RTA distortion measurement problem on REW 5.20.14

[Antonio Di Motta]

I noticed a problem on the distortion measurement, the THD measurement of an external BF generator changes with the activation of the internal REW generator even if not connected to the audio input.

I have attached two photos where it is shown that with the same spectrum without active generator the distortion is correct (0.025%) while activating the generator the THD measurement is considerably lowered (0.00054%).

The two generators are set to the same frequency only that the external generator connected to the audio input the actual frequency is about 981.3Hz while the internal REW generator not connected the actual frequency is 999.76Hz.

A greeting

Antonio Di Motta

 

[John Mulcahy]

That is the intended behaviour, per the help: If the REW signal generator is playing a sine signal or tone burst the generator frequency is used as the fundamental frequency of the input, otherwise the highest peak is used to determine the fundamental. That ensures the correct frequency is used when the REW generator is the source even if the fundamental is not the highest peak in the measurement or the peak is misshapen. It also provides a means of forcing a fundamental frequency when using an external source under such conditions.