Abstract: The influences on phase noise of direct digital frequency synthesizer with truncation of the phase information stored in the accumulator,truncation of sine words in look-up tables and finite bit resolution of DAC are analyzed,It is shown that the effect of DDS chip itself on the phase noise of output signal can be neglected. The hardware constitution of a low phase noise frequency synthesizer using AD9854 and the measured performances of system phase noise and spurious spectrum are presented.