Real-world signals usually contain noise. A system can introduce additional noise into the signal. THD + N measures signal distortion while taking into account the amount of noise power present in the signal. Measuring THD + N requires measuring the amplitude of the fundamental frequency and the power present in the remaining signal after removing the fundamental frequency. The following equation yields THD + N.
where N is the noise power.
A low THD + N measurement means that the system has a low amount of harmonic distortion and a low amount of noise from interfering signals, such as AC mains hum and wideband white noise.
As with THD, you usually report the results of a THD + N measurement in terms of the highest order harmonic present in the measurement, such as THD + N through the third harmonic.
The following equation yields percentage total harmonic distortion + noise (%THD + N).
