Progresses have been made in the development of digitalized and miniaturized rubidium atomic frequency standard (RAFS). In the traditional RAFS circuits, the 6 840 MHz microwave signal is mixed with the 5.312 5 MHz signal generated by the frequency synthesizer to obtain 6 834.687 5 MHz microwave signal, which is used to excite rubidium atom transition. Early RAFS frequency synthesizer used a large number of discrete analog devices, and showed disadvantages such as low degree of digitization, complicated parameter optimization and large physical size. The direct digital synthesizer (DDS) scheme used currently generates the 5.312 5 MHz signal directly, but often needs to multiply the 10 MHz signal. It has the disadvantages of low spectral purity and high phase noise. This paper introduces a digital frequency synthesizer solution that generates the 5.312 5 MHz signal. The design eliminates the need for a 10 MHz multiplier circuit when using a DDS device. It has the advantages of high spectral purity, low phase noise and adjustable output frequency/phase.