Patent GB-1170787-A (GB)
Title: Broad-band Signal Transmission Circuits
Inventor: SEIDEL HAROLD
IPC: H04B3/00
Filed: 1966-11-17
Abstract: 1,170,787. Amplifiers &C. WESTERN ELECTRIC CO. Inc. 17 Nov., 1966 [18 Nov., 1965], No. 51474/66. Heading H3W. [Also in Division H1] To permit the processing, e.g. the amplification or delay, of broad-band signals in equipment of inadequate bandwidth, a signal transmission system includes a lossless four-port filter for dividing the broad-band input signal into two overlapping sub-bands each of which is separately processed before being recombined with the others in a second lossless four-port filter to provide a processed broad-band output signal, the total relative phase-shift experienced by the overlapping components of the subbands being arranged to be substantially zero. In the system shown in Fig. 1, the quadrature four-port filter 10 splits the input signal into two sub-bands which have a relative phase difference of #/2. After separate passage through amplifiers 14 and 15, and the addition of a # phase shift to one of the sub-bands, passage through a second, similar quadrature four-port filter 11 recombines the two subbands. The filter 10 and 11 may be constructed as shown in Fig. 2, in which the input signal is applied to a port a of quadrature hybrid 20 to be divided between branches 22 and 23, in each of which that portion of the signal of bandwidth f 1  to f 2  is passed by the filters 24, 25, the signals in the two branches thereafter being recombined in quadrature hybrid 21 to provide an output at port b. The f 2  to f 3 portions of the signals from hybrid 20 are reflected by the filters 24, 25, and recombine in hybrid 20 to provide an output at port b of that hybrid which is #/2 out of phase with the f 1  to f 2  output signal. Three sub-band and four subband systems are described (Figs. 3 and 4, not shown); in these, in order to maintain phase coherency, each branch of the circuit is provided with the same number of similar filters together with their associated 180 degree phase shifters. Alternatively, since filters associated with non- adjacent sub-bands have very little effect upon the phase dispersion of frequencies falling within each others bands, their effect may be adequately approximated by low order phase equalizer networks producing the required time delay, to provide a simplified circuit (Fig. 5, not shown) in which each branch includes compensating filters corresponding only to its adjacent sub-bands. In order to permit the use for each sub-band of signal processing devices, e.g. amplifiers, or acoustic grating dispersive delay lines, operating over the same frequency band, all the sub-bands may be down-converted by heterodyning in mixers to the same frequency band before processing, and thereafter up-converted to form the original sub-bands before they are recombined to form the processed output signal (Fig. 6, not shown); in this system additional time delay networks must be included to compensate for the varying delay differences arising between the subbands due to the fact that they are all amplified at the down-converted, rather than their original sub-band, frequencies. Reference is made to the use of 180 degrees, instead of quandrature, four-port circuits.