GB640986A - Improvements in or relating to arrangements for transmitting electric signals occupying a wide frequency band over narrow band circuits - Google Patents

Abstract

640,986. Transmission systems. STANDARD TELEPHONES & CABLES, Ltd. Feb. 7, 1947, No. 3709. [Class 40 (iv)] A broadband signal, i.e. 100-6000 cs., is divided into several frequency bands for transmission over separate channels having narrow frequency limitations, i.e. 300-3200, frequencies outside the "cut-off" frequencies of the channels being shifted for transmission by two successive stages of frequency changing in one of which the oscillator frequency may be adjusted to suit the cut-off frequency of the channels. The signal is applied via the amplifier 5 and passes via the low pass filter 6 and the hybrid coil 7 to the modulators 16 and 10 having oscillators 17 and 11 of 11.5+fc. (the cut-off frequency) and 12 kc/s. respectively, a pad 9 being introduced in the path carrying the lower frequencies to equalize the levels of the signals over the two channels. The outputs from the modulators 16 and 10 selected by the band-pass filters 18 and 12 having pass bands of 8.8-11.7 and 5.7-11.9 respectively are applied to the modulators 19 and 13 passing to both channels, bands of 300-3200 cs. which are each demodulated at the receiving end by two stages of demodulation, and combined in the hybrid 28 to reproduce the original signal. The L.P. filter 26 removes the upper sideband and programme leak from the demodulator 24, and the filter 27 prevents beating and intermodulation between unwanted products of the two channels, while pad 33 restores the two sections of signal to their proper relative levels. The oscillator 8 generates test frequencies, say 1 kc., with prominent 2nd and 4th harmonics for transmission over the channels, the received signal being checked on the monitoring circuit 36 for frequency shift.