Software (REW) can correct for soundcard induced distortion in a loopback configuration whereby the harmonics are suppressed down to the noise floor achieved by the test configuration. The harmonic suppression can be done manually across the typical range of harmonics, or can be an automated process (a lot easier!). REW can assess the noise floor behaviour, as noise floor can be interpreted and described in different ways, and 'high' doesn't really mean anything.
thanks trobbins. I must look into this further. I did some kind of sound card calibration through REW. However, I assumed this was just to set the level. Does that calibration also correct the noise floor/distortion error as well? I've been reading the lengthy documents. I've probably missed the error correction calibration file.
I must have done something wrong. After calibration of the sound card, I open RTA and loopback the sound card to take a 1khz reading. I still see high 2nd and 3rd order harmonics. I'd assume the error correction file would zero those out.
I'm looking at page 271 of the REW document.
I'm looking at page 271 of the REW document.
The calibration process is a loopback test that derives a cal file to normalise the frequency response for constant gain and phase across the bandwidth selected - ie. to give a 'flat dc to daylight' frequency response. That is not related to the harmonic suppression process, or related to how the noise floor can be assessed.
The harmonic suppression process is just for a single test sinewave frequency and signal level, and the supression outcome will vary as the signal frequency or magnitude is varied. So this is not related to a step related sweep or other process unless each step is done separately and re-calibrated for harmonic null.
Noise floor is very dependent on the processing set up, such as the fft process window and config, and the averaging mechanism used. For sinewave plotting, coherent averaging is a further processing technique to suppress noise between harmonics. This is all quite a detailed and technically imposing topic, and can take quite some learning curve to better appreciate.
The harmonic suppression process is just for a single test sinewave frequency and signal level, and the supression outcome will vary as the signal frequency or magnitude is varied. So this is not related to a step related sweep or other process unless each step is done separately and re-calibrated for harmonic null.
Noise floor is very dependent on the processing set up, such as the fft process window and config, and the averaging mechanism used. For sinewave plotting, coherent averaging is a further processing technique to suppress noise between harmonics. This is all quite a detailed and technically imposing topic, and can take quite some learning curve to better appreciate.
Perhaps look at response when signal level is varied. Keep reading the help topics and gaining experience as settings are changed.