Calculating Noise Floor from Export Frequency Series

QA40x
1

I am having an issues with calculation of a noise floor value from the exported frequency series. I have both inputs to the QA403 grounded. I run a 64K FFT, 8 averages and stop the acquisition. The dBV measurement shows a value of -117.82dBV. When I do a sum of the exported frequency values I come up with 116.23dBV. I am thinking they should be much closer. I have tried this a few other configurations and my calculated value is always higher than the value shown in dBV measurement window. Same test with an open +input to the QA403, the dBV measurement is -86.63dBV and my calculated value is -84.87dBV. Is there some correction factor this is applied to the value in the dBV measurement window?

2

Just checking you know the difference between spectral density (for noise) and a signal spectrum (for discrete tones)?

3

I believe I do but it sounds like you see a point that needs clarification. In asking my question I am trying to find why my calculation does not match what is calculated by the QA403 software. Not sure that I see where spectral density is relevant to determining the difference in the two calculations.

4

My question comes from work I am doing translating changes in the full bandwidth noise floor of an input sine wave as it relates to magnitude and character of clock jitter.

5

Hi @rmod, there are correction factors that must be applied based on the window: These are energy correction factors and amplitude correction factors. In the link below, you’ll see the “first principles” on how take data and convert to meaningful RMS energy calcs and also displayed peaks.

PS. When computing RMS you don’t just sum the frequency values. You need to square each bin (not including bin0 which is DC), sum those, and then take the sqrt. See around line 107.

Momentarily switch to a rectangular window and that will make half your problem go away. I note the gap in your measurement is 1.6 dB, which is roughly the ECF for Hanning at Hamming. If you switch to RECT and gap goes to zero, then your issue is ECF

6

Hi Matt,

Thanks! that is just what I needed. I will this a try and let you know what I get.