A mixer multiplies two signals in the time domain, producing sum and difference frequency components. This allows frequency conversion — moving signals from one frequency band to another. Mixers are at the heart of every superheterodyne radio, radar, and wireless communication system.

Basic Operation

An ideal mixer with RF input at \(f_{RF}\) and local oscillator (LO) at \(f_{LO}\) produces output at \(f_{IF} = f_{RF} \pm f_{LO}\). The desired sideband is selected by the IF filter; the unwanted image frequency falls at \(f_{LO} - f_{IF}\) (for a low-side LO) and must be rejected.

Conversion Loss and Noise Figure

Passive mixers (using diodes or FETs) have conversion loss — the IF output is weaker than the RF input. Typical conversion loss: 5–8 dB for a diode ring mixer. The NF of a passive mixer equals approximately its conversion loss. Active mixers provide conversion gain (0–15 dB) at the cost of higher DC power and potentially more distortion.

Image Rejection Mixer (IQ Mixer)

An IQ (in-phase/quadrature) mixer uses two mixers with LO signals 90° apart, producing in-phase (I) and quadrature (Q) baseband outputs. When the outputs are combined correctly, the image frequency is cancelled. Image rejection of 30–40 dB is achievable in practice; better rejection requires careful amplitude and phase balance.

Spurs and Isolation

Real mixers generate mixing products at \(mf_{LO} \pm nf_{RF}\) (spurious responses or "spurs"). Mixer datasheets specify LO-to-RF, LO-to-IF, and RF-to-IF isolation to characterise feedthrough. Double-balanced mixers provide better isolation and spur rejection than single-ended designs.