and the addition and subtraction by moduation and filtering. Where one of the frequencies f1, f2 &c. is too low for efficient radiation, it may be transmitted as a modulation of one of the other frequencies; or a low frequency may be generated independently at the signalling station. Intermediate relay stations may be interposed between the control station and the signalling stations, and these relay stations may also generate or modify the control frequencies. Fig. 1 shows diagrammatically one arrangement of the stations, the central station 1 radiating one or more frequencies which are picked up at the stations 2, 3, 4, 5 and after the necessary frequency modification are utiaized in the transmitters T2 - - T5 and receivers R2 - - R,. Stations 2, 3 may be arranged for duplex working, transmitter T2 signalling to receiver R3 at one frequency, and transmitter T3 to receiver R2 at a different frequency. Fig. 4 shows the circuits at a typical signalling-station. An aerial 48 receives the control frequencies from the central station and is doubly-tuned by branch circuits 49, 51 to the two control frequencies f1, f2. A circuit 50 tuned to f, is connected to an amplifier 53 and thence to an harmonic producer 55 having its output circuit tuned to the l<th> harmonic, of frequency lf1. This is then modulated by a local source of comparatively low frequency f3 and produces a frequency lf1+f3, which is selected by tuned circuits 59, 60 and applied to a second modulator 61. This modulator also receives current of frequency f2 from the second branch circuit 51 through an amplifier 54. The output circuit 63 thus carries, among others, a current of frequency Lf1+f3+f2, which is selected by a tuned circuit 64 and applied through an amplifier 66 to a modulator 67. The signal to be transmitted, shown as arising from a telephone circuit 84 is also applied to the modulator 67 through a balanced transformer 83 and an amplifier 87, the signal-modulated current passing to the transmitting aerial 89 through a filter 88. The frequency f2 passes from the amplifier 54 to an harmonic producer 68 and the m<th> harmonic (of frequency mf2) is selected by circuits 69, 70 and combined in the modulator 71 with the frequency f, from the other branch. The circuit 75 thus applies to the demodulator 76 a frequency mf2+f1, which is identical with the signalmodulated frequency received on the aerial 77 and therefore produces a low-frequency signal current which passes through a filter 80 and amplifier 81 to the transformer 83 and thence to the telephone line 84. The aerials 48, 77 are so arranged as to be unaffected by the radiat;on from the local transmitter 89. In some cases; a single aerial may replace the aerials 48, 77. When one of the control frequencies is radiated as a modulation of another control frequency, the modulation frequency is obtained at the signalling station by rectification. The harmonic generators, modulators, de-modulators, amplifiers, and rectifiers referred to are preferably of the thermionic type.