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A simple and practical photonic approach for microwave instantaneous frequency measurement has been proposed and experimentally demonstrated. The unknown microwave signal with its frequency to be measured is carrier-suppressed modulated on the lightwave by a Mach–Zehnder modulator biased at the minimum point. And thanks to the inherent complementary transmission responses of the Mach–Zehnder interferometer (MZI), the fixed relationship between the amplitude comparison function and microwave frequency to be measured is established. By monitoring and processing the optical powers of the two output ports of the MZI, the frequency of the microwave signal could be easily estimated. The proposed scheme is experimentally verified and has many advantages, such as simple structure, low cost, fast response, practical prospect, tunable measurement range, and high resolution.