Master oscillator requirements for coherent radar applications are derived from a consideration of the mainlobe width and sidelobe level objectives of the Doppler aperture function of the overall radar system. These parameters are related to the linewidth and sideband level of a carrier as modulated by the differential oscillator phase during a signal transit time. Both periodic and random phase modulation are considered. For the latter, techniques are given and illustrated by example for transforming oscillator stability characteristics measured in the time domain into a spectral form useful for radar design.