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| Measuring Distortions with the Scope:What you see is not what you really have.. |
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| Martin72:
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.... |
| rf-loop:
--- Quote from: Martin72 on December 27, 2022, 10:06:11 pm ---Hi.... Following one of Sinisas hints I´ve played with the vertical resolution between 100mV and 200mV/Div., same FFT settings as before. Two pics, one with 100mV/div. (as the measure before) and one with 130mV/div. Judge by yourself.. --- End quote --- You can also do same but take all averaging off and watch screen more time than just one eyes blink because we do not have good min-max feature in this FFT application, and not much else either. After enough watching... take a piece of paper or nice cloth and make curtains to hide these very low level peaks and "problem" is "solved". :- // (= if the measurement instrument can't show enough truth then instrument must not display it) Btw, voltage band change between 100│102mV/div. (and 1.00│1.02V/div) Using this have minimal other effects than just band bottom and band top, so minimal change in signal level in ADC but different front end noise figure. |
| Performa01:
--- Quote from: rf-loop on December 28, 2022, 07:06:18 am ---Btw, voltage band change between 100│102mV/div. (and 1.00│1.02V/div) Using this have minimal other effects than just band bottom and band top, so minimal change in signal level in ADC but different front end noise figure. --- End quote --- ... because of vastly different PGA gain, which in turn could also lead to a different distortion level of the PGA (which probably is the main source of nonlinearity in a scope frontend). |
| rf-loop:
--- Quote from: Performa01 on December 28, 2022, 09:00:33 am --- --- Quote from: rf-loop on December 28, 2022, 07:06:18 am ---Btw, voltage band change between 100│102mV/div. (and 1.00│1.02V/div) Using this have minimal other effects than just band bottom and band top, so minimal change in signal level in ADC but different front end noise figure. --- End quote --- ... because of vastly different PGA gain, which in turn could also lead to a different distortion level of the PGA (which probably is the main source of nonlinearity in a scope frontend). --- End quote --- Yes, this can apparently suspect. :) More research is needed. |
| switchabl:
--- 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. |
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