  Phase noise of the frequency synthesizer is one of the main limitations to
the short-term stability of microwave atomic clocks. In this work, we
demonstrated a low-noise, simple-architecture microwave frequency synthesizer
for a coherent population trapping (CPT) clock. The synthesizer is mainly
composed of a 100 MHz oven controlled crystal oscillator (OCXO), a microwave
comb generator and a direct digital synthesizer (DDS). The absolute phase
noises of 3.417 GHz signal are measured to be -55 dBc/Hz, -81 dBc/Hz, -111
dBc/Hz and -134 dBc/Hz, respectively, for 1 Hz, 10 Hz, 100 Hz and 1 kHz offset
frequencies, which shows only 1 dB deterioration at the second harmonic of the
modulation frequency of the atomic clock. The estimated frequency stability of
intermodulation effect is 4.7*10^{-14} at 1s averaging time, which is about
half order of magnitude lower than that of the state-of-the-art CPT Rb clock.
Our work offers an alternative microwave synthesizer for high-performance CPT
Rb atomic clock.
