Why THD+N goes up at mid-band audio frequency range?

Hi Jay

     The digital sample rate and the PWM frequency are all fixed when we testing this THD diagram. From digital processing theory and also PWM theory, if the output frequency more close to the sample or PWM frequency, there would be more distortions. So the THD would be higher when the frequency goes up. But when the frequency higher than 6.67KHz, it's 3rd harmonics would beyond 20KHz, that is the audio range. We use an additional low pass filter at 20KHz to get rid of noise that is beyond audio range, that's standard method to test audio amplifier. So you'd see when frequency higher than 6.67KHz, THD will become small again, that is because the filter takes effect.