Cold Cathode Gas Tubes for Telephone 

 Switching Systems 



By M. A. TOWNSEND 



(Manuscript received September 4, 1956) 



Cold cathode gas tubes perform both switching and memory functions in 

 telephone switching systems. One measure of the performance of a switching 

 diode is the switching voltage gain, defined in terms of the characteristics of 

 the demce. Some of this gain must be sacrificed in order to increase the switch- 

 ing speed, in a way which is analogous to the gain-bandwidth property of a 

 conventional amplifier. In this paper, methods of achieving a high switching- 

 voltage gain are described in terms of the gas discharge processes. An example 

 is given of an application of these principles to a tube for use as a switch in 

 series with the talking path in an electronic telephone switching system. 



INTRODUCTION 



Gas discharge tubes have found extensive use in telephone switching 

 and other digital systems. Most of these applications take advantage of 

 the fact that both switching and memory can be provided by a single 

 gas discharge device. The switching characteristics result from the fact 

 that the device is an essentially open circuit when the gas is not ionized 

 and a closed circuit when the gas is ionized. The memory function is 

 possible because the tube can be held in a high current condition, once 

 it is ionized, by a voltage which is too low to initiate this conduction. 

 Thus a triggering signal which ionizes the tube is "remembered" until 

 the holding voltage is removed and the tube is allowed to de-ionize. 



In some applications, gas tubes are used as switching devices in series 

 with voice frequency circuits. For this purpose, the tube must offer a low 

 impedance to audio frec^uency signals in addition to meeting require- 

 ments of switching and memory. 



This paper first describes some switching characteristics of gas tubes 

 considered as circuit elements. Desirable performance objectives are es- 

 tablished in terms of these device characteristics. Following this, physi- 

 cal processes within the tube are described as they relate to circuit per- 



755 



