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A photonic approach for instantaneous microwave frequency measurement over a wide bandwidth is proposed and experimentally demonstrated based on the fading effect of the fiber. In this scheme, three lights with different wavelengths and three sections of optical fiber are used to construct the frequency-dependent amplitude comparison functions (ACFs), using a phase modulation along with an intensity modulation. The unknown microwave frequency can be determined from the intersection of three ACFs. The method can not only provide extended measurable range of the microwave frequency, but also improve the accuracy by choosing an ACF with a large slope. The measurement errors as demonstrated in the experimental results are within ±90 MHz in the frequency range of 0.5–20 GHz.