100 BELL SVSTlUr TECHNICAL JOURNAL 



from the two lateral branches. The })ortion applied to the calibrated 

 variable phase shifter at the top of the hgure becomes the homodyne carrier. 

 The remaining portion is applied to a balanced crystal modulator- through 

 a second variable phase shifter which need not be calibrated. The latter 

 was introduced in order that the phase of any modulated power reflected 

 due to an imperfect balance in the modulator could be shifted so that it 

 would be in quadrature with the homodyne carrier and would, therefore, 

 not produce an audible signal in the detector. 



The portion of the power which enters the modulator is modulated by a 

 signal derived from an audio-frequency oscillator. The suppressed-carrier, 

 double-sideband signal which leaves the modulator is applied, after a certain 

 amount of attenuation, to the input of the device to be measured. Probes 

 are provided at the input and output of the latter for sampling the signal. 

 Provision is made for connecting either probe to a crystal detector of the type 

 used for detecting an amplitude-modulated signal. 



The homodyne carrier emerging from the calibrated phase shifter is 

 attenuated to a level of about one milliwatt and is applied to the crystal 

 detector. The output of the detector is connected to an audio-frequency 

 amplifier terminated by a pair of headphones or an output meter. An 

 attenuator may be placed between the amplifier and the detector as an aid 

 in measuring the magnitude of a transfer impedance. 



The procedure for adjusting the apparatus and measuring phase is as 

 follows: 



With both sampling probes disconnected from the detector the variable 

 phase shifter between the oscillator and modulator is adjusted until the 

 output of the detector is zero. This balances out the effect of any signal 

 reflected by the modulator. The input probe is then connected to the 

 detector and the calibrated phase shifter is adjusted until the signal disap- 

 pears in the audio output. When this occurs the homodyne carrier is in 

 quadrature with the signal sidebands, and the resultant signal applied to the 

 detector is equivalent to a phase-modulated wave having a low modulation 

 index, and consequently is not demodulated by a detector of the type used 

 here. 



The input probe is then disconnected from the detector and the output 

 probe connected. The phase shifter is again adjusted for a null in the audio 

 output. The difference in phase between the two adjustments of the phase 

 shifter is equal to the phase shift between the input and output of the 

 device. If the probes are not located exactly at the input and output termi- 

 nals of the unknown it may be necessary to make a correction in the meas- 



- (', F. Edwards, "Micruwave Conveners," Proc. /. A'. E., Vol. 35, Xo. 11, pp. USl- 

 11<)1; November 1947. 



