We demonstrate a simple scheme of 6.835 GHz microwave source based on the sub-sampling phase lock loop (PLL).A dielectric resonant oscillator of 6.8 GHz is directly phase locked to an ultra-low phase noise 100 MHz oven controlled crystal oscillator (OCXO) utilizing the sub-sampling PLL.Then the 6.8 GHz is mixed with 35 MHz from an direct digital synthesizer (DDS) which is also referenced to the 100 MHZ OCXO to generate the final 6.835 GHz signal.Benefiting from the sub-sampling PLL,the processes of frequency multiplication,which are usually necessary in the development of a microwave source,are greatly simplified.The architecture of the microwave source is pretty simple.Correspondingly,its power consumption and cost are low.The absolute phase noises of the 6.835 GHz output signal are-47 dBc/Hz,-77 dBc/Hz,-104dBc/Hz and-121 dBc/Hz at 1 Hz,10Hz,100Hz and 1 kHz offset frequencies,respectively.The frequency stability limited by the phase noise through the Dick effect is theoretically estimated to be better than 5.0 × 10-14τ1/2 when it is used as the local oscillator of the Rb atomic clocks.This low phase noise microwave source can also be used in other experiments of precision measurement physics.