692 



BELL SYSTEM TECHNICAL JOURNAL 



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100-130 KC 

 OSCILLATOR 



1 



LOW PASS 

 FILTER 



AMPLIFIER 



6 6 



PHASE 1 



0.5-30 KC 



PHASE 2 

 0.5-30 KC 



Fig. 8 — Circuit diagram of a heterodyne type two-phase oscillator, the output 

 frequency of which is continuously variable from 0.5 to 30 kilocycles. The output 

 of each phase and the 90-degree difference between the two phases are practically 

 constant over the frequency range. 



quency oscillator is connected to the common branches of the two push- 

 pull modulators. The fixed frequency oscillator is connected in series 

 with the grid circuits of the two modulators. The resistance-capacity 

 networks shown in the circuits of the fixed frequency oscillator are pro- 

 vided to produce phase shifts of 90 degrees between the two series 

 voltages of the two modulators. In the same manner as that discussed 

 before in connection with the null method measuring circuit, the phase 

 shift introduced to the fixed frequency is maintained in the beat fre- 

 quency output, so that the phase difference of 90 degrees is preserved 

 in the outputs of the two modulators when the variable frequency 

 oscillator goes from about 100.5 to 130 kilocycles. The outputs of the 

 two phases are connected to the test amplifier and to the wattmeters as 

 shown in the preceding Fig. 7. 



Comparison of the Methods 

 Measurements of transfer factors by the two methods outlined 

 above were found to be in agreement within the error of measurement. 

 The visual method as developed was capable of use over only a very re- 



