MEASURING SYSTEM FOR V I DFX) 227 



oscillators must always dififer in frequency by a constant amount. This 

 difference is maintained at 31 kc by the control of the master oscillator 

 over the slave oscillator frequency. 



\'ery briefly, the scheme consists in applying the fixed local oscillator fre- 

 ijuency, /, of the master oscillator, to an automatic frequency control 

 rircuit which produces an output frequency/ + 31 kc. / + 31 kc is then 

 modulated with variable local oscillator frequency, f — F, oi the master 

 oscillator, resulting in an output of frequency F + 31 kc. Frequency F, 

 formed by modulation of/ and/ — F, is the master oscillator frequency. 



In the automatic control circuit, frequency / is compared with that of 

 a controlled oscillator, by detecting their difference in a modulator. The 

 nature of the control is such, that any deviation of this difference from 31 kc 

 causes the frequency of the controlled oscillator to change in the direction 

 which eliminates the deviation. While it is simpler to compare / and the 

 controlled oscillator frequency directly, in the slave oscillator the compari- 

 son is made between the outputs of tripler circuits energized from the latter 

 frequencies. In this way more complete isolation is realized between / 

 and the controlled frequency than would be afforded with only buffer am- 

 plifiers. Because of the triphng, it follows that the oscillator must be 

 controlled according to the departure of the difference between the tripler 

 circuit frequencies from 93 kc. This, however, has the advantage of 

 avoiding the generation of 31 kc anywhere in the automatic frequency 

 control circuit, which could, by spurious modulation, cause the / + 31 kc 

 output to be contaminated with small traces of frequency/. The necessity 

 for exceptional purity of / + 31 kc output arises in the measurement of 

 high losses where minute amounts of F at the /^ 4- 31 kc input to ''.S" 

 and "X" modulators may produce appreciable error. 



Owing to phase tracking requirements between "5" and "X" inter- 

 mediate frequency channels, and to the frequency dependence of the phase- 

 shifter calibration, it is necessary to maintain the intermediate frequenc\- 

 as closely as possible to the precise value, 31,000 cycles. The permissible 

 deviation from the correct value has been limited to ±1 cycle. This pre- 

 cise control is maintained in the presence of 10 kc changes in /, which may 

 occur when the setting of the 0-10 kc interpolation dial of the master 

 oscillator is varied in the course of measurement. 



Figures 5A and 5B illustrate the automatic frequency control and hetero- 

 dyne circuits of the slave oscillator. The frequency of oscillator 10 in 

 Fig. 5 A is controlled by the reactance tubes 11 and 12. Reactance tube 12 

 is actuated by direct voltage from frequency discriminator 16, so that it 

 controls oscillator 10 according to frequency error. Frequency error is 

 the difference between the input frequency to discriminator 16 from am- 

 plifier 9, and 93 kc, the frequency of zero voltage output from the dis- 



