Patent GB-294213-A (GB)
Title: Improvements in or relating to transmission systems for use in electric signalling systems
IPC: H04L27/04
Filed: 1928-07-19
Abstract: 294,213. Marconi's Wireless Telegraph Co., Ltd., (Assignees of Hansell, C. W.). July 20, 1927. [Convention date]. Keying and modulating systems ; multiplex signalling ; fading, preventing.-A radio transmitter is controlled by a remote keying-means and an alternating-current land line, the key preferably operating to change the frequency in the land line. The alternating-current modulates an intermediate-frequency wave in a carrier-suppression modulator, and only one side-band is passed on to a further modulator. The modulation is thus effected in two or more stages, one side-band being passed on from stage to stage. Some of these stages may be at the keying end of the land line. For reducing fading, a frequency wobble may be introduced at either end of the line, and the period and extent of the wobble may be under the control of the keying-operator. Multiplex signalling may be effected by using- different frequencies for the alternating-current in the line. Frequency-multipliers and amplifiersmay be used after the modulation stages. In Fig. 1, the land line 6 includes a key 4 and an A.C. generator 2 at the operates end, and is coupled at its other end to a balanced modulator 10 supplied by a carrier-wave source 8. The output is taken through a filter 12, which selects one of the side-bands, and an amplifier 14 to an aerial 16. According to the invention, the selected side-band is, as shown in Fig. 3, applied to a balanced system 10 and used to modulate a higher frequency supplied by a crystal controlled oscillator 8. From the output, one side-band is selected and passed on to an amplifier or frequency-multiplier 40 by a series-connected crystal 30 resonant to the desired side-frequency. The capacity effect of the crystal electrodes is neutralized by a condenser 32. To enable a narrow band of frequencies to be passed further crystals 34, 36 having adjacent resonant frequencies are connected in paralle 1 with the crystal 30. A tunder circuit 38 is connected in the grid circuit of the valve 40, and the output is taken to further amplifiers or frequency-multiplieis 14 and to the aerial 16. Fig. 5 shows an arrangement in which one stage of modulation together with a frequencywobbling device is arranged at the operator's end of the land-line 6. The keyed alternating-current in the circuit 50 is used to modulate a wave of higher frequency supplied by an oscillator 52 in a carrier-suppression modulator 60. A side-band selected by a filter 54 passes to the land-line 6. The oscillator 52 is controlled by a crystal 56, one electrode of which 62 can vibrate under the control of a spring 65 and an electromagnet 68. The coil of this magnet is included in a tuned circuit maintained in oscillation by a valve 72. Vibration of the electrode 62 wobbles the frequency of the oscillator 52, and the period and extent of the wobble are adjusted by the tuning- condenser 76 and by shifting the tapping-point 78. The frequency wobble may be introduced at the transmitter, and at an intermediate or at the final carrier frequency stage; but preferably the wobble is introduced and controlled at the operator's end of the land-line so that it may be varied in response to requests from the distant receiving-operator. In multiple working, the wobble of each signal channel may be independently controlled, as shown in Fig. 6. The frequencies in the land-line 6 are determined by oscillators 80 tuned by condenses 100. Each oscillator is keyed by a key 86 connected across part of an iron-core inductance 82 in its oscillatory circuit. The wobble generator 90 energizes a coil 88 coupled to the coil 82, and is driven by a motor 94 supplied by a direct-current power line 96. The wobble period is varied by a rheostat 98, and the extent of the wobble by a rheostat 92.