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Measuring Distortions with the Scope:What you see is not what you really have..
Sighound36:
I have only just read this thread our workload currently is quite manic so as we have a quality audio analyser UPV66 have set up the unit to measure its own internal high precision genny @ 1 khz ref voltage is 1V 20Hz-20Khz FFT 256K 4 averages, Van hann window
On the Wavepro HD I used max bandwidth 4Ghz, NO filters ( I have ERES and digitial filter package) as close as I can get to Martin's set up here, ground is on 600Ohm output on the genny selected. SA set to Von han / power spectrum / RBW20Hz. 1Mohm input 1m long RG59U 22 AWG. with an XLR<>BNC adapter, I didn't de embed cable impedence tonight.
Next image same setting etc just with 10 averges
The following image now spots 200Ks and 1Ms sampling same HTB then the 10 avergae of that as well.
Then 5MS memory points and 25MS/s sampling rate & that average
Last pairing 200Meg points of memory & 1GS/s rate.
Finial image the noise sat on the genny output with it off!
Martin72:
Hmm..
So the first pictures show the "truth", because actually there should be no harmonics in the amplitude range... ???
Sighound36:
Hi Martin
I think it with either NTICO or Sinisa? who made the observation about what's really buried in the noise.
Its Sunday evening maybe I have not been as accurate as I could have been? The analogue gennerator is the precision version so it should be pretty clean. Again we are back to how good is the set up/opertor and actual ability of the equipment to deliver a result that is as free of measurment uncertanty as possible.
If you have specfic set you wished checked happy to do this.
Sighound
Martin72:
Hi,
I'll try to finish the oscillator as soon as possible or use the generator from my neutrik A1 to recreate your measurements.
From my (probably too simplistic) thinking, if a signal with say -100dB thd is fed in, there should be nothing to see in the displayed FFT when it goes "down" to -80dB.
Martin72:
--- Quote ---I'll try to finish the oscillator as soon as possible
--- End quote ---
The missing parts ordered today at Mouser, the sum has already become three digits. ;)
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