A microwave synthesizer apparatus features very low phase noise, fine frequency resolution and wide tuning range coverage. The microwave synthesizer apparatus utilizes a fundamental offset source in an offset phase lock loop (PLL) to translate an output signal Fout to a lower IF signal Fif for locking to a low frequency interpolation signal Fint. The use of the fundamental offset source instead of the conventional multiple frequency offset signal from a comb generator or sampler results in superior phase noise and spurious performance. The synthesizer comprises a main signal loop having a main loop VCO that produces an output signal Fout and an offset signal loop having an offset VCO that produces an output signal Fos. The signals Fos and Fout are mixed in the main loop to control the frequency of the signal Fout. The main loop VCO and the offset loop VCO preferably are YIG-tuned Oscillators (YTOs) that share a main coil. Moreover, the main YTO and the offset YTO preferably have a common housing and further, each of the main YTO and the offset YTO has a separate FM coil. The use of a dual YTO in the microwave synthesizer apparatus minimizes overall cost and power consumption of the synthesizer by combining the dual YTO in the single package. In one embodiment, the microwave synthesizer further comprises a mode selection feature that selects between operation of the synthesizer in an offset or dual loop mode and a variable divider or single loop mode. In another embodiment, the microwave synthesizer further comprises a selectable frequency divider that produces the output signal Fos with a smaller frequency step size.