GB937949A - Improvements in or relating to carrier current communication systems incorporating repeaters - Google Patents

Abstract

937,949. Unattended repeaters. STANDARD TELEPHONES & CABLES Ltd. July 8, 1960 [July 9, 1959], No. 23609/59. Class 40 (4). In a carrier communication system, noise generation in a repeater amplifier is tested by transmitting from a station to the repeater amplifier a signal of characteristic frequency, amplifying such signal in an auxiliary amplifier at the repeater, modulating the output of the auxiliary amplifier with a sample band of frequencies from the output of the repeater amplifier, and transmitting a band of modulation products to the station. A two-way repeater 2, Fig. 1, transmits high frequencies from East to West and low frequencies from West to East. Band pass filter S passes test frequency S originating at West. A rectifier 5 acting as a frequency doubler is connected between filter S and a second filter 2S. Filter 2S transmits a portion of the frequency doubled signal to West through repeater 2, the doubled signal being appropriate to the high frequency direction of transmission. The output from filter S (or from rectifier 5) is also connected by auxiliary transistor amplifier 6 to modulator M. Band pass filter N feeds the modulator M with a band of noise from the output of the repeater amplifier, this band being in a part of the frequency spectrum away from the normal transmission bands, which part would be attenuated by the transmission path. A band of modulation products from modulator M is selected by filter R and fed to West. Fig. 2 shows the repeater comprising hybrids 11, 14, 15, 16, low pass filters 12, 18 passing only frequencies below about 105 kc/s. and high pass filters 13, 17 passing only frequencies above about 112 kc/s. Hybrids 14, 16 have low transmission loss in the main signal paths to and from amplifier A1 and high transmission loss in the side paths to equipment 19. The characteristic signal selected by filter S and originating at West is in the range 100 to 105 kc/s. and passes via symmetrical hybrid 20 to equipment 19. This comprises a low pass filter 21, auxiliary transistor amplifier A2 and modulator 22 which is fed by amplifier A2 and by the output from repeater amplifier A1 via filter 23 passing noise in the band 290 to 300 kc/s. This band is modulated with the output from amplifier A2, and a band of signals-from modulator 22 of frequency about 190 kc/s. and of band width 10 kc/s. is selected by filter 24 and is returned to West. Non-linear distortion and intermodulation.- Non-linear distortion in amplifier A1 is estimated by transmitting a frequency of 145 kc/s. together with a characteristic frequency of 105 kc/s. from East. The harmonic at 290 kc/s. is selected by filter N and, after modulation with the characteristic frequency, is measured at West. Alternatively a test frequency of 97 kc/s. is transmitted by West, together with the characteristic frequency of 105 kc/s., and the harmonic of the test frequency at 291 kc/s. passes through filter N and is measured at West. Information on intermodulation is obtained by sending a selected pair of test frequencies and measuring the level of specific intermodulation products. Specification 656,188 is referred to.