In the coherent-on-receiver radar system using the magnetron source, frequency synthesizer is employed as a STALO(Stable Local Oscillator) to keep the intermediate frequency stable. In this paper, X-band(8.4GHz~9.7GHz) single loop frequency synthesizer is designed and implemented by an indirect frequency synthesis technique. Phase comparison is performed by a digital PLL(Phase-Locked Loop) chip and the loop filter is designed for the low phase noise. The effects of loop component characteristics on the output phase noise are analyzed for single loop structures, and the calculated results are compared with the measured data.