Patent GB-916525-A (GB)
Title: Improvements in or relating to broadband communication systems
IPC: H04B7/17
Filed: 1960-12-07
Abstract: 916,525. Radio signalling. WESTERN ELECTRIC CO. Inc. Dec. 7, 1960 [Dec. 28, 1959], No. 42099/60. Class 40 (5). In a broad band communication system, repeater stations include means for adjusting the level of the signal radiated therefrom in accordance with the level of the signal received at the next station. For this purpose a a sample of the signal level received at each repeater station is sent to the immediately preceding station on an auxiliary channel and there compared with a reference level derived from the incoming broad band signal level. The system described uses travelling wave tubes and pulse code modulation is employed. The Figure shows the portion of a repeater station corresponding to the west-east direction of transmission, the auxiliary channel, which contains the information for controlling the east-west portion being separated from the incoming signals by a band-pass filter BPF, and the remainder of the broad band signal being amplified by travelling wave tubes 64, 66, and separated into the corresponding sub-channels by filters 68 to 78. The output of each filter is supplied to an input level detector 80, followed by a variable attenuator 82 and an output level detector 84, and then recombined for amplification by the travelling wave tube 86. The auxiliary channel is supplied to a frequency-changer 88 receiving the output of an oscillator 90 and the intermediate frequency is amplified at 92 and demodulated at 96. The demodulator provides separate outputs relating to the signal level for each of the six sub-channels derived from the next repeater to the west, and this information is supplied to the east-west portion of the repeater. The output of the amplifier 92 is also supplied via a limiter 94, frequencychanger 98 and filter 100, to a travelling wave tube 102, to act as an auxiliary channel carrier, level control signals from the east-west section being supplied to the tube 102 by a modulator 104, for transmission to the next repeater to the east. An isolator 106 prevents interaction between the auxiliary channel and the broad band channel, and the signals are combined at 108 for application to the antenna 110. A modulator 112, corresponding to the modulator 104, accepts the level control information from the detectors 80 for transmission via the auxiliary channel of the east-west portion to the next repeater to the west. The level control information from the next repeater to the east, which is associated with the eastwest auxiliary channel is supplied by a demodulator 114, the six output levels being supplied respectively to A.G.C. units 116 where a comparison is made with the signal level detected by the corresponding detector 84 in order to control the attenuators 82. A detector 117 samples the level of the combined outputs and via a unit 120 is arranged to control the cathode current and thus the gain of the tube 86, and this control signal is also supplied to the A.G.C. units to compensate for the change in gain of the tube 86. In this way the life. of the travelling wave tube is extended since it oper ates at full power only in the presence of maximum fading in the propagation path.