Cold Cathode Tubes for Transmission 

 of Audio Frequency Signals 



By M. A. TOWNSEND and W. A. DEPP 



(Manuscript received August 13, 1953) 



Cold cathode gas filled tubes have been extensively applied as electronic 

 switching elements in the telephone system. In general, these applications 

 have been limited to control circuits. The usefulness of these tubes can be 

 further extended by making them capable of carrying voice frequency signals. 

 The transmission properties that are required of the tube for this use are 

 considered. It is shown that troublesome oscillatory noise can be eliminated 

 and that the insertion loss of the tube can be reduced to a low value. Fur- 

 thermore, by a special design of cathode a stable insertion gain of a few db 

 may be realized. Other requirements on bandwidth, power and distortion are 

 satisfactorily met. Thus, these tubes are potentially useful in coordinate type 

 switches in which voice frequency signals must be rapidly switched. 



INTRODUCTION 



Cold cathode glow discharge tubes have found increasing usefulness 

 as two-valued switching elements in telephone and other automatic con- 

 trol systems. In many applications the tube functions as a simple control 

 element which either does or does not pass current, depending on the 

 control signals applied. The output signal is in the form of a voltage or 

 a current which can be used to trigger other tubes or operate relays.^ 



In other types of circuits the glow discharge tube may be used as a 

 switch in series with a transmission path for audio frequency signals. 

 When used for this purpose, the tube not only must fulfill the SNNitching 

 requirements but also must meet an additional set of requirements 

 which may be realized by controlling the dynamic properties of the dis- 

 charge. This paper describes some of the characteristics of cold cathode 

 tubes which have been developed for use as switching elements in series 

 with voice frequency telephone transmission circuits. 



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 



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