• Content Count

  • Joined

  • Last visited

About MitchG

  • Rank

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. Thanks @attila I'll check out that release when available . I've added more info on the noise source file I use and also checked results when a 5th order 20kHz analog filter is added in the test chain.
  2. I've updated the article .. a bit wordy but hopefully it's reasonably accurate:
  3. Sorry to keep posting responses on this thread but I hope to update the article on using Discovery specifically for wideband noise measurements relevant to audio application. The AD TN noise measurements (included in latest beta release) in the frequency domain should agree with rms noise measured in the time domain using the scope. The default sample mode setting in the FFT is DECIMATE whereas the default mode for scope is AVERAGE sampling. When the noise values are compared between FFT and Scope, the results agree almost exactly provided both are set at DECIMATE or AVERAGE. For exampl
  4. Thanks again @attila I think i understand now. For noise analysis it seems to me that AVERAGE sampling makes more sense. For example in the 96kHz white noise sample I have been using, the TN results displayed for DECIMATE are all the same (56mVrms) independent of the captured frequency range of 100kHz, 50kHz or 20kHz and with default 4097 bins even though the white noise content is uniform from DC to about 48kHz where it has sharp drop-off. Conversely with AVERAGE sampling, the TN results change (as I'd naively but intuitively expect) with capture range like: 100 kHz: 54mVrms
  5. Ok so I guess I still don't understand the vertical scale for noise purposes in the FFT window. Here's a simple example (based on article in link above). Same white noise source input .. from a 24bit/96kHz wave file so (as seen below) noise cutoff is at 48kHz. When I change the FFT freq range displayed from 20 to 40 to 100kHz with same bin# over each range, why is the TN the same? (and identical to the scope total noise measurement). And also because the bin frequency width (resolution) increases with freq range , I expected the bin value would be higher as the range increases but its alm
  6. Thanks @attila Just ran this beta and the NF and TN are good now and the TN doesn't depend on the sample window type (corrected for ENBW) as expected but the NF is still the averaged value, not adjusted for ENBW. Final question is about SNRFS. Is the full scale here: 5.518Vpp = 2.76Vp = 1.95Vrms So if I switch sample windows shouldn't I expect to get the same value? The values I get are all different. For a TN value of 330mVrms I'd expect a SNRFS of 20*log(1.95/0.33) = 15.4dBFS This is exactly 3dB higher than the Rectangular window value of 12.4dBFS (with ENB
  7. Thanks @attila. I forgot to mention that the window type should add a correction factor P (sampling spread in adjacent bins) to get exact agreement of total noise using FFT bin summing and scope total rms noise: TN = sqrt( sum(Bin^2)/P) The default window in my Spectrum Analyzer options is Flat Top which has P=3.83 .. a lot of overlap between adjacent bins. I had used Rectangular earlier (P=1) so correction wasn't required. For Hann window, P=1.5. Current NF don't seem to include that correction factor.
  8. Yes!! Thank you so much! I did notice the Sqrt2 difference and it was driving me crazy. Agreement is perfect now. Might be a good idea to explicitly add that NF definition (with /count) to the documentation showing that the FFT NF measurement is an average value of all the sample bin values. Are the other definitions such as SN correct ? Or do SNR and THD, for example need to be corrected with 3dB? Is the next software release out yet? For audio measurements, its usually the TOTAL noise value (typically in 20kHz range as in 210 mVrms above) that's useful TN = sqrt( sum(Bin^2))
  9. Ok here is a specific example: Fairly high flat noise spectral source. Waveforms DC to 20kHz range with 1025 bins. Using units of RMS (V). The FFT NF measurement = 4.66 mVrms (or -46.6dBVrms using those units) I measured total noise using Waveforms scope as 210 mVrms which agrees closely with measurements from a calibrated sound card measurement. I then exported all the FFT bin measurements (in Vrms units) and using Excel computed the sum of the squared bin values: Sqrt(Sum(Vbin^2) which gives 209 mVRMS in exact agreement with the time domain scope measure
  10. Thanks. I have checked the signal level calibration and the dBV level is accurate. For the noise test signal, I used a white noise source signal generated by Wavelab Pro at a good high level (well above the noise floor of Analog Discovery) and measured its total rms noise voltage in dBV (16 Hz to 20 kHz with LPF and HPF) using the Discovery scope and another measurement. They agree very well. I'm confident this measurement is accurate. The measured results are in my last link. How does the FFT noise measurement (NF measured value) relate to the time domain measurement within a specified
  11. Thanks. I have looked at that documentation but I'm still a bit confused about the NF description in the doc versus what the NF measurement displays in measurements. To be specific, here's a simple example which I think shows how to get the total integrated noise floor over a specified bandwidth (20 kHz in the example) starting with the NF value from the FFT and compares this total with the total real-time rms noise measurement with good agreement. This seems to be what the linked Audio Precision article says. Noise Floor and total integrated noise
  12. I have used Analog Discovery 1 extensively for various audio measurements (Bode plots, photonic measurements, scope noise measurements etc.) and have just started to use the Waveforms Spectrum Analyzer. I have made quite a few real-time noise measurements using a very low noise well calibrated 24 bit/96kHz sound card down to the uVrms level using a steep 46kHz low pass filter and also a similar 20kHz 5th order LPF. So these are time-domain analog measurements within a well defined "noise bandwidth". With sufficiently high noise levels (say +20mVrms well above the ~ 0.5mV noise floor of the Di
  13. Here is an article on using a USB scope in the restoration and testing of an older (1946) AC/DC tube radio, and some safety precautions to follow: