DIRECT MEASUREMENT 



33 



SAMPLING 

 CAVITY 



REFERENCE 

 CAVITY 



Figure 2.7. The Bimbaum refradomeler. 



of the sampling cavity of approximately 9.4 kc/s if the nominal operating 

 frequency is 9400 Mc/s. 



For the measurement of small scale variations of refractive index, a 

 modified Grain refractometer utilizing a 10.7-Mc/s center frequency with 

 a 200-kc/s linear range has been used [29]. Scales of 1 to 20 A^ units for 

 full scale deflections of a 1-mA chart recorder have been used. 



The Birnbaum refractometer [32], illustrated in figure 2.7, applies the 

 resonance principle in a somewhat different fashion. Both the reference, 

 or sealed, cavity and the sampling, or open, cavity are passive elements. 

 The cavities are slightly different in frequency and are of the transmission 

 type having crystal detectors at the output. 



A single klystron, frequency modulated by a sawtooth voltage and 

 having an output frequency linear with time, excites each cavity in se- 

 quence. During the frequency excursion of the klystron, the respective 

 cavity resonances are excited, and a pulse formed at each crystal detector. 

 Since the resonant frequencies of the cavities differ, the two output pulses 

 will be displaced in time. If the modulation on the klystron is periodic, 

 then the output pulses will be also displaced in phase, the displacement 

 being a function of the difference in resonant frequencies. Any change in 

 the refractive index of the contents of the sampling cavity will alter the 

 phase difference between the two pulse trains. The relative phase be- 

 tween the two outputs can then be measured by an electronic phase meter. 



