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A photonic approach using a single laser source and two quadrature optical filters (QOFs) is proposed to perform instantaneous frequency measurement (IFM). In the proposed approach, two QOFs with in-phase and quadrature filtering responses are designed. The wavelength of the laser source is aligned at the peak of the in-phase filtering response and at the -3-dB point of the quadrature response simultaneously. Two quadrature power ratios are then derived, from which the microwave frequency within full free spectral range (FSR) of the filtering responses can be estimated. Thanks to the use of a single laser and two QOFs, the number of required laser sources is greatly reduced, especially for a bank of IFM systems. The wavelength alignment and the single sideband and carrier-suppressed (SSB-CS) modulation are also considerably simplified. In the proof-of-concept experiment, measurement errors less than ±0.4 GHz are observed within the range of 20~36 GHz.