A Simplified Photonic Approach to Measuring the Microwave Doppler Frequency Shift

A novel approach for measuring the microwave Doppler frequency shift (DFS) based on a dual-drive Mach-Zehnder modulator (DDMZM) is proposed and demonstrated. The value and direction of the DFS can be simultaneously measured with high precision by using a reference signal. In the DDMZM, the transmitted signal and reference signal are applied to one radio frequency (RF) port, while the echo signal is applied to the other RF port. Then, the generated optical signals are sent to a low-speed photodetecter, and the beat frequency between the transmitted and echo signals is equal to the value of DFS, while the direction of DFS can be distinguished by comparing the two beat frequencies generated by the transmitted signal with reference signal and the echo signal with reference signal, respectively. The DFS from -100 to +100 kHz at the carrier frequency of 10, 14, and 18 GHz is successfully measured, and the maximal error is less than 1.0 Hz.