Products > Test Equipment
Measuring Distortions with the Scope:What you see is not what you really have..
rf-loop:
--- Quote from: 2N3055 on December 28, 2022, 03:03:00 pm ---
--- 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 ---
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
--- End quote ---
In this regard, I just previously suggested using paper or a piece of fabric to make curtains. Then just close a curtain in front that covers the things that shouldn't be taken so seriously. Of course, one could also make the curtains in another way... like Keysight, for example. This way you can conveniently cover it up and no one will make a fuss about "strange" signal spikes somewhere deep...
Maybe someone should suggest that... you could easily implement a noise generator there to satisfy the users, because then the lies will be removed --- oops --- hidden.
Detlev:
Hello everyone,
for comparison I made an FFT of my SDG6022x with the RTB2004 (RTB2K-COM4). The values of the 6022 are very similar to those of the 2122X
Many greetings
Detlev
Martin72:
Like the comment function of the RTB...
By the way, I remembered that I still have something on the shelf with which I can measure THD.
My Neutrik A1.... 8)
With this puppy it should be possible to get "more trustful" values.
mawyatt:
--- Quote from: Detlev on December 28, 2022, 09:12:37 pm ---Hello everyone,
for comparison I made an FFT of my SDG6022x with the RTB2004 (RTB2K-COM4). The values of the 6022 are very similar to those of the 2122X
(Attachment Link)
Many greetings
Detlev
--- End quote ---
Thanks for the plots!!
If you check the earlier posts we provided, it's likely that the DSO is contributing more of the higher frequency harmonics than either AWG. This is because these AWGs provide pretty good signal fidelity from 16 bit DACs and thus more so than the typical general purpose bench top DSO can cleanly resolve.
Best,
Performa01:
My bad – In my initial comments I simply forgot that the linearity of the average scope frontend (including SDS2000X HD) is not up to the task of characterizing the harmonic distortion of an SDG1000X or 2000X.
--- 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 ---
If you look up the datasheet of PGAs that can be used for a scope like this, such as LMH6518, you will find the harmonic distortions specified somewhere between -44 and -50 dBc. Considering this, the SDS2000X HD still does a fairly decent job.
The FFT is still useful for many tasks, including distortion measurements within the usual range of interest, which hardly ever exceeds -60 dBc except for high end audio. There is a reason, why specialized audio analyzers exist.
You just have to take into account that the SFDR of a general purpose scope frontend might be only about 60 dB in practice – many (if not most) of the more affordable old SA boat anchors haven't been much better btw.
For instance, there is no restriction for single tone narrowband measurements, like phase noise or modulation spectra, where especially the SDS2000X HD will give you an exceptional dynamic range that clearly exceeds the ~72 dB that you could expect from the 12 bits.
The THD of a general purpose oscilloscope frontend is usually not specified in the datasheet. On the other hand, the Picoscope 4262 has a guaranteed linearity of 16 bits – but that is certainly not a general purpose instrument, with its 16 bit converters and limited bandwidth of just 5 MHz. Yet this is the way to go if you need to measure down to -96 dBc and don't want to be restricted to the audio frequency range like with soundcards.
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