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Resolution enhancement limits number of samples

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Resolution enhancement limits number of samples

Postby CoyoteDavid » Sat Feb 02, 2013 8:57 am

I am using a 3406B Picoscope. When I turn on any resolution enhancement with the requested number of samples set to greater than 2M samples, it immediately changes the requested number of samples down to 2M. If I put it on auto trigger, and have the traces running, it will let me change the number displayed in the requested samples box to as high as I want, but it will always use less than 2M samples. Then if you hit STOP, or START, or almost any other control, the requested number suddenly changes back to 2M samples. The instructions say that you should increase the number of samples to offset the effect of the filtering, but don't say anything about Picoscope limiting the number of samples available when using resolution enhancement.

To recreate, do the following:
1) Setup single channel A, default setup watching compensation signal at 2 ms/div
2) Set samples to request 10M samples (I get 10,000,005 samples)
3) Open channel A menu and select 8.5 bits resolution
4) The requested samples immediately changes to 2 MS, and it uses 1,250,005 samples.
5) Now try increasing the number of requested samples. The control lets you change the requested number in the box, but the actual samples remains at 1,250,005.
6) Hit any other control, like the AC/DC setting, A menu control, or any other meny button in the channel row, and the requested number reverts back to 2 MS. The actual samples remains at 1,250,005.

Another strange behavior, after this test, do the following:
1) Stop scope trace.
2) Set resolution back to to 8 bits
3) Set requested number of samples to >2M (like 10M samples for instance)
4) Open menu for channel A. ==> Requested samples immediately changes to 2M
In this case, you don't even have to change the requested number of samples, just hit the A channel menu and it immediately changes the requested number of samples.
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Re: Resolution enhancement limits number of samples

Postby Hitesh » Wed Feb 06, 2013 10:39 am

Hi David,

PicoScope applies some form of limit to the samples so that when measurements/filtering/resolution enhancement is being carried out, the software is able to still continue capturing data without loss in performance.

When the resolution enhancement is set to 0.5 bits, a moving average of 2 values is used.

With regards to the second feature, this is due to the fact that the settings are only applied once the the software has started running again.

I will feed this back to our Technical Authoring team.

Thanks,
Hitesh

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Re: Resolution enhancement limits number of samples

Postby CoyoteDavid » Mon Feb 11, 2013 9:04 am

Thanks for the reply.

It would seem that if I am using resolution enhancement, that I would want to use the maximum number of samples possible. That makes the sample rate faster, which in turn reduces the effects from the averaging, like limiting the frequency bandwidth that I can capture. As it is now, it I try to use the 12-bit mode I end up severely limiting the bandwidth that I could otherwise capture. To get 12 bits, it has to average 256 samples. So if I want to sample 50 milliseconds of a waveform, at 8 bits I can get 1 GS/s. With 12-bit, I get 31.25 MS/s only due to the number of samples being reduced from 50M to 1.5625M, which combined with the 256 average gives a square edge an 8 microsecond rise time. With the 1 GS/s I would still see about a 250 nanosecond rise time.

If I change my sweep rate to 200 us/div, then I can get my expected bandwidth back, since the sample rate then goes to 1 GS/s. However, then I can only capture 2 milliseconds of data, as opposed to 50 milliseconds.

i guess I am mainly wondering if I am interpreting this correctly. It appears that using the resolution enhancement takes away the deep capture benefit of the PicoScope.

-David
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Re: Resolution enhancement limits number of samples

Postby ziko » Thu Feb 28, 2013 3:45 pm

Hi David,

You are correct, with filtering and measurements the number of samples is limited to 2MS. This in turn will limit your sample rate, depending on your time base. If there was no limit it would affect the overall performance of the software and hardware really.

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Re: Resolution enhancement limits number of samples

Postby CoyoteDavid » Thu Feb 28, 2013 6:30 pm

What do you mean by "affect the overall performance"? Limiting to 2 MS is quite surely limiting the performance, but that can't be what you mean, since that is what you are doing to supposedly NOT limit the performance... One of the advantages I was expecting from this scope, based on its specifications, was to be able to take a 20 millisecond high resolution (12 bit) scan at 1 GS/s sample rate, but I can't (with the PicoScope). The reason I can't is because the SOFTWARE is indeed "affecting the overall performance of the software and hardware really". Really? What am I missing here? Is it the update rate, and if so, I "really" don't care if the update rate is slower in that case, I know I am transferring lots of data. I would rather it be slow in that case, than not work at all!

As a suggestion, could you put that limitation in the specification that you publish for the scope?
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Re: Resolution enhancement limits number of samples

Postby ziko » Fri Mar 01, 2013 12:20 pm

Hi,

One thing I was made aware of today is that if you capture the data in its raw form and then save it. You can later open up the file and then apply the resolution enhancement without limitation.

This limit is applied when in real time to ensure the software will work well on most PCs (even on a humble netbook!). In terms of hardware it may affect the dead time between captures in real time. That being said I will pass these comments to our software team. I have already spoken to our technical author with regards to the limitation.

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Re: Resolution enhancement limits number of samples

Postby CoyoteDavid » Fri Mar 01, 2013 5:37 pm

Hi Ziko,
Thanks for the response. I am using the scope with a Core I7, so have plenty of CPU on the PC side. It would be kind of sad to have to limit the fast PC's usage due to the other slower PCs. The user always has the option to easily limit the number of samples if on a slow PC. However, the situation I wanted is a single capture, so it would still be useful even if the PC took a long time to process the data. Much more useful than not allowing it at all!
-David
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Re: Resolution enhancement limits number of samples

Postby Martyn » Fri Mar 01, 2013 6:15 pm

Just capture the single shot of data, with resolution enhancement turned off, and then just enable it afterwards. You don't need to save it and then reload it.
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