A Method of Measuring Phase at Microwave Frequencies

I T H the increasing interest in wide-band amplifiers and circuits for microwave communication systems the measurement of the transfer phases of such components lias 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 oscillator is divided into two portions. One portion is applied to a balanced modulator where it is modulated by an audio-frequency signal. The suppressed-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.