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Measuring Distortions with the Scope:What you see is not what you really have..

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Martin72:
Hi,


--- Quote ---Note that you are trying to characterize a 16 bit generator with a 12 bit oscillscope.
--- End quote ---

I realise that, but the scope should not show anything that is actually not there, and that is the current situation.
And I don't think it's because you get a 16 bit signal with a 12 bit scope(the very most user got 8bit ones).
Approaches have already been shown as to what the problem could be, and that worries me at least.
Not that you generally have to doubt everything that the FFT function provides or that you first have to make an increased effort instead of just roughly adjusting it and that's it.

A very interesting and important topic, it should be separated from this thread, otherwise you won't find it again so quickly.
Regarding the original topic here:
And then I wonder what else you can compare between the models that doesn't need to be doubted because you did it with a scope?

rf-loop:

--- Quote from: switchabl on December 28, 2022, 11:14:08 am ---
--- Quote from: Martin72 on December 28, 2022, 12:03:16 am ---One of the findings one could derive from this would be of a fatalistic nature, namely that one can forget the FFT function with oscilloscopes.
Is this really the case, or do you have to try a little harder to get a more credible result?
And if so, in which direction one would have to go for it....

--- End quote ---

Not at all, in the same way that the frequency measurement function is not useless just because it does not have the same resolution and accuracy as a standalone frequency counter. An oscillscope is not a replacement for an audio analyzer (neither is a spectrum analyzer btw).

Every instrument creates some distortion of its own (not only in the ADC but more importantly the front-end as well). It will depend on the frequency and amplitude of the input signal and the vertical setting as well. And unfortunately, it is usually poorly characterised in oscilloscopes (the SDS2000X HD datasheet only says SFDR >= 45dBc which is not very helpful). Once you get close to those distortion limits, the error bars increase. The distortion of the generator and the receiver do not necessarily add in a straightforward way either. In some cases they may cancel partially and a noisier input signal may show less distortion because of dithering effects.

The SDG2000X has pretty clean output in the audio range (significantly better than the datasheet specs at most settings). Almost all oscilloscopes will struggle there, with some exceptions like the remarkable but specialized PicoScope 4262. Note that you are trying to characterize a 16 bit generator with a 12 bit oscillscope. Now that doesn't tell you anything definite about linearity at all but IMHO it should give you pause and make you proceed with careful consideration and healthy scepticism.

What can you do to improve your measurements? First, know your instrument and its limitations. You can try to measure an oscillator that is known to have distortion levels significantly better than your oscilloscope as a reference. That also allows you to find the settings that give you the best results. A quick sanity check is to add some in-line attenuation (on a spectrum analyzer, change the input attenuator setting). If the relative levels of the harmonics change, you will know that the distortion is from the receiver.

You can also add an external notch filter to attenuate the fundamental. This will enable you to measure down to very low distortion levels.

--- End quote ---

Well said!

Also example I have used several kind of spectrum analyzer over tens of years. Most (all) of them display "fake" signals.  Non input related spurs and then input related. Look generator harmonics with SA and wonder what harmonics are generated or affected due to SA own generations... and so on. Simple: There is not ideal instruments on this Tellus, not in history, not now and never in future. Every instrument display sum of errors mixed with some kind of truth what we do not know.
With FFT display human eye see these so... omg...so big false peaks. User need just understand and just know his instrument limits and lies and not believe everything.

Martin72:
OK, so we have the philosophical approach already.
Do not trust what you see, use your imagination.
Btw, this is what it should look like to me, a 1Khz square wave, steadily decreasing harmonics.

rf-loop:

--- Quote from: Martin72 on December 28, 2022, 01:43:29 pm ---OK, so we have the philosophical approach already.
Do not trust what you see, use your imagination.
Btw, this is what it should look like to me, a 1Khz square wave, steadily decreasing harmonics.

--- End quote ---

Yep, roughly so.

You can now zoom in and look just 577th harmonic and check 576th and 578th... then measure these and tell "truth" and think why these levels are just right or how they are... (not seriously...  :D )

2N3055:

--- Quote from: Martin72 on December 28, 2022, 12:31:01 pm ---Hi,


--- Quote ---Note that you are trying to characterize a 16 bit generator with a 12 bit oscillscope.
--- End quote ---

I realise that, but the scope should not show anything that is actually not there, and that is the current situation.


--- End quote ---

Well scope doesn't show anything that is not there in normal time domain mode.....

FFT is math mode... In math mode there are no limits on result.
You can make math channel where you multiply a signal by 10E6 and you will get weird stuff.

Like others said, with SA you need to characterise for spurs to know if signal peaks are real.

Whole thing stems from the fact that input noise is really low. On my Keysight 3104T noise floor is so high you cannot see spurs at -60dBm

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