Measurement Accuracy

I have a feeling the THD+N and SNR numbers displayed at the top don’t jive with the measurements. Matt, can you have a look?

Hi @Dave_MacKinnon, Your THDN is -60 dB. Your SNR is 60 dB. You an see your harmonics are -87 dBc. But the SNR measurement is telling you the noise is -60 dBc. And so, your measurement here is dominated by noise. I see your attenuator is on, and when the attenuator is on the noise floor goes up (because your max input level goes up, and DR remains roughly constant)

If you short the inputs of the QA402 in +24 dbV full scale input mode, the noise floor is -87 dBV (20 khz, no weighting). So, you need to learn the noise floor of your current configuration (just short the inputs without changing anything, and note the RMS dBV)

And then, unshort and connect the DUT. Enable the N-D measurement (this shows noise, but ignores fundamental and ignores any distortion components) and that will tell you the DUT noise measurement. If that is higher than your shorted measurement, then you know the noise is coming from the DUT.

If that is the close to the same as the QA402, then you know that means the measurement is limited by the QA402 noise floor. In that case, you’d want to attenuate the input externally with a low-impedance attenuator and then use the QA402 in a low-noise mode (eg 0/6/12/18 dBV input range)

But, the measurements shown look correct given the data on the display.


Thank you! That’s exactly what I needed. I’ll follow those instructions.