Microwave interferometer for phase and response time measurements

We report the development of a microwave interferometer using a quadrature intermediate frequency (IQ) mixer designed to measure the relative phase change and response time of microwave devices tested in the K_a and upper K bands (22-40 GHz). The interferometer is currently used to test liquid crystal based devices. The system allows for the application of an AC bias beyond the amplitude/frequency limitations imposed on vector network analyzer bias ports. Our IQ mixer based design uses bias signals ranging from 0 to 100 V peak-to-peak in a frequency range from DC to 100 kHz. This range of bias signals is necessary to properly test the response of microwave devices designed with liquid crystal materials. The setup enables us to measure changes in the output phase of the device as a function of both the voltage and frequency of the applied bias signal. The setup also measures the phase difference as a function of microwave frequency and response times for the device under test. Our system can be integrated into a stand-alone test setup without the need for a vector network analyzer.