Harmonic mixer

The harmonic mixer and subharmonic mixer are a type of frequency mixer, which is a circuit that changes one signal frequency to another. The ordinary mixer has two input signals and one output signal. If the two input signals are sinewaves at frequencies f₁ and f₂, then the output signal consists of frequency components at the sum f₁+f₂ and difference f₁−f₂ frequencies. In contrast, the harmonic and subharmonic mixers form sum and difference frequencies at a harmonic multiple of one of the inputs. The output signal then contains frequencies such as f₁+kf₂ and f₁−kf₂ where k is an integer.

Background

The classic frequency mixer is a multiplier. Multiplying two sinewaves produces just the sum and difference frequencies; the input frequencies are suppressed, and, in theory, there are no other heterodyne products. In practice, the multiplier is not perfect, and the input frequencies and other heterodyne products will be present.

An actual multiplier is not needed. The significant requirement is a nonlinearity, and at microwave frequencies it is easier to use a nonlinearity rather than an ideal multiplier. A Taylor series expansion of a nonlinearity will show multiplications that give rise to the desired higher order products.

Design goals for mixers seek to select the desired heterodyne products and suppress the undesired ones.

Diode mixers.

Overdriven diode bridge mixers. Drive signal looks like odd harmonic waveform (essentially a square wave).