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| A newbie needs some help with my first DSO |
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| Someone:
--- Quote from: awakephd on May 30, 2024, 02:06:03 pm ---To take a practical example, if I am trying to decode some messages on an SPI bus, here is what I think I am understanding: On the Siglent, I will need to show "all" of the messages on the screen to capture them - even though at that point they will be so squished together than I can't discern the content of any of the messages. Then, once capture, I can zoom in and scroll through to see the actual content of the messages. Have I got it right? --- End quote --- This is where you might want to explore the sequence/history mode, which can capture packets each on their own acquisition "screen" without all the quiet/empty bits in between. That will be quicker to step through but depends if there is enough gap between the data to cleanly separate them. Or use triggers to capture the message/signal/packet/byte that you are specifically interested in. Or as posted above, have the zoom window open so you can see both the long and short timeframe. Using the horizontal timebase control alone is just one way to work, and unlikely the best way to navigate the data. --- Quote from: awakephd on May 30, 2024, 02:06:03 pm ---One more possibly related question. I feel rather dumb for asking this, but ... I think I have just begun to realize that the "trigger point" of the waveform on the screen is actually in the center of the screen (by default), meaning that the scope is showing me events that happened before the trigger. Is that correct? Unless I have completely forgotten what I have done in the past with my old analog Tektronix, this is very different behavior, as the analog scope starts the sweep at the trigger, meaning that anything prior to the trigger simply cannot be shown. Clearly this is a huge advantage for the DSO! And if I understand correctly, I can adjust the trigger point left and right, so that I see less of what happened prior to the trigger and more after, or vice versa. --- End quote --- This is where the deep/long memory is an advantage, you can put the trigger as far to the right as the memory will support! Viewing data from before the trigger with your own little time machine. |
| Someone:
--- Quote from: awakephd on May 30, 2024, 02:06:03 pm ---The link to Dave's video was actually very helpful for me in making sense of what memory depth is. I confess that i was assuming it would work more like the Keysight, which is part of why I was having trouble understanding - I hit "stop," but there was no data past the end of the screen. I think I am now understanding that, for the Siglent and a few others, what is captured is only what fits on the screen at a given moment - thus, one must zoom out, capture, then zoom in to see detail. Right? Maybe? --- End quote --- No, there is no extra detail from capturing a longer screen and then zooming in. If anything it presents the risk of capturing less high frequency detail if increasing the timebase caused the scope to reduce the sample rate (due to hitting its memory depth limit). In run mode (not single capture mode) the keysight models do not capture around the visible window all the time, it's a "bonus" that occurs some times. Some other brands do it all the time, but that slows the update rate and triggers/information may never reach the screen: |
| nctnico:
You have to realise though that when doing protocol decoding, you don't want multiple trigger events in a single screen because the protocol decoding can decode one trace only. So typically triggering is set to normal and the signals are generated by instructing the DUT to do so. All in all a very slow process which doesn't need fast waveform updates at all. In most cases you want a record which is as long as possible. Capturing beyond the screen helps to look at the cause & effect of what has been captured. For example: sometimes there is a surprise and having more data is extremely helpfull to catch would could become an elusive bug, early on in the development process. But even without a need, capturing beyond the screen typically makes it not necessary to mess with the oscilloscope settings before having the DUT sending new data. Just scrolling left / right a bit to bring the section of interest on screen is enough. |
| bdunham7:
--- Quote from: awakephd on May 30, 2024, 07:01:33 pm ---But now that I know, it all seems so simple, and I feel so delightfully empowered ... --- End quote --- Lots of memory points are handy on occasion, but to really be able to use an oscilloscope you have to understand triggering. And that is not simple. Most scope work (or at least a lot of it) can be done with a few thousand points. I even have one with less than 1k points and while that can be a limitation, it is pretty good at what it does. |
| Someone:
--- Quote from: nctnico on May 30, 2024, 10:38:07 pm ---Capturing beyond the screen helps to look at the cause & effect of what has been captured. --- End quote --- You're really going to push your single minded position that this is the true/only/correct way to do things? While conveniently avoiding mentioning the use of a zoom window to see everything simultaneously without any hidden surprises... and people wonder why the signal to noise is so low here. :horse: |
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