Abstract: A microwave input signal whose frequency is to be determined is mixed in a mixer with a local oscillator (LO) signal and the resultant difference signal is applied to a low pass filter. The frequency of the LO signal is controlled by a digital processor that samples the output of the low pass filter. Initially the LO signal is at zero. Thereafter, the frequency of the LO signal is sequentially stepped through the fundamental and, then, the harmonics of a principal oscillator signal having a fundamental frequency F.sub.R. When the difference frequency falls within the direct counting capability of a digital counter, the digital processor opens a main gate for a short period of time, and a first difference frequency (F.sub.1) count is made and stored. The frequency of the LO signal is then offset by a known amount, and a second difference frequency (F.sub.2) count is made and stored -- if this F.sub.2 count is within the direct counting capability of the digital counter. Alternatively, if this F.sub.