IQ imbalance

The flaw that appears when a radio's two quadrature channels — the in-phase 'I' and the quadrature 'Q', meant to be perfect twins 90° apart — turn out slightly mismatched. Modern radios split a signal into these two paths so they can recover information from both sides of the carrier. The trick only works if the two paths have identical gain and are exactly 90° out of phase; any difference in amplitude or any deviation from 90° is IQ imbalance.

The damage is an 'image': energy from the wanted signal leaks into its mirror frequency, and an unwanted neighbour's image leaks onto you, raising the noise floor and smearing the constellation of QAM symbols. Image-rejection ratio measures it; without correction, a few percent of gain mismatch or a couple of degrees of phase error limits you to perhaps 30–40 dB rejection — often not enough, so transceivers run continuous digital IQ calibration to push it higher.