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| Two Tone Test with Scope and SA |
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| SilverSolder:
--- Quote from: rf-loop on August 04, 2020, 03:31:30 am ---[...] Two tones sources isolation. Is it isolated when one function generator digitally combine two tones - really. [...] --- End quote --- Are you saying, we might really be measuring the distortion of the function generator itself if we do that? |
| gf:
--- Quote from: SilverSolder on August 04, 2020, 04:22:17 am --- --- Quote from: rf-loop on August 04, 2020, 03:31:30 am ---[...] Two tones sources isolation. Is it isolated when one function generator digitally combine two tones - really. [...] --- End quote --- Are you saying, we might really be measuring the distortion of the function generator itself if we do that? --- End quote --- That's why you need to take care, cross-checking the signal on a SA, in order to verify that the a priori distortion of the signal is significantly lower than the distortion of the scope under test. Note that the SA is not supposed to be the DUT for the test which is subject of this thread. Testing a high dynamic range SA is much more demanding. |
| 2N3055:
--- Quote from: SilverSolder on August 04, 2020, 04:22:17 am --- --- Quote from: rf-loop on August 04, 2020, 03:31:30 am ---[...] Two tones sources isolation. Is it isolated when one function generator digitally combine two tones - really. [...] --- End quote --- Are you saying, we might really be measuring the distortion of the function generator itself if we do that? --- End quote --- 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, |
| mawyatt:
As the OP this request was to help show how good or bad the scope front ends are and and how well they preserve the ADC performance. Having AWGs available at reasonable cost, and modest SAs allows the Two Tone IMD Test to be implemented by just about anyone that's seriously interested, and many folks certainly did contribute...so thanks :-+ There's no need for a complex expensive setup as long as the input signal source IMD is at least 10~20dB better than what you are measuring. The AWGs have shown very good performance even when the two tones are combined digitally (I assume this for the Siglent AWG, very nice feature). My goal was to help decide on which Digital Scope to purchase for my developing home lab now that I'm retired and this cost is out of my own pocket :) Hopefully it may help others interested in new instruments, or just evaluating the instruments they already have on hand. So many thanks to all that have contributed and helped me spend my retirement savings ::) BTW just purchased the Siglent SDS2102X Plus, wanted the 2104X Plus but they are not in stock here is US. Soon will be the AWG and SA, and maybe a programmable PS. Best, |
| jjoonathan:
Is SDG6000X WaveCombine digital? Mine clicks a relay when I engage it. I suppose it's probably range related. In any case, the resulting signals are quite nice! |
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