Whereas in a fundamental frequency mixer
the output frequency (
) contains the sum and difference of the fundamental tones of the two input signals (
), in a subharmonic mixer
the output signal is the sum and difference of the fundamental and one of the harmonics
of the two input signals such as:
. Subharmonic mixers are often used in direct-digital, or zero IF, communications system in order to eliminate the unwanted effects of LO self-mixing which occurs in many fundamental frequency mixers.
A subharmonic mixer that has two switching stages is used to improved mixer gain in a direct downconversion receiver. The first switching stage mixes a received RF signal to an intermediate frequency that is one-half the received RF signal frequency. The second switching stage mixes the intermediate frequency to baseband. By connecting the two switching stages in series, current is reused and harmonic content from the first stage is fed into the second stage thereby improving the mixer gain.