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Problems with Triggering & Capturing with 2203

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Problems with Triggering & Capturing with 2203

Postby Big 10 Inch » Mon Jul 28, 2008 6:29 pm

Hello,

We are using the 2203 with 6.0.15.2 to capture a range of PSI that is reported back to the scope in a range of 0.25V to 4.25V. Our software specs are set as follows:

-Collection time: 20 ms
-Number of samples: 200 kS
-Trigger: Probe B, rising edge, threshold = 1V, pre-trigger = 70%

Out of roughly 8 attempts, we have only received 2 traces that look as they should (see attached 20080726-shot 3 txt.txt file) and the other 6 were garbage (see attached 20080726-0001.txt file). The data in the good trace is about (with only slight variations) what we would expect to see every time. The numbers in the table have been converted to PSI with a custom probe setting; 0.25V = 0 PSI and 4.25V = 150 PSI. Our trigger point is for probe B at 1V (~28 PSI), yet (as you can read) in the garbage trace we never saw above 1.1 PSI.

My questions are as follows:

-Why does the waveform not pass through the trigger point? It would make sense to me to see a plot of 1V (~28 PSI) for probe B at time ZERO.
-Do we have a problem triggering? Why are we getting these garbage traces?
-Any other suggestions?

Any light you can help shine would be greatly appreciated. Thank you in advance.

Best regards,
Alex

PS. I cannot upload the psdata files to the website.
Attachments
20080726-0001.txt
These are the garbage traces we are receiving more often than not.
(214.38 KiB) Downloaded 206 times
20080726-shot 3 txt.txt
This is a trace as we would expect to see.
(225.48 KiB) Downloaded 292 times
Last edited by Big 10 Inch on Wed Jul 30, 2008 3:23 am, edited 1 time in total.
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PDFs of the Traces

Postby Big 10 Inch » Mon Jul 28, 2008 6:37 pm

Printed the graphs to PDF files...
Attachments
Good Trace.pdf
PDF of one of the good traces
(76.51 KiB) Downloaded 208 times
Bad Trace.pdf
PDF of one of the bad traces
(73.38 KiB) Downloaded 216 times
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Postby hillp » Mon Jul 28, 2008 9:27 pm

Why would the "garbage trace" even trigger in the first place? It's barely up above the 0 psi (250 mV) baseline for the entire captured trace, not even close to the 1000 mV trigger point.
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Postby picojohn » Tue Jul 29, 2008 8:34 am

Alex,

Could you zip the 'good' and 'bad' psdata files and send them for my attention to support@picotech.com

Please also paste this post link in the email to remind me of the problem.

Regards
John
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You've got mail...

Postby Big 10 Inch » Tue Jul 29, 2008 4:25 pm

You've got mail...
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Postby hillp » Tue Jul 29, 2008 10:09 pm

Could this have anything to do with the resolution enhancement feature? There are several warnings about it in the help file, with such ominous words as "flat line" (the "bad traces" are almost that), but it doesn't give a whole lot of guidance except to try different amounts of resolution enhancement.
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Postby picojohn » Wed Jul 30, 2008 1:04 pm

Alex,

Examination of your files indicate that the behaviour in your application is due to the trigger hysteresis level of the PS2203.

The choices are to increase the trigger threshold to around a minimum of 12 counts above the level of the dip in the trigger waveform (around 32 PSI?) or, upgrade to the PS2205 which supports advanced triggers.

Regards
John
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Postby Big 10 Inch » Wed Jul 30, 2008 1:50 pm

Thank you for the response, John.

Any explanation as to why the waveform never pass through the trigger diamond?

I will e-mail a couple other psdata files to you of other "good" traces from when we were figuring out the best trigger settings. I use the word "good" loosely as we had good numbers but improper trigger settings (you'll noticed it captured the rising pressures before the steep rises as seen in the previously emailed psdata files). They didn't capture what we were looking for and triggered early.

Is this a trigger hysteresis problem as well?

Thanks,
Alex
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Postby hillp » Wed Jul 30, 2008 3:37 pm

What does "12 counts above the level of the dip in the trigger waveform" mean? Is this 12 of the 256 count range of the 8-bit 2203 scope? And what is the significance of the "dip" in the trigger waveform?
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Postby hillp » Wed Jul 30, 2008 5:20 pm

Another question - how was it determined that this is a trigger hysteresis problem? I ask because the "bad trace" captured never exceeded about 1 psi (0.28 VDC) anywhere in the trace. The trigger level was set at 28 psi (1 VDC). With the scope set at -2 to +2 VDC, the trigger setpoint is 18% of the scope's range above the highest measured signal, or about 46 counts on an 8 bit scope. Yet it still triggered.

