A tunable all-optical microwave generator based on period-one (P1) dynamics of an external optically injected distributed-feedback laser diode (DFB-LD) is proposed and demonstrated. A DFB-LD implements the functions of light source, microwave frequency selection, and optical–optical modulation. To overcome the frequency drift in P1 oscillation process, a set of reference frequencies from a fiber ring laser (FRL) is introduced. Once the P1 oscillation signal and the modes of FRL are coupled, the whole system is closed and a stable frequency determined by P1 oscillation can be established. In the experimental verification, tunable photonic microwave signals with frequencies from 6.42 to 21.36 GHz are obtained. The measured single-sideband phase noise is −83  dBc  /  Hz at 10-kHz offset, which is operation frequency independent.