1372 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



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 com- 

 pared 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 introduc- 

 tion 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 con- 

 tribute 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. This requires that the 

 open impedance of the individual switch be of the order of hundreds of 

 megohms. 



2. Bandwidth — ^It is desirable that the electronic switch transmit 

 faithfully frequencies of 300 to 3,500 cycles per second. 



3. Power Output and Distortion — ^ Since the impedance of the elec- 

 tronic switch may vary with the current passing through it, distortion 

 may be introduced when the current savings are large. On the other hand, 

 without the proper current swing, insufficient power Avill be delivered to 

 the load impedance. Telephone circuits need to handle powers of the 

 order of a few milliwatts with a harmonic distortion less than a few 

 per cent. 



4. Noise — ^The noise introduced into a telephone switching system 

 by an electronic switch should not be noticeable to a subscriber. This 

 means it should be below 10~^ or 10"^ watts. 



5. Stability — The properties that have been considered above must 

 be highly stable with time. In central office use, such devices might be 

 used for periods of ten to forty years. 



STATIC CHARACTERISTICS 



A common form of glow discharge tube comprises a pair of metal 

 electrodes in a glass envelope which is carefully evacuated and filled with 



