Products > Test Equipment
Measuring Distortions with the Scope:What you see is not what you really have..
<< < (25/35) > >>
mawyatt:
Think the context here where "Function Generator" is mentioned more than likely infers a AWG, which can have a respectable THD+N metric as shown by a few.

Best,
Performa01:

--- Quote from: elecdonia on January 05, 2023, 07:52:28 pm ---Several posts in this topic display the FFT harmonics from the output signal of a "function generator." Typically the ratio of fundamental/harmonics shown is no better than -50dB for function generators. ...

--- End quote ---

Thank you so much for giving us valuable insight in the obscure field of audio engineering.  We learn something new every day!

In case you did not understand Tautechs response: This thread originally was (and somehow still is) about the distortion in AWGs (Arbitrary Waveform Generators), which have been shown to provide decent distortion levels down to -80 dBc.

In case you did not read the posts within this thread: unsurprisingly, it has been found that the linearity of the frontend of an average general purpose DSO cannot keep up with the modern AWGs, hence we get much worse distortion figures than what the generators are actually capable of.

Hint: the latter fact was the reason why the OP decided to change the thread title.
Martin72:

--- Quote ---Hint: the latter fact was the reason why the OP decided to change the thread title.
--- End quote ---

Exactly.
It actually started "harmlessly", with an FFT comparison between SDG1000X and 2000X.
rf-loop noticed (in a neighboring thread) that something could not be right and so it began...
What I noticed (later), the FFT of the two looked pretty much the same, which possibly already indicates an "error".
I find that very interesting what happens there and must read me again the pages here to create perhaps a summary of why you can not trust your eyes (apart from that you know the actual values according to the data sheet).
Also, we have not yet illuminated whether it gets "better" when you feed in a signal that has a not so low THD.
I had briefly addressed this at work or discussed with a senior developer and he said flatly, that comes from when you measure "digital" with "digital".... :D
Oh well, the display of my Neutrik A1 has an illumination, it is only totally weak:
EL foil, over 20 years old... ;)
Replacement bought, should come next week and then I need no more flashlight. 8)

elecdonia:

--- Quote from: Performa01 on January 05, 2023, 08:21:21 pm ---
--- Quote from: elecdonia on January 05, 2023, 07:52:28 pm ---Several posts in this topic display the FFT harmonics from the output signal of a "function generator." Typically the ratio of fundamental/harmonics shown is no better than -50dB for function generators. ...
--- End quote ---
In case you did not understand Tautechs response: This thread originally was (and somehow still is) about the distortion in AWGs (Arbitrary Waveform Generators), which have been shown to provide decent distortion levels down to -80 dBc.
--- End quote ---
Somehow I missed the word "arbitrary" when I first read this topic. I must re-read all 5 pages more carefully.
"Arbitrary waveform" implies a digital system with D-A converter at the output. Today 16 and 24-bit DACs with excellent linearity and very low noise are widely available and cheap. I'm not surprised that -80dB distortion ( .01% THD ) is feasible for an AWG. Perhaps I'm showing my (advanced) age because my concept of "function generator" applies to analog devices limited to producing sine, square, ramp, & triangle waveforms?
AWG systems are a whole different animal.


--- Quote ---In case you did not read the posts within this thread: unsurprisingly, it has been found that the linearity of the front end of an average general purpose DSO cannot keep up with the modern AWGs, hence we get much worse distortion figures than what the generators are actually capable of.
--- End quote ---
I fully agree. This is one reason why it is so useful to connect the oscilloscope to the output signal from a THD analyzer (or notch filter) so that the oscilloscope isn't required to process the fundamental.
In any case one must first measure the signal source (AWG) with the analysis device (DSO) to determine the baseline distortion/noise level before putting a device-under-test into the signal chain.
Performa01:
Now we are on the same page!  :-+
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod