930 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



Sin(!0 the maximum value of E is obtained when Z is at a maximum: 



Z_ = ^ = ^ (2) 



provided co/^co2 is near unity. 



Probe and Grid Circuit of Oscillator 



The oscillator frequency is determined by the resonant frequency 

 of the tank circuit. Fig. 3 shows that the tank comprises the capacitance 

 of the probe plus a fixed capacitance and an inductance. The probe which 

 is assumed to be a parallel plate condenser has a capacitance which is 

 inversely proportional to the plate separation. 



Let: d be plate separation of probe, di be separation at resonance 

 (where / = /a), 



d 

 di 



Then the variable capacitance of probe is given by: 



n _ fi di _ Ci 



l^p — <- 1 -^ — 



d X 



and the total capacitance of the tank by: 



C 



Cq -{- Cp = Co + — , 



X 



where Co = fixed capacitance of tank including fixed part of probe 

 capacitance. 



Let: 



then 



(-9 





and 



1 



LiCc 



LiCo(l + K) 



where w and 0)2 are radian frequencies of the oscillator at probe separa- 

 tions d and di respectively and Li is the effective inductance of the tank 

 circuit. It follows that: 



