[...] Two tones sources isolation. Is it isolated when one function generator digitally combine two tones - really. [...]
Are you saying, we might really be measuring the distortion of the function generator itself if we do that?
Of course you will measure distortion of the function generator if it is worse than scope or SA noise floor..
I agree with Rf-loop that this is not serious testing... but...
We did get some interesting and real data from it despite not being done by rigorous method.
Not accurate data, but more of a glimpse of order of magnitude of some behaviour.
First thing I discovered is that digital WaveCombine on my SDG6000X does respectable job on low frequencies at 0dBm . Also most of the scopes shown here, inexpensive and expensive, has shown that pretty much none of them have any appreciable intermod distortion above noise level and resolution of scope A/D, at frequencies and levels tested..
This makes me curious to actually sit down and do more comprehensive test, first to figure out IMD of SDG6000X across frequencies and output levels using digital WaveCombine, to figure out how well it works when D/A isn't full scale, and when switching attenuators, at lower levels when S/N ratio gets low or distortion when I get to higher levels and amplifiers start distorting...
Also here we could see difference in FFT implementation on different scopes, and where are the limitations...
So not a waste of time.
But it is all, so far a discovery journey to learn something about your equipment (Thanks to Mawyatt who brought it up as a topic). This type of testing is very common in industry for much more than just mixers and RF, like he said, but his bringing it up here made it visible to many who didn't use it before for this purpose. And it sparked some very good posts, so all very good and well...
If we would like to make standardized test, we would have to define it. Which would need some sort of goal or target result expectations (what do we want to accomplish), some sort of technical measurement equipment spec (what equipment and what spec we would need to do it), and measurement method (what needs to be measured and how exactly).
Problem is, that could be specified, but it would vary for different purposes (IMD distortion of audio front end, or 100MHz scope or 1Ghz scope) and would soon evolve into IEC type of standard, that not many would be able to follow, if not for any other reason than lack of equipment necessary.
Regards,