A novel frequency synthesizer scheme is proposed. The Direct Digital frequency Synthesizer(DDS) is utilized as the reference of the Phase Locked Loop(PLL) to realize the fine step. The fast frequency hopping is realized by adding a new circuit in the PLL. The frequency expansion is realized by frequency doubling, frequency division and filtering. The frequency range can realize 2-18 GHz and the frequency hopping time can be in the scale of microsecond. The technical difficulties in the scheme and design are analyzed and the improvement methods are given. The frequency synthesizer has good measured indexes and strong practical value in engineering, and can be expanded to other radar systems.