Frequency Error, Quadrature Modulation (QM)

In quadrature-modulated (QM) systems, frequency error refers to the difference between the specified carrier frequency and the actual measured carrier frequency.

At baseband frequencies, frequency error in QM systems is a complex signal impairment that manifests itself as a rotation of the locus of symbol coordinates about the I/Q plane. A fixed frequency offset appears to be a rotation at a constant angular velocity. Frequency error can be either positive or negative in sign, indicating the direction of the rotation.