An atomic clock of the type having a cell filled with an active vapor through which is projected a light beam. A detector of the projected light provides corresponding detector signals to a microprocessor. An rf frequency synthesizer provides a microwave signal to a microwave cavity adjacent the cell and also provides a clock standard output signal. The rf frequency synthesizer includes a fractional-N frequency synthesizer which compares signals from a voltage controlled crystal reference oscillator and a voltage controlled oscillator. The fractional-N frequency synthesizer is operable to provide an output control signal to precisely lock the voltage controlled oscillator microwave output signal with the voltage controlled crystal reference oscillator signal. The fractional-N frequency synthesizer also periodically causes a predetermined desired dither in the microwave signal.