A photonic approach to generating a phase-coded microwave signal is proposed and demonstrated. The main principle is to beat the encoded coherent optical sideband to obtain high-frequency, high-coding rate, low-noise encoded microwave signals. The proposed technique, which is simple and conducive to integration, can adapt to different coding rates, generate phase-coded microwave signals with tunable frequency, and solve the bottleneck problem of traditional electronic approaches. The principle is discussed in detail. Mathematical models are developed to consider perturbation on the generated coded signal caused by the phase fluctuations of the microwave driving signal and the optical carrier. The required fiber Bragg grating notch filter is fabricated, and 20 GHz and 25 GHz phase-coded microwave signals are experimentally generated, respectively. The experimental results agree well with theoretical values, and it is proved that the proposed method improves the pulse compression capability.