Measuring noise

HI there,
I am thoroughly enjoying using my QA401 - great hardware!

I would like to know what are the best parameters for measuring/quantifying amplifier noise? I prefer unweighted measurements and use a 20-20 000Hz bandwidth - I am presuming that because bandwidth is digitally derived this 20-20kHz is genuine noise bandwidth and i do not have to take into account any filter slopes used to define the bandwidth like in an analogue analyser.

Should I use averaging? I am not looking for repetitive phenomena just noise quantification, so it would seem averaging should be off…or does averaging simply affect the display of the noise and not its power/amplitude?

Should I use any windowing? It seems to me that any window (except rectangular of course) will bias the measurement.

What does the N+D Level actually measure?

On a separate note was reading your notch filter post and wondered would it be possible to export FR measurements in the same way so as to use them as self-correcting weighting files? My QA401 is 0.2dB at 20kHz and I would like to get it flat to 50kHz for measuring phono preamps.

That is correct: To measure in 20 to 20K bandwidth the measurement is done in the frequency domain, and the only points used are those within the region you specify.

If you specify weighting, then the weighting is applied to the frequency domain AND THEN the noise measurement is made between the indicated bandwidth points you’ve specified.

If you noise is just noise (and not some signal near the noise floor), then averaging won’t change it much. You can test this by setting averaging to 0, and looking at the noise measurements that are reported over 5 or 10 cycles. And then bump up averaging–it won’t change much.

If it’s noise, the windowing shouldn’t matter. Windowing matters when you are dealing with a signal that might be discontinuous at the window edges.

The N+D (noise + distortion) measures the level of the noise and the harmonics BUT NOT the fundamental. For example, if you connect the L+ output to the L+ input, and short the L- input, and turn the generators off, you will measure a noise level (“RMS L”)around -109.5 dBV or so. This is the noise level of the generator (which is about 6 dB higher than the noise level of the analyzer input). The N+D will be the same because there isn’t a fundamental present.

Then, turn on the generator at -10 dBV. There you will see the N+D increase about 0.5 dB. That is because the N+D measurement has detected a fundamental (there should be a green “F” at the bottom of the display as shown below) and the N+D has thus been degraded about half a dB by the presence of the visible 2H and 3H.

Not currently. You could create a compensation file by hand and specify points at 10, 20, 30, 40 kHz, etc and get pretty close. But I agree it would be nice to be able to export a correction file.