Products > Test Equipment

Oscilloscope Zoom Out Quirk

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Someone:

--- Quote from: EEVblog on May 10, 2020, 11:10:38 am ---To summarise, I think STOP mode or Single Mode should ultislise all available hardware memory regardless of current memory setting and at the fastest sample rate. This is clearly what Keysight have opted to do.
And perhaps that's a better description of this issue, "Capture memory depth in STOP/Single mode".
--- End quote ---
nctnico's corner case is when there isn't a repetitive trigger to create that larger capture. That "magic" capture when pressing stop only happens under specific circumstances so it can't be relied upon in regular use.

The argument against always having full memory depth for single captures is what do you do when the pre trigger buffer isn't full yet, but the user pressed single? Throw away possibly the only trigger?

nctnico:

--- Quote from: Someone on May 10, 2020, 11:01:19 am ---
--- Quote from: nctnico on May 10, 2020, 09:40:59 am ---And I don't get how fast waveform updates suddenly become relevant in this thread; capturing beyond the screen has no downsides. Just set the memory shorter one way or another and you have more waveforms/s and/or history / segmented recording.
--- End quote ---
Waveform update rate is limited by letting the acquisition length expand around the visible window, to claim there is no downside in doing so is plainly ridiculous. You like having manual control of memory depth and it expanding around the screen, we keep hearing this and get it, but your justifications are nonsense, and you throw it out as some big issue when it really isn't.

--- End quote ---
It seems your understanding of how an oscilloscope works is severely lacking. If you set the memory to a shorter depth then acquisitions take less time so you can achieve higher waveforms/s. I just want to have the choice between many waveforms/s OR using the full memory. I don't get why not having that choice is somehow better.

EEVblog:

--- Quote from: Someone on May 10, 2020, 11:12:19 am ---All that extra memory around the aquisition window kills waveform update rates (a tradeoff which is completely acceptable for some uses). Here is the plot from a Tek DPO4000:

Extending the acquisition buffer each side of the trigger outside the window on the screen is shown with dotted lines toward the right. It knocks orders of magnitude off the update rate.

Although there are more intelligent memory management options, scopes are rather simple/dumb. They won't redraw/paint another trigger until the full memory depth is filled. So at 5GS/s and 10M memory depth, even a perfect zero overhead, instant trigger will only run 500 updates/s.
--- End quote ---

This is why Keysight are clever in how they do it. They know that in trigger/Auto continuous update mode you don't care about extra data outside the screen, you just want the fastest update rate possible. Only when the user specifically requests STOP or Single mode does the scope then go "a-ha, I don't care about update rate any more, so I'm going to make one last capture using all my available memory, just in case the user wants to do a zoom outside the display window. And I pay no penalty do doing this! Ain't I clever!"

2N3055:

--- Quote from: EEVblog on May 10, 2020, 11:10:38 am ---
--- Quote from: 2N3055 on May 09, 2020, 05:06:53 pm ---This shows what I'we been explaining. 3000T is capturing only screen full of data from trigger to trigger. It is only when you press stop that it will reassemble some data before begin of the screen, and it will keep capturing until it runs after memory. What I suspect is that it literally goes from RUN to STOP, by capturing one SINGLE capture on next trigger after you press STOP (Just with half of the buffer because of state of capture engine at the time, the ping-pong buffers).

I presume it behaves like that, because, if press stop now, while it's waiting for next trigger, it doesn't get any additional data:
(Attachment Link)

--- End quote ---

That sounds right. It's doing just the screen worth in Normal or Auto mode because it needs the fastest update rate possible. There is no point capturing continuous updating data outside the screen if the user can't see it.
So yes, doesn't surprise me that this only works in STOP or Single mode, because these modes are, practically by their usage definition, analysis modes were you may want to expand the timebase to view that extra data. So when the scope has been instructed perform STOP or Single mode it makes sense to then capture the entire available memory length regardless of what the user has set, because update rate no longer has any relevance.

Come to think of it this way now, there should be absolutely no reason why the likes of Siglent could not implement this easily with a firmware update. The likely reason they haven't is because they just haven't thought of it before.

To summarise, I think STOP mode or Single Mode should ultislise all available hardware memory regardless of current memory setting and at the fastest sample rate. This is clearly what Keysight have opted to do.

--- End quote ---

I absolutely agree. Keysight made a very nice trick here, and I would like it to be on all scopes. But history mode (to keep hundreds of previous triggers automatically) is also very nice (i have it on Pico).
Ideally, it should be user's choice.


--- Quote from: EEVblog on May 10, 2020, 11:10:38 am ---And perhaps that's a better description of this issue, "Capture memory depth in STOP/Single mode".

--- End quote ---
That is very good observation, very on point.

Someone:

--- Quote from: nfmax on May 10, 2020, 11:15:02 am ---
--- Quote from: Someone on May 10, 2020, 11:12:19 am ---
--- Quote from: EEVblog on May 10, 2020, 10:56:35 am ---
--- Quote from: nctnico on May 10, 2020, 09:40:59 am ---And I don't get how fast waveform updates suddenly become relevant in this thread; capturing beyond the screen has no downsides.
--- End quote ---
It's only relevant if designing in this feature was done at the sacrifice of update rate in the design.
Obviously with Keysight having the fastest update rate and having this feature, that's obviously not a trade off that needs to be made in basic scope design.

--- End quote ---
Except they are mutually exclusive, the bonus full length capture of the megazoom scopes only occurs when there is another trigger after pressing stop. If you're trying to capture long record lengths of short bursts between infrequent events, pressing stop won't help.
--- End quote ---
You should use Single mode in that case
--- End quote ---
NO SHIT SHERLOCK! Or use a zoom window on the full length acquisition window so you can see it all at once without having to zoom in and out.

Can you not see the noise building way above the signal at this point of the thread?

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