A Method of Measuring Phase at Microwave Frequencies 



By SLOAN D. ROBERTSON 



A method of measuring microwave phase differences is described in which it 

 is unnecessary to compensate for amphtude inequalities between the signals 

 whose phases are being compared. The apjmratus descril^ed is also suited for 

 the measurement of the magnitude of a transfer impedance as well as the phase. 



WITH the increasing interest in wide-band amplifiers and circuits for 

 microwave communication systems the measurement of the transfer 

 phases of such components has become a necessary procedure. A commonly 

 used technique for measuring phase at microwave frequencies is to sample 

 the signal at the input and output of the device to be measured and to obtain 

 a null balance between the two signals by varying the phase of one signal by a 

 known amount. If the two samples are not of nearly equal amplitudes, it is 

 necessary to attenuate the larger one with an attenuator of known phase 

 shift. The latter operation presents difficulties. 



A method of phase measurement has been developed which overcomes 

 these difficulties by permitting measurements to be made with samples of 

 unequal amplitudes. The method uses the homodyne detection principle 

 and operates in the following manner: The output energy of a signal oscil- 

 lator is divided into two portions. One portion is applied to a balanced 

 modulator where it is modulated by an audio-frequency signal. The sup- 

 pressed-carrier, double-sideband signal from the modulator is applied to the 

 device to be measured. As before, means are available for sampling the 

 signal at both the input and output of the device. The other portion of the 

 oscillator power is fed through a calibrated phase shifter and is applied to a 

 crystal detector in the manner of a local oscillator in a double-detection 

 receiver. The signal samples are then alternately applied to the crystal 

 detector where they are demodulated by the action of the homodyne carrier. 

 In each case the phase shifter is adjusted so that the audio signal is a mini- 

 mum in the detector output. This occurs when the phase of the homodyne 

 carrier is in quadrature with the signal sidebands. The difference in [)hase 

 between the two adjustments of the phase shifter is equal to the phase dif- 

 ference between the two samples. 



Figure 1 shows the apparatus used for measuring phase in this manner. 

 Radio frequency power from a suitable oscillator is applied to the ll-plane 

 branch of an hybrid junction' where it divides and emerges in equal portions 



1 W. A. Tvrrell, "Hybrid Circuits for Microwaves," Proc. I. R. E., Vol. 35, Xo. 11. pp. 

 1294-1306; Noveml)er'l947. 



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