Abstract: Chip-scale atomic clock (CSAC) mainly includes an RF source, a physics package and other peripheral control circuits. The RF source which is related to the short-term stability of the CSAC plays an important role in CSACs. The technology of digital phase-locked loop is used to realize the 4.596 GHz RF source in this paper. The RF source is made up of three parts, including the fractional-N frequency synthesizer chip, the voltage-controlled oscillator (VCO) and the loop filter. Advantages of digital phase-locked loop used in the microwave signal source are low phase noise and pure spectrum. Furthermore, as the fractional-N frequency synthesizer chip is programmable, the output frequency can be swept by configuring the values of R divider and N divider, making the frequency control of CSACs easy. With the related formula, parameters of the loop filter are calculated approximately, and a loop filter with 300 kHz loop bandwidth and 55 phase margin is designed. Finally, The whole RF source based on the digital phase-locked loop is simulated, fabricated and tested. The result shows that the phase noise of the RF source is -74.02 dBc/Hz at 300 Hz offset, which meets the requirement of the RF source for CSACs.