Abstract: The design method and correlated principles of a X-band frequency synthesizer are described. This frequency synthesizer with wide frequency coverage, low phase noise, and low spurious is achieved by combining Phase locked loop (PLL) and mixer. Main technical specifications of the frequency synthesizer are as follows: frequency range of 9.8GHz～10.8GHz, frequency step of 5MHz, the phase noise is better than -85dBc/Hz@1kHz, -88dBc/Hz@10kHz, and spurious suppression is better than 60dBc. The final test result proves that this frequency synthesizer not only can ensure low spurious but also improve the phase noise level. It can be widely used in communication and test systems.