Measurement of THD vs THD+N problem

I am having a problem with my QA401 software. When I run a test using the AMP THD VS Frequency plugin and select the THD + N setting with a start frequency of 10 Hz and a stop of 20 kHz I get plot that graphs the result to the full bandwidth of 20 kHz. When I preform the same test but do not select THD + N the graph result stops at just below 10 kHz.

This problem started at some point a couple of months back when I upgraded to the latest version of the software. I have de installed and reinstalled the software but still have the problem. Any suggestions?

The traces in red and black were done with THD+N and the orange and green traces were done with just THD. No other changes made.

Hi txturbo,

Can you please try:

  1. Setup for single-ended loopback
  2. File->Reset Startup Defaults
  3. Make sure +6 input range
  4. Set the plugin settings as follows


The graph should similar to this:

And with same settings but THD+N enabled I get:

Note the flatness of THD and THD+N across the spectrum.

On your plot, a THD or THDN measurement at 20 kHz tone doesn’t really make too much sense if your Distortion Measurement Stop is set to 20 kHz (sorry if I misunderstood your post and let me know if why that is true isn’t clear). But in short, if you really want the ability to measure the at least to the 3rd harmonic. So, if your stop frequency is a 6K tone, the 3H would be at 18k, which is below the measurement stop specification of 20k.