An intermittent frequency synthesizer for fast duty-cycled receivers is presented in this paper. Different from state-of-art techniques which try to eliminate the initial phase error that degrades the intermittent frequency detection, a new frequency detector is proposed to maintain an accurate frequency detection regardless of the initial phase error. Moreover, an averaged fraction division scheme (AFDS) is integrated in the synthesizer to improve the frequency resolution. The frequency synthesizer is implemented and verified by using discrete devices and an FPGA. The measurement results show that the frequency detector provides a frequency detection accuracy of 0.88%, and the synthesizer achieves a frequency resolution of 0.625 MHz when controlled by a duty-cycled signal with a 6- \mu \text{s} period and a 2- \mu \text{s} time duration. The frequency synthesizer can perform an intermittent frequency hopping from 160MHz to 140MHz while requiring a period of 430~\mu \text{s} without disturbing the VCO's oscillation.