ELECTRONIC RELAY TESTER 929 



to 10 volts with 10 volts appearing when the armature is operated. 

 A typical voltage-displacement curve is shown on Fig. 4. This output is 

 applied to the grid of the "zero set" cathode follower and its bias is set 

 so the output will be from approximately —2 volts to +2 volts. This 

 voltage is applied to the horizontal dc amplifier of the scope. 



Theory of Operation of Gauge 



Since the capacitance probe is in the tank circuit the frequency of the 

 oscillator changes with variations in the separation of the probe elec- 

 trodes. Hence a displacement of the moving electrode causes a change 

 in the frequency but no change in the amplitude of the alternating 

 current through the pentode plate. The radio frequency voltage that 

 appears across the coupling impedance is the product of the plate cur- 

 rent and the coupling impedance. 



Since the current is constant in amplitude: 



E = hZ, 



where E = rf voltage applied to rectifier, /o = rf plate current from os- 

 cillator, and Z = coupling impedance. 



Since a change in E is due to a change in the oscillator frequency 

 and the resultant change in Z; 



Let / = K- = oscillator frequency, 



2ir 



/2 = ;r— = resonance frequency of coupling circuit. 



2 



T 



Q = 



and 



002jL'2 



f 

 7=1--. 



J2 



where L2 = inductance of coupling circuit, and R = resistance of 

 coupling circuit. 



The coupling impedance can be written : 



At resonance Zmax. = ^iL-iQ. 



