Patent GB-1001402-A (GB)
Title: A switch particularly for use in communication systems
IPC: H03K17/76
Filed: 1962-03-27
Abstract: 1,001,402. High frequency switching system; diode gating circuits. WESTERN ELECTRIC CO. March 27, 1962 [March 28, 1961], No. 11571/62. Headings H3P, H4J and H4R. A switch for selectively connecting an input channel with one of a plurality of output channels comprises a plurality of gates, having means interconnecting them with the input channel, each gate being connected to an output channel, each gate in one state presenting a low loss T- network, and in its second state forming with the interconnecting means a band-pass filter. By thus incorporating the capacities of the gating elements, as well as other stray capacities into the structure of the band-pass filter their adverse effects may be eliminated. The invention is thus of particular use for switching high frequency channels. In the gate, Fig. 3, if terminal L3 is biased positively with respect to L4 diodes 9, 10 conduct and their resistances together with resistance 25 form a low-loss T pad for signals passing from L1 to L2 the characteristic impedance being matched to the input by choice of resistor 25. With L3 biased negatively with respect to L4, diodes 9, 10 are non-conducting and diode 25 conducts, so that a high loss is introduced between L1 and L2. Moreover the chokes 20, 22 together with the capacities of the diodes and any stray capacities are equivalent to parallel L-C networks connected across the input and output paths. The interconnecting networks between the gates and the channels are of the form shown in Fig. 4, and comprise a shunt inductance 28 with series arms on either side, each comprising an inductance and a capacitance. Such an arm in combination with the L-C circuit mentioned above constitutes a bandpass filter. The invention is applied to the system shown schematically in Fig. 1A in which input channels A-F are normally connected to outgoing regular channels, but any of which may be connected to a spare or " protection " channel X or Y. In the normal case it is required to terminate the input channel with its characteristic impedance such as 2a. (This is the impedance 4 in the subsequent description). To utilize the invention the input channel is connected to terminals 41, Fig. 4 and the outputs 30a, 30b, 30c are connected respectively to a matching impedance 4, and to interconnecting networks leading to the two protection channels. These latter networks are similar to Fig. 4, except that they have six inputs (the other five being connected to similar gates for the other input channels) and one output leading to the protection channel. When the regular channel is in use only the gate leading to the matching impedance 4 is " on," and the equivalent circuit is that of Fig. 5, the input being connected to the matching resistor 4 by low losspad 33 via a band-pass filter comprised of the series impedances 26, 27, 29a, 30a and shunt impedances 28 and 32, the latter representing the effects of the two " off " gates. When a protection channel is in use only the gate leading to that channel is " on " and the equivalent circuit is that of Fig. 6, the only difference compared with Fig. 5 being that the low-loss pad 33 is now connected via a further arm of an interconnecting network and shunted by the remaining five " off " gates (associated with the other input channels) to form a further band-pass filter. Where only one protection circuit or less than six input channels are provided, dummy circuits may be provided. Specification 973,733 is referred to.