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REVIEW - Rigol DS2072 - First Impressions of the DS2000 series from Rigol

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marmad:

--- Quote from: WVL_KsZeN on December 18, 2013, 06:58:10 pm ---Was wondering if anyone could confirm this bug aswell:

--- End quote ---

Like most DSOs, ALL measurements from the left menu are software measurements on display memory (NOT sample memory) - they can only be considered accurate if you can see the wanted measurement parameter clearly defined on the displayed waveform.

OTOH, the counter from the Measure menu is a hardware frequency counter - and will be accurate, regardless of the displayed portion of the waveform.

WVL_KsZeN:
ofcourse, but there's more than 100 oscillations on the display.. no need for >1% error.

And if it's only depending on the display, how does changing the mem depth make such a big difference? the displayed signal is exactly the same.

@ auto mem depth


@ 56mpts mem depth

Bugware:

--- Quote from: marmad on December 18, 2013, 07:02:34 pm ---
--- Quote from: Bugware on December 18, 2013, 06:52:48 pm ---Sure that is a scaling problem. But I think all data in the display memory must be scaled?! So maybe there is no change in the update rate, so my thought...
--- End quote ---

There is the normal scaling done between sample memory and display memory. The scaling for statistics is clearly done after this (since it's doubling lines), as part of the overlay (adding measurements, screen icons, etc).


--- Quote ---I see. Sorry I have misunderstood the bug #17 in the list. So then this is only a confirmation of that. ;)  BTW. the offset is not only with AC Coupling but also for LF and HF Reject Coupling...    8)
--- End quote ---

Yes, I knew, but the bug clearly affects filtered triggers - so fixing it for one will likely fix it for all - but I added *filtered* to the bug description to make it clearer.

--- End quote ---

Thank you for your explanations. I have now understood the backgrounds. Again learned something! This helps newbies like me very much.  :-+

WVL_KsZeN:
yes.. other measurement also seem to be affected. I think it's weird that the mem depth is having an influence, how could that be? As you can see by my screenshots, they're almost 99.9999% identical, so the scope is not (only) measuring 'on the screen'.

This is weird? a post by Marmad seems to have disappeared?

marmad:

--- Quote from: WVL_KsZeN on December 18, 2013, 07:12:52 pm ---ofcourse, but there's more than 100 oscillations on the display.. no need for >1% error.

And if it's only depending on the display, how does changing the mem depth make such a big difference? the displayed signal is exactly the same.

--- End quote ---

The memory depth changes the amount that the DSO decimates to the display memory - either 13.8MB decimated to 700 bytes - or 56MB decimated to 700 bytes - in your images. You can imagine there will be changes in the frequency decimated to because of rounding errors.

Personally, I would NEVER use frequency from the measure menu (because of accuracy) - just the counter.

But it seems we have this discussion about the flaky measurements on DSO every few months. The truth of the matter is this:

Unless you have an expensive DSO which measures in sample memory, DSOs suck at measurements. All oscilloscopes, traditionally, have been crude measurement devices (on old analog scopes we counted graticule lines): they are, first and foremost,  VISUALIZATION devices. Now, DSO manufacturers have figured out a cheap, simple, and fast way to tack on measurements by using display memory - but honestly, if measurement accuracy is important - use an instrument designed for accurate measurement - or - capture/save the sample memory and measure using external software.

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