ELECTRONIC RELAY TESTER 



927 



cient gain to provide adequate deflection voltage to the horizontal plates 

 of the scope. 



The oscillator comprises a 6AU6 pentode tube and an associated tank 

 circuit. The tube is connected as an electron coupled oscillator with the 

 energy for the oscillations being supplied by the screen circuit. The plate 

 output is a modulated current that passes through the screen and sup- 

 pressor grids. Considerable isolation of the coupling circuit from the 

 oscillator is obtained by this arrangement. 



An antiresonance circuit similar to the tank circuit is used to couple 

 the plate of the oscillator tube to the rectifier. Since the electron coupled 

 oscillator acts as a high impedance generator or constant current source 

 the voltage across the coupling impedance is proportional to the value 

 of the impedance. Hence as the oscillator frequency is shifted by a change 

 ill the position of the armature a corresponding shift occurs in the ac 

 voltage across the coupling impedance and rectifier tube (6AL5). The 

 output voltage of the rectifier also shifts with the input to the rectifier 

 since the changes are slow compared with the time constant of the out- 

 put circuit of the rectifier. 



In normal operation the output of the rectifier may vary from G volts 



