The error vector and power amplifier distortion

The RMS value of the error vector magnitude (EVM_RMS) is a useful system-level figure of merit for the accuracy of digitally-modulated signals. At the circuit level, the nonlinearity of a power amplifier is often given by the amplitude distortion (AM-AM) and phase distortion (AM-PM). When the amplifier input is undistorted, there is a simple relationship between the amplifier distortion and the error vector at the output: the error vector is the difference of the measured signal and a reference (ideal) signal, while the AM-AM and AM-PM are found from the ratio of the measured and reference signals. This relationship enables the EVM time series to be predicted from the AM-AM and AM-PM. In the frequency domain the spectral regrowth at the amplifier output is equal to a portion of the error vector power spectrum. A frequency-domain distortion power ratio may be estimated from the appropriate time-domain average of the EVM. These relationships are validated experimentally for a class AB amplifier and π/4 DQPSK signals