Hi, Creep. I suspect this has already been addressed at this point, but since you asked me directly...
Mark_O: can't one just turn off the other channels to check how the signal looks in one channel mode if you get a feeling something might be off? Then if you have confirmed it, you can safely turn on the rest of the channels knowing that what you see is the accual signal. Of course, this assmums that one is triggering off the channel in question.
Would there be any accual problems going about it this way?
Yes, this would work. And no, there are no actual problems with doing so. One could either do it preemptively, using a single enabled channel to probe the DUT, looking for high-frequency content. And even use the FFT to check for higher amplitude content in the no-no part of the spectrum. OR, just proceed without, and only fall back to check if your testing suggests that something may be amiss.
P.S. I know that that would be a hassel and I'm not saying it's convenient. Nor am I saying I understand this stuff all that well, it's just an idea I got while reading the previous comments.
Your understanding is fine, and it doesn't have to be a huge hassle. It is an extra thing to remain aware of, and could cost you some time. And time is often something a professional engineer is trying to optimize. But the 1000z-series are versatile scopes, and very usable.
If I were using one in 4-channel mode, but in too big a hurry to check, I'd just enable the 20 MHz BWL. And if that were insufficient, I'd just use one of my other DSOs, that didn't have that constraint. But though I have quite a few scopes, only a couple of my Tek's and one LeCroy exceed the Rigol's capabilities on 4-channels. And they're not nearly as compact.
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One thing folks need to keep in mind is that people like Marmad are simply trying to provide an education, by calling attention to the limitations of the instruments we're using. The educators aren't saying "don't even try to use it", or "the thing is worthless". Just that you can't go in blindly.
If you think that's not possible or even likely, consider this scenario. Someone who reads the EEVBlog see's the discussion on the 1054z, and the great price, and decides to pick one up, and hacks it to a 1104z model, with "100 MHz bandwidth". And, like pascal_sweden, assumes that a 100 MHz scope is always fine for examining 100 MHz signals. So he hooks it up to the embedded system he's working on, and starts looking at the SPI bus. Since the 1000z series supports all 4 of the required channels (MOSI, MISO, CS, and CLK) he can do that easily.
Nowadays, SPI busses aren't just 2 MHz, or 5, or 8 MHz. I've got one here that's running at 60 MHz, and I wouldn't be surprised to hear about faster. So let's say he's looking at that signal on his embedded system. He's perplexed, because he can't get the darn thing to decode properly. It
should work (a la, pascal), because it's "only" 60 MHz. But there are numerous problems that prevent it. First off, the scope isn't capturing fast enough to determine where the edges of the pulses are, with any certainty. (And the Rigol can't sync to the clock... it's async.) But there's also a lot of large-amplitude spectral content,
way past the Nyquist limit, that the Rigol will do nothing to ameliorate. That will fold over into the passband, and depending where the trigger levels are set, could be of high enough amplitude to completely corrupt the decoded stream on the SPI bus. But even rare, sporadic, corruption is extremely undesirable, if not totally unacceptable.
Now he's going to be pretty unhappy. He bought something thinking he knew what it could do for him. And it's not working. So the first conclusion is that it's broken. We've seen that here more than once, and I saw it all the time back when I was discussing similar topics on the RC Groups forum.
"The scope's no good. It doesn't work." And lots of time wasted. All because folks were unaware of the limitations of the test instruments they're using.
Having that knowledge in advance will enable them to work smarter, not harder, and get better results they can be confident about. And spend less time scratching their heads, trying to figure out what's wrong with their DSO, and more time focusing on what's wrong with their circuit/system. That is the goal, after all.