Cold Cathode Tubes for Transmission of Audio Frequency Signals

TRANSMISSION REQUIREMENTS OF AN ELECTRONIC SWITCH When an electronic switch is used as a substitute for a pair of metallic contacts, a number of requirements must be met in order that voice 1371 1372 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1 9 5 3 frequency currents be faithfully transmitted. These are as follows: 1. Gain or Loss--'The impedance level of telephone voice frequency circuits is usually of the order of several hundred ohms. The impedance of any device inserted in this circuit should be sufficiently small compared to this value in order to restrict the insertion loss to less than 1 or 2 db. Of course, the impedance levels of the circuits may be raised to higher values. Aside from the extra cost of transformers, the problems of noise pickup and crosstalk become bothersome when this is done. Since no amplification of the signal from the telephone set is normally used in local transmission circuits, any value of loss is highly undesirable. In fact, since the use of the electronic switch may require the introduction of other circuit elements which introduce loss, it would be desirable that the electronic switch provide a small amount of gain. The above discussion has considered the gain or loss of the switch in its "closed" condition. Between two busy circuits, there may exist paths consisting of one or more "open" switches. Each of these paths may contribute crosstalk into the circuits. Therefore, impedance required of an individual switch must be high. For a large coordinate switch, there will be a very large number of these undesired paths.