34 MEASURING THE RADIO REFRACTIVE INDEX 



The phase meter in use is merely a multivibrator having a constant ampli- 

 tude output. The pulse from each cavity alternately switches the multi- 

 vibrator "on" and "off." The width of the constant amplitude output 

 pulse from the multivibrator is a measure of the time difference between 

 pulses. The resulting train of constant amplitude variable width pulses 

 is then applied to appropriate recording circuits. The Birnbaum refrac- 

 tometer is adaptable to multi-cavity operation. A single klystron may 

 be used to sweep simultaneously a number of spaced sampling cavities. 

 A single reference cavity permits simultaneous comparison between the 

 several sampling cavities, and important consideration when the scale and 

 form of refractive index variations are desired ([27]. 



The accuracy of the Birnbaum instrument is dependent upon maintain- 

 ing a linear frequency sweep. This problem can be circumvented in the 

 manner of Sargent [16], who modified the Birnbaum refractometer to 

 operate as a microwave hygrometer. A servomechanism is used to tune 

 the sampling cavity to the resonant frequency of the reference cavity. 

 The servomechanism positions a tuning probe in the sampling cavity. 

 The depth of penetration is the measure of the refractive index of the 

 contents of the sampling cavity. This technique minimizes the depend- 

 ence on the sweep characteristics of the klystron. 



The Vetter refractometer [20], figure 2.8, virtually eliminates the de- 

 pendence of the refractometer on electronic characteristics and shifts the 

 limitations to the cavities themselves. In addition, while previous in- 

 struments are primarily relative refractive index indicators, the Vetter 

 refractometer was developed as an absolute refractive index device by 

 using klystron stabilization techniques [33, 34, 35, 36]. 



Figure 2.8 is a simplified diagram of the Vetter refractometer illustrat- 

 ing the basic principle of operation. The reference cavity is excited by a 

 klystron which is modulated by a small sine voltage on the repeller. Any 

 output at the fundamental modulating frequency at the detector of the 

 reference cavity is compared in phase to the original modulating signal, 

 and an error signal is applied to the repeller of the klystron to lock the 

 center frequency of the klystron to the reference cavity. At coincidence, 

 the fundamental disappears in the output of the reference cavity. The 

 same klystron excites the sampling cavity. Any fundamental appearing 

 at the output after phase comparison to the modulating signal develops 

 another error signal. This error signal is used to drive a mechanical 

 servomechanism which tunes the reference cavity to the resonant fre- 

 quency of the sampling cavity with concomitant shifting of the klystron 

 center frequency. This then is a double loop system; the reference cavity 

 controls the klystron; the sampling cavity controls the reference cavity. 



Tuning of the reference cavity is accomplished by a motor-driven 

 probe that penetrates the reference cavity. The design of the probe is 



