PERFORMANCE 1NDE,X OF QUARTZ PLATFIS 229 



meter and in the oscillator under test are identical for the same amplitude 

 of oscillation. By this means, the PI meter directly indicates the effective 

 oscillator capacitance, C/. The amplitude of oscillation must be duplicated 

 in as much as Ct is not independent of the amplitude in most oscillators. 



15.40 Description of Oscillator Generating "ci" 

 The generator plays no part in the theory of PI measurement as it could 

 be replaced by a signal generator or any other suitable source of radio fre- 

 quency energy. It is convenient, however, to utilize the voltage appearing 

 across Ca as an input to an amplifier whose output represents the generator. 

 This in effect constitutes a feedback oscillator whose frequency is controlled 

 by the crystal under test. Initial consideration of the over-all charac- 

 teristics of the PI meter oscillator leads to the following requirements. 

 The oscillator must, 



1. Be capable of oscillating all crystals usable in other oscillator circuits. 



2. Be capable of operating the crystal over a wide range of shunting 

 capacitances in order to duplicate all the frequencies of oscillators 

 now in the field. 



3. Be capable of permitting high degrees of AVC control in order to 

 maintain the generator voltage constant while the frequency is ad- 

 justed for reasonance. 



If the generator voltage, Ct , is constant, resonance of the crystal circuit 

 is essentially indicated by maximum crystal current, and oscillation is 

 maintained at that resonant frequency. The adjustment to obtain maxi- 

 mum current is such that the phase shift throughout the oscillator loop is 

 Itth where « = 0, 1, 2, 3, etc. As previously described the phase shift and 

 resulting frequency of oscillation are varied by a tuned circuit. The 

 generator voltage, d , is held constant by an automatic amplitude control 

 similar to the automatic volume control which is often applied to amplifiers. 

 The manual control of the magnitude of the generator, g, , is provided by 

 an adjustment of the bias voltage of the automatic amplitude control circuit. 

 In this way the maximum or start gain is independent of the setting of the 

 amplitude control. 



Automatic amplitude control (commonly referred to as automatic volume 

 control, A VC) of an oscillator may be applied by the separation of the limiter 

 from the linear amplifier. This means that in order to apply a high degree 

 of AVC to the PI oscillator (Fig. 15.4), the input voltage of the limiter 

 must be above the threshold of limiting by an amount exceeding the variation 

 in the j3 path caused by the AVC control. This enables the limiter to 

 absorb the changes in the gain of the linear amplifier such that )u/3 = 1 at 

 all times. 



The time constant of the limiter is fast compared to that of the AVC 



