Issue with Triggering from a filtered channel?

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Issue with Triggering from a filtered channel?

Postby hexibot43 » Tue Oct 21, 2014 5:45 am

I just ran into something unexpected. I had a really noisy channel B. I was using an Amp probe to sync via #1 fuel injector on Channel B, and Firstlook on channel A.

2006ChevroletSilveradoTriggerProblem.jpg


I had turned filtering on for Channel B because The noise on The channel was over shadowing . I couldn't get the trigger to occur on what I expected to be the actual trigger event.

So my question is, is the trigger still based on the UN-filtered RAW version of channel B? Even though I am looking at a nice clean filtered Channel B? Or is there something else I'm missing? I just couldn't get the trigger point to line up with the actual injector pattern. This is the first time I've had this happen. We were in a hurry so I didn't have a lot of time to sit there trying to get a cleaner sync signal. We were running the vehicle without coolant. Tear down of the engine had already begun when we decided to sample with the Firstlook device.

Thanks, MaB
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Re: Issue with Triggering from a filtered channel?

Postby Steve Smith » Wed Oct 22, 2014 3:22 pm

Hello and thank you for the post.

Looking at the nature of how the current waveform "floats" brings me onto grounding of the scope.

We have connected, a First look sensor (no ground) and a current clamp (no ground utilising an internal battery)

Could we try to ground one of the scopes remaining channels to ensure a good ground reference.

The current waveform requires zeroing/zero point to provide the correct value measurement.

The noise present on the current clamp is of a very high frequency and suggests this could be from an external power source. How was the PC powered, and was the scope connected via the Pico blue USB lead?

Moving onto the trigger error, unfortunately the world was against you on this capture given the noise and floating manner of the signal on channel B.

You correct when you mentioned filtering, filtering is applied "post capture" and so the rising edge trigger would still be "pre triggered" by the noise even though you are looking at the cleaned signal.

Her you would have to utilise other trigger conditions at your disposal, but for me the noise reduction would be paramount, and once resolved the rising edge trigger would be sufficient.

i hope this helps, take care.......Steve
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Re: Issue with Triggering from a filtered channel?

Postby hexibot43 » Fri Oct 24, 2014 2:20 am

Looks like my bad again. I didn't ground it. You are right. The truck will be running tomorrow with new cam and lifters. I'll get another chance at it. And now I understand why the trigger wasn't working. Another reason to not turn on filtering. Thanks, Mab.
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Re: Issue with Triggering from a filtered channel?

Postby hexibot43 » Tue Jun 02, 2015 7:39 pm

I keep running into this issue, and have been waiting awhile before continuing the discussion. I just recently had to verify proper control of an alternator that is controlled via demand by the PCM. I've never really wanted to understand this kind of system design, but gotta work with what you get. I've always liked alternators that are self contained. Anyway, back to the real issue. I still find I'm having trouble getting a good trigger. I had good grounds all at the negative battery terminal. I am using three Pico test leads with all three grounds connected together, and then connected to the negative terminal of battery. I kept the cabling as far away from everything as possible.

Here are two images again. First with filtering off ->
Ford_Focus_2003_Petrol_No_Filter.jpg

Secondly with the filter ON ->
Ford_Focus_2003_Petrol_Triggering_filtered.jpg

With the filter on you would not expect to see it triggering on what appears to be nothing. The trigger doesn't coincide with the "filtered" wave pattern.

Is there no way to filter these transients from the trigger? There are times when I would like to be triggered by transients , and then there are times when I'd like to completely disregard them. I'm interesting in timing events and not the noise. Would shielding my cables be the direction I should be going? And if that is the case, please point me to where to get the "right cables". I would love to see an option to not trigger on these transients. I'm looking to speed up my time working on cars with noise. Things like CAM and CRANK readings where yes I want to have the option to see the transients but disregard them in favor of locking a pattern in to verify timing events..

Thanks,

MAB
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Re: Issue with Triggering from a filtered channel?

Postby Joller79 » Sun Nov 15, 2015 9:40 pm

Hi.
Did you solve the problem?? I got the same issue, at times...
I got the software on 2 different laptops, one with a slower amd e1 processor, and it does this ecakt thing, at times. My other tablet, none problem whatsoever. So maybe u got a computing issue??
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Re: Issue with Triggering from a filtered channel?

Postby Alan » Mon Nov 16, 2015 5:37 am

Hi,

As Steve says the filter is applied in software so is done after the hardware trigger in the scope.

If you have a PicoScope 4425 / 4225 then you also have an additional hardware filter that is before the trigger. Its in the channel options drop down menu. Its good for getting rid of noise like this which I suspect is picked up in the vehicle wiring which is not as well screened as our test leads.

Another option is the advanced triggering (almost certain this is on the 4423 as well). A pulse width trigger could be used to trigger on the the signal but not on the short duration spike.

Regards
Alan
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Re: Issue with Triggering from a filtered channel?

Postby Joller79 » Thu Nov 19, 2015 1:57 pm

wll, sorted out the reason on MY laptop..
in preferences, and powercontrol, it was set to 10 frames a second, tried putting it at 60, it helped a bit...
put that one back to 10,, and the tricker was put to repeat, instead of auto, and then the signal triggers perfect everytime..
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