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Pocket-Sized 6 GHz 1 TS/s ET Scope

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SJL-Instruments:

--- Quote from: KE5FX on March 01, 2024, 05:07:02 pm ---Looks like a job for a median filter to me.  Or if you want to get a little fancier...

--- End quote ---

--- Quote from: joeqsmith on March 01, 2024, 04:41:45 pm ---Doing a vertical scan that requires cells above and below to be active, we are already loosing a fair amount of good data.  I would need to do a horizontal scan as well.  It could certainly be done but what a mess...   

--- End quote ---
Yes, horizontal correlation is the way to go. There is no way to distinguish statistical noise from a rare event using only the information in one PDF. A horizontal median filter is a simple way to do this. The approach we mentioned before is theoretically better-conditioned than the median filter, but the gist is the same - temporal correlation.

A general-purpose image despeckle algorithm isn't really sufficient for this application, since we're dealing with quantitative data. The results should be accurate and unbiased, whereas an image despeckle just needs to visually resolve the problem.

joeqsmith:
Attached is the PAM4 (CSV) data I used to create the above plots for those who want to try their hand at cleaning up the speckles.  Each line is CRLF terminated.  Header should explain the format.     

SJL-Instruments:
We found a simple, bias-free way to remove 90% of the speckle. Essentially, we just enforce monotonicity on each CDF. In detail, any neighboring pair of points that are in the wrong order are set to their average. This might need to be repeated a few times until the entire CDF is monotonic.

Attached are before/after images, plotted on the same color scale. This will be implemented in software v2.5.12, and long-term will be done in firmware v14.

Beyond this, we don't think it's a good idea to try filtering the data in any way. It's better to just increase the CDF settings and gather more statistics.
The monotonicity filtering is based on an inherent mathematical property, and will not alter any already-valid CDF data, so it's always OK.
But any other filtering method will introduce artifacts in some cases.

In the long-term, the speckle issue can be completely eliminated with a dual-comparator design. If/when we introduce a new model of the GigaWave (or a dedicated SI analyzer), we will implement this.

SJL-Instruments:
We've just released minor software update v2.5.12 which significantly improves the speckle issue (and fixes several UI bugs).

The corresponding manual has been updated to revision H12. We have significantly expanded Section 2 to add detailed guidance on choosing appropriate CDF sampling settings.

As always, let us know if anything in the manual is unclear.

joeqsmith:
Comparing the latest 2.5.12 with the earliest version I have, 2.5.3.  Both in demo mode.  Attempting to set intensity to give the same shading. 

IMO, these speckles are always going to raise questions for the user if they are dealing with a scope or a signal problem.  Looking forward to the updated firmware.  I have not tried to bump the triggers above 30k as suggested.   


--- Quote ---In the long-term, the speckle issue can be completely eliminated with a dual-comparator design. If/when we introduce a new model of the GigaWave (or a dedicated SI analyzer), we will implement this.

--- End quote ---

I am sure you were aware of the speckles early on in the design phase.   Most likely before even starting on the hardware.  I envision the signal processing was simulated first, but maybe not.    I am curious if you knew changing the architecture would have solved it, why didn't you just change it.   Was the added cost really that big of a factor?   

Had the dual-comparator approach been used, how would it have effected the sweep speed compared to your future firmware approach? 

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