In a quadrature modulated (QM) system, the ideal QM waveform is the sum of the I and Q components of a signal, as shown in the following equation.
i(t) is the baseband I waveform
q(t)is the baseband Q waveform
The practical QM waveform differs from the ideal QM waveform. A generalized adjusted QM waveform can be expressed as the following equation.
The vector sum of the in-phase DC offset and the quadrature -phase DC offset results in origin offset.
The origin offset, measured in dB, is the distance between the ideal constellation center to the actual constellation center, as shown in the following figure.
Before measuring EVM, the symbols are corrected for origin offset.
I/Q gain imbalance refers to the difference in scaling between the I and Q components of I/Q data.
Quadrature skew describes a complex signal impairment of the I and Q components that are not perfectly orthogonal.
Quadrature skew can be either positive or negative, with the sign indicating the orientation of the error, as shown in the following figure.