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BELL SYSTEM TECHNICAL JOURNAL 



frequency operation. This advantage more than offsets resulting problems 

 of modulator design and automatic frequency control. 



Conforming with the definitions of insertion phase and transmission, the 

 measurement system compares, with respect to phase and amplitude, the 

 outputs of two transmission channels energized from the measurement 

 frequency source, one of which serves as a standard or reference channel, 

 while the other contains the apparatus under test. This is illustrated by 

 the block drawing in Fig. 2. 



For loss measurements the range attenuator I (Fig. 2) is set at db. 

 Measurement frequency F from the master oscillator is appHed to both 

 standard "5" and unknown "X" channels through spUtting pad /. The 

 voltages at "S" and "X" modulator inputs, points A and B respectively 



Z. 



% 1 >/\A/' * • — 



T 



INSERTION LOSS 



OR 

 INSERTION GAIN 



= 20 LOG 



INSERTION PHASESHIFT = Z-E^-Z-E, 

 Fig. 1 — Definition of quantities measured. 



in Fig. 2, differ with respect to phase and amplitude because of the trans- 

 mission differential introduced between the two channels by the apparatus 

 under test. By frequency conversion in the "S" and "X" modulators 

 these amplitude and phase differences at frequency F are translated at 

 points C and Z> to a constant intermediate frequency, 31 kc. The second 

 input to the "5" and "X" modulators, of frequency F + 31 kc, is supplied 

 by the slave oscillator which automatically tracks at constant 31 kc dif- 

 ference with respect to the master oscillator. By selective filtering, only 

 the difference frequency appears at the modulator outputs C and D. 31 kc 

 has been chosen as the intermediate frequency, primarily on the basis of 

 filtering requirements in the modulators. The detector (Fig. 2) compares 

 the voltages of the "X" and "S" channels at K and L as to magnitude 



