A microwave frequency synthesizer includes a phase locked loop with dual reference sources. One source is coupled into the loop by a mixer and provides multiples of 100 MHz at a frequency range of, for example, 2 to 4 gigahertz which is provided by a crystal controlled oscillator and provides a final multiplied output frequency offset by 100 MHz to 200 MHz from the final desired output frequency. A second low frequency variable frequency synthesizer is coupled into the phase locked loop by a pha...