In atom interferometer gravity experiments, Raman laser preparation commonly uses an optical phase-locked loop method, in which the master and slave laser beat signal is mixed with a 6.8 GHz microwave signal source firstly, then frequency discrimination is performed with direct digital frequency synthesis signal generator, and lastly the feedback signal obtained is used to control the low-noise Raman optical output. So the phase noise of Raman output will directly affect the sensitivity of the atom interference gravimeter. This design uses the STM32F103C8T6 microcontroller to program and control the LMX2594 digital phase-locked loop chip, and obtains a 6.8 GHz microwave signal source through the phase-locked loop frequency synthesis technology. The final experimental results show that the phase noise of the microwave signal source is 65.2 dB@1 Hz, and -95.3 dB@1 kHz, Allan deviation (ADEV) is 2.72 × 10^-11@1 s, and the output power is greater than 10 dBm. When the pulse interval time (T ) is 100 ms, influence of signal source on sensitivity of the atom interference gravimeter is 8 × 10^-8 m/s²/Hz^1/2, and on the resolution of the atom interference gravimeter is 2×10^-8 g@600 s. It is shown that the design has the advantages of high frequency stability and low phase noise, which can meet the requirements of microwave reference sources for atom interferometer.