A mixer multiplies the RF signal by an LO signal. For a fundamental mixer, the IF contains the sum and difference frequencies: f_IF = |f_RF - f_LO| (and f_RF + f_LO).

Image frequency
For a given LO and IF, two RF frequencies produce the same IF: f_LO + f_IF and f_LO - f_IF. The unwanted one is the image. Solutions:
- Pre-selection filtering before the mixer (easier with a higher IF, which pushes the image further away).
- Image-reject mixer architectures (Hartley, Weaver) using quadrature LO and phase shifting; image rejection depends on amplitude and phase balance.
- Zero-IF (direct conversion) receivers avoid image filtering but introduce DC offset, flicker noise, LO self-mixing and IQ imbalance issues.

Spurious responses
Real mixers produce products at m x f_RF +/- n x f_LO. Spur charts or spur tables help select LO and IF frequencies that keep strong low-order spurs out of band. Planning a frequency plan with a spur calculator is a standard early step.

LO leakage and isolation
LO-to-RF isolation matters because LO energy leaking back out of the antenna can violate emissions limits and, in zero-IF receivers, self-mix to DC. LO-to-IF isolation affects the IF filter requirements. Double-balanced mixers suppress LO and RF at the IF port by symmetry.

Key metrics
- Conversion loss/gain.
- Noise figure (for passive mixers, roughly equal to conversion loss in single-sideband terms).
- Input IP3 and P1dB.
- LO drive level requirement (for diode mixers the "level" designation indicates the LO power needed).
- Port isolation.

Passive versus active
Passive diode or FET mixers usually offer high linearity and broad bandwidth; active mixers (such as Gilbert cells) provide conversion gain and lower LO drive requirements.