REGENERATION THEORY AND EXPERIMENT 



689 



CIRCUIT 

 UNDER 

 TEST 



^ 



f ± lOOO'v 



ATTENUATOR 



PHASE 



"shifter 



ATTENUATOR 



PHASE _ 

 SHIFTER 



P>^^f 



-^^ 



^-nAA/^ 



AMPLIFIER 

 DETECTOR 



w 



Fig. 5 — Schematic diagram of the null method used to measure the transfer factor. 



connected across the input is given by 



PP.cos (27r-1000/). 



If now these two waves are to be made to cancel, there must be a dif- 

 ference in the attenuation of the two branches equal to the ratio 

 Po/Pi, and a difference in the phase shift equal to T 0. The change in 

 sign of the phase angle introduced by setting the beat oscillator above 

 or below the test frequency is most conveniently handled by setting 

 the carrier oscillator consistently on the same side of the test frequency 

 in making a run over the frequency range. 



By using a high gain amplifier preceding the detector, the precision 

 may be made great, limited only by circuit noise and by interference. 

 The attenuators and phase shifters are calibrated separately. It should 

 be noted that any difference in the transfer constants of the two de- 

 modulator circuits may be compensated by an initial adjustment which 

 is carried out by paralleling the input terminals of the two demodula- 

 tors across a source of electromotive force. With the particular type 

 of phase shifter used the phase shift may be changed without altering 

 the attenuation, so that the two settings for amplitude and phase may 

 be made independently. 



