To meet the test requirements of a large capacity modern military communication system.Requires the synthetic signal generator to have very low phase noise,high rate,high deviation and high accuracy frequency modulation performance.But the traditional synthetic signal generator can’t meet the requirements.In this paper, the design and principle of a low noise synthesis FM signal generator is introduced, including a phase locked loop and a frequency locked loop.This paper focused on the implement method of low noise delay line discriminator and the functions of decreasing signal phase noise.how to overcome the limitation of the PLL and FLL to FM performance and how to realize high rate,high deviation frequency modulation by using the key design technology of many paths frequency modulation ,digitized FM, high rate high deviation linear FM etc. The signal phase noise is -125 dBc/Hz(1 GHz,10 kHz offset),the maximum FM rate is 10 MHz,the maximum FM deviationthe is 10 MHz.It is incomparable to the traditional synthetic signal generator.