COLD CATHODE TUBES FOR AUDIO FREQUENCY SIGNALS 1377 



to the fact that some of the electrons have gained sufifcient energy for 

 excitation. A slight further increase in anode-cathode distance usually 

 results in the anode glow changing from a uniform layer to a "ball'' of 

 glow. When this occurs, oscHlations of several volts amplitude appear 

 across the tube terminals. These oscillations result from a sequence of 

 events which is initiated when the electrons gain enough energy in passing 

 through the anode fall region so that they may ionize. A small number of 

 ions generated in this region will, because of their relatively low velocity, 

 enable a large electron current to flow without developing space charge! 

 This then mil reduce the voltage appearing across the anode fall and 

 greatly reduce the number of ionizations taking place. As soon as the 

 recently produced ions leave this region the voltage drop across this 

 region increases causing the ionization to build up again. This alternate 

 building up and decaying of ion density results in the observed oscilla- 

 tion which is ordinarily in the frequency range from 0.5 to 20 kilocycles 

 per second. 



This oscillation usually cannot be prevented by external circuit means. 

 However, by proper choice of anode-cathode spacing, type and density 

 of the gas filling, and to lesser extent the geometry of the anode, a tube 

 can be made which is free from anode oscillations. The main restriction 

 that this puts on tube design is the limitation of breakdown voltage. 



IMPEDANCE 



From the previous discussion of transmission requirements it is clear 

 that one of the most important properties of a gas tube is the impedance 

 presented to small ac signal currents superimposed on the steady dc op- 

 erating current. At low frequencies, these signals cause the voltage across 

 the tube to vary in accordance with the static characteristic. The im- 

 pedance of the tube to these signals is almost entirely resistive and is 

 equal to the slope of the static characteristic. At higher frequencies, how- 

 ever, there is a lag in the adjustment of the voltage across the discharge 

 to the changes in current. Hence, at these frequencies, the impedance of 

 the tube may have both resistive and reactive components. This is illus- 

 trated in Fig. 4. 



The small superimposed signals result in current- voltage loci which are 

 ellipses. In current range HI at 200 cps, the position of the ellipse corre- 

 sponds to a negative resistance in series \nth an inductance. The negative 

 resistance changes rapidly with frequency and as shown at 2,000 cps, it 

 may be positive. Because of the rapid variation of impedance, both \nth 

 frequency and current, this range of currents is not generally useful for 

 dependable transmission of voice frequency signals. 