In the distant past (but within the last 5 or 6 years) I've had difficulties with erratic triggering (ADC40, ADC11/10) that seemed to have been fixed, at times, by NOT using a custom range, that is, just staying with the straight voltage range and converting the data in a spreadsheet.

Can you comment?
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Postby picojohn » Fri Aug 01, 2008 8:26 am

Hi Alex,

I’ve noticed that you are applying a full 4-bits of resolution enhancement which will have a significant low-pass filtering effect on the displayed signal. Further, it is highly probable that the scope is triggering on an area (such as spike) that is removed from the displayed signal as a result of the filtering.

My assessment from your 'good trace' is that you require a trigger level of 30-31 psi for reliable triggering, and that is due to the interaction of your waveform with respect to the PS2203 trigger hysteresis of 6-7 psi (10-12 LSB).


NOTE for Hillp:
Thank you for your comments, however, I would ask that you start a new thread or submit an email to our helpdesk ([url]support@picotech.com[/url]) as it is quite difficult to address two customers with diverging issues. As my prime aim is to resolve the initial problem presented by Alex, I thank you in advance for your understanding.

Regards
John
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Sorry for the Confusion...

Postby Big 10 Inch » Fri Aug 01, 2008 4:10 pm

picojohn, sorry for the confusion. hillp is part of my team (and frankly, more familiar with our setup than I am)...
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Postby hillp » Sun Aug 03, 2008 2:16 am

Sorry, Picojohn, that I wasn't clear enough in my questions to make it evident that I was addressing this current topic. I'll try again:

RE: resolution enhancement
Thanks for the tip on the resolution enhancement; we'll take this out completely and see what the traces show. Are there any tips on how much we can use? The events we're capturing are not very easily generated; they are one-shot DC voltage patterns, not a repetetive signal at all. Of course we will keep ETS off in any case.

RE: triggering
Is there any place I could educate myself on the details of the PS6 triggering mechanism and its hysteresis? I know what hysteresis is in electromechanical systems, like a knob on a potentiometer that slips a little before it turns the shaft, thus leading to different resistance on the pot for the same knob position, depending on the direction the knob was last turned. I think I learned from your latest post that the displayed trace after triggering is not the same as the data used to trigger the scope, at least with resolution enhancement operational. The hysteresis and advanced triggering writeups in the PS6 help file talk about a hysteresis trigger feature in advanced triggering, but not about normal triggering and how it works with regards to the amount of signal movement necessary to trigger. This may become important as we substitute other pressure transducers (such as 0-25 mV output over 0-50 psi) and adjust the scope range to compensate, thus changing the number of counts/psi.

My comment about not using custom ranges in earlier versions of Picoscope was meant to ask if the range conversion from volts to psi should have any effect on triggering reliability. I did notice in PS6 that the trigger level is still specified in volts, even though the signal is being displayed in psi per our setup.

Thanks.
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Postby picojohn » Wed Aug 06, 2008 2:24 pm

There is no straightforward advice on the use of resolution enhancement, other than to try 0.5 a bit at a time.

Section 7.7.2.1 of PS6 user guide and the following link http://www.picotech.com/education/oscil ... ering.html provide an explanation of trigger hysteresis and, in the case of the PS2203, the hysteresis is currently fixed at 10 LSB of the selected range.

As suggested previously, you may wish to consider a unit such as the PS2205 which allows advanced edge triggers in which you may define your own hysteresis levels.

With reference to your final comment, as your probe allows PicoScope to manage the range selection automatically, any such effects would be minimised.

The trigger level remains in volts as that does not change in practice. Nonetheless, the scaled value may be determined using a marker on the trigger diamond.

Regards
John
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Postby hillp » Tue Aug 12, 2008 4:24 am

Let's see if I have this right for the 2203 scope:

The trigger level that I set in the trigger boxes (for instance 1 volt rising edge) arms the trigger when the signal reaches it (1 volt), and then the trigger actually fires when the signal reaches 10 least significant bits above the trigger level? This would be at 1.156 volts for a chosen range of -2 to +2 VDC. (4 volts range over 256 least significant bits gives 15.6 mV/LSB. 10 of these is 156 mV.)
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