Patent GB-830304-A (GB)
Title: Repeaters for amplifying signals of two different frequency ranges travelling in opposite directions
IPC: H04B3/38
Filed: 1957-12-20
Abstract: 830,304. Two-way transmission systems. WESTERN ELECTRIC CO. Inc. Dec. 20, 1957 [Dec. 28, 1956], No. 39647/57. Class 40 (4). A repeater, e.g. for a carrier system in which a single amplifier 40, Fig. 2, is used to amplify simultaneously high-frequencies of one direction (W-E) and low-frequencies of the other direction (E-W) comprises two hybrid frequency branching networks 18, 19. Each such network, e.g. 18, comprises two hybrid devices 20, 21 whose outputs PP<1>, SS<1> are connected by low-pass filters 24, 27. These filters are mutually inverse, i.e. when fed with signals outside their pass-band reflect them with 180 degrees phase difference. In the following description the arms A, B, P, S of the hybrids will operate as follows:-An input signal to A causes equal signals of the same phase on P and S. An input signal to B causes the emission of oppositely phases signals on P and S. In-phase signals on P and S reinforce in A and oppose in B. Out-of-phase signals on P and S reinforce in B and oppose in A. A high-frequency signal arriving at W splits equally to produce in-phase signals in arms P, S of hybrid 20. These signals are reflected 180 degrees out of phase by filters 24, 27 so that on re-entering the hybrid they combine to emerge from arm B. The signals then enter hybrid 22 and after emerging out of phase in arms P, S are reflected back in phase by filters 25, 26 so that they combine and emerge from arm A and pass to the amplifier 40. On emerging therefrom they are similarly treated by hybrids 21, 23 and emerge at E. Lowfrequencies arriving at E, however, produce inphase signals in branches P<1>, S<1> of hybrid 23, which signals through filters 25, 26, recombine in hybrid 22 to produce an output in arm A which feeds amplifier 40. The output thereof splits in arms P<1>, S<1> of hybrid 21, passes through filters 24, 27 and recombines in hybrid 20 for emission at terminal W. A detailed circuit, Fig. 5 (not shown), illustrates the circuit, using the normal type of hybrid coil with a centre-tapped winding. In a second embodiment, Fig. 6, pairs of hybrids such as 20, 22, Fig. 2, are replaced by a single transformer having two centre-tapped windings. Thus high-frequency signals incoming at W split in the lower winding of T1 to provide inphase signals to filters 66, 67 which then reflect them back out of phase, so that they reinforce and are induced in the upper winding of T1. After reflection at 64, 65 with a mutual 180 degrees phase shift, they are now emitted over the centre tap of the upper winding for amplification at 68. The amplifier output is treated in a similar manner by transformer T2 and emerges at E. Low frequencies incoming at E split in the upper winding of T2, pass through filters 64, 65 and combine in the centre tap of the upper winding of T1. After amplification at 68 they split in the lower winding of T2, pass through filters 66, 67, combine at the centre tap of the lower winding of T1 and emerge at W. Undesired feed-back due to differences in transmission characteristics of the mutually inverse filters may be minimized by oppositely poling one of the windings of T1 with respect to the corresponding winding on T2. In a third embodiment, Fig. 7, both high-pass filters 76, 77 and 88, 89 and low-pass filters 78, 79 and 86, 87 are used, each pair being mutually inverse. Thus high-frequency signals at W are reflected by low-pass filters 78, 79 to induce signals in the upper winding of T3 whence they pass via transformer 100 to amplifier 90 and thence via highpass filters 88, 89 and T4 to low-pass filters 86, 87, whence they are reflected back for emission at E. Low-frequency signals from E pass by T4, reflection from high-pass filters 88, 89, lowpass filters 86, 87, transformers 102, 100, reflection from high-pass filters 76, 77, transformer 100, amplifier 90, transformer 83, reflection from 88, 89, transformers 83, 73, low-pass filters 78, 79, transformer T3, reflection from 76, 77, transformer T3 to terminal W. Resistances 74, 75, 84, 85 are adjustable to balance out discrepancies in transmission characteristics of the networks. The filters of a pair are preferably identical and of the " antimetric " type i.e. the product of their input impedances looked at from either end is real. These filters are then connected in opposite directions in the appropriate pair of hybrid limbs. This method facilitates accurate matching of the pairs of filters. In the case of microwave signals these may be obtained by using identical filters, one of which is positioned #/4 further away from its hybrid than the other. In these cases "magic- T " hybrids may be used.