Noise Levels between Scope Models 3243 vs 4425

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Noise Levels between Scope Models 3243 vs 4425

Postby FioranoCars » Mon Oct 17, 2016 1:01 pm

While doing some Can-Bus work (yes the No-Gear Bull), we noticed that the 2 scopes we have gave remarkably different results, in terms of noise!

3 Series
Pico3series.png
3 Series full screen


Pico4series.png
4 Series



Up until doing these screen shots, I thought it was just noise, and to that extent the 2 captures (and screen shots) are taken directly using the same leads, same USB2 cable, same laptop, with least wiggle during swap over possible. All 4 channels were earthed for both in the same place (as we have different earth needs for the 2 scopes - so needed to rule that out) ... As near identical as we could manage.

Pico3series-zoom.png
3 Series Zoom


Pico4series-zoom.png
4 Series Zoom


However, sample settings did change between swapping of scope, although only noticed this now doing the screen shots! ...

We have had this noise issue before (well noticed it), both from using Pico Templates and our own templates/existing captures, but also from scratch set up when switching devices during tests.

In this scenario we started with the 3243 and swapped over to the 4425 (manual reconnected and device selection needed as the auto connected can't swap device upon USB unplug - replug ... can this be fixed, even with a warning of device type swap?, so we don't loose the settings? :twisted: )

So, now I do have some possible logic as to the noise levels being differnet (a 20fold higher sampling rate), so the 3series is just missing the extreme peaks, although surely there would be a 1 in 20 chance of catching the peaks at the higher level? :idea:

The 3423 is using 200ns Sample interval, 5MS/s sample rate = sample size of 10,000, while the 4425 then uses (based on same starting template) Sample interval 10ns, Rate 100MS/s = Sample size 200,000) ?!

This all came about as we were concerned at noise levels leading to corrupt messages (as we still have one ECU being problematic in this regard), but based on the Diagnostics all talking to each ECU and otainign valid payload data, without errors or DTC, then the noise was finally rule out (or at least not primary to our issue - although certainly wasted a couple of man days chasing it :evil: :evil: :evil: )

So a few questions:
- Why does the sample rates change between device changes, as we started with the 3 series (the less capable), surely the lower setting could be used on the 4 series, why would they change?
- Why does the 3series never pick up the extreme's if it only about 20times better sampling surely the peaks are still peaks and once in a while we'd see them (and we don't)?
- Is the noise a factor? Should we be concerned?

Ok, I know very few users have a 3series and 4 series, but swapping devices and device side by side comparisions should be a little better.

I still have an alergic reaction tot he usb cable discconection loosing all the data that's unsaved (please fix this, just disable the start/record button, as viewing and setting up should be possible without the scope plugged in)

Keen to hear other observations and learn a little more :oops:

Thanks
Richard
Attachments
USB2 Pico4series.psdata
4 series capture
(4.44 MiB) Downloaded 10 times
USB2 Pico3series.psdata
3 series capture
(592.49 KiB) Downloaded 10 times
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Re: Noise Levels between Scope Models 3243 vs 4425

Postby Alan » Mon Oct 17, 2016 2:27 pm

Hi,

We do have issues at time when faster hardware reveals things that were already there (ie revealing previously hidden problems) but not visible before but having said that the noise levels look rather high with the 4425.

If you are still connected can you only collect one channel on your 3423 - this way the sampling rate on the 3423 should jump to 20MS/s, the 5MS/s is not really enough to see the detail on CAN. This may make the signal look closer to that of the 4425 but bear in mind the 3423 has a lower analog bandwidth as well.

I suspect that if you also enable a 10MHz filter for the matching channel on the 4425 then the waveforms will look pretty similar. So to recap try 1 channel on the 3423 to get the 20MS/s vs collecting on all 4 channels on the 4425 but have a 10MHz filter set on the 4425. If you can post both of those it would be great.

At the moment I think this noise really is there in the vehicles wiring - its a sharp pulse of around 20MHz that repeats every 35us or so - its odd.

As for the USB cable disconnection I agree its annoying but its not as simple to overcome as it first seems - to ensure you get a fast screen update rate when using deep memory the scope hardware effectively creates the image to be shown on screen and transfers that. In other words until you select file / save most of the data is actually only in the scope not on the PC. When you unplug the USB cable to the scope the power and data has gone.

Regards
Alan
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Re: Noise Levels between Scope Models 3243 vs 4425

Postby FioranoCars » Tue Oct 18, 2016 1:17 pm

Thanks Alan

Ok, will try this later.

Noise:
The sharp pulse, it's not mains to the laptop or a trickle charger, as we've already ruled out external power, removed the charger, and left the latop on battery only. Removed all OBD and breakout boxes (using the Pico OBD breakout), so the only non-production car element connected is the probes from the Scope. All Pico leads :-)
The noise continues once the car goes to sleep.

Any thoughts?

USB Cable I'll post a separate thread under software!

Thanks
Richard
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Re: Noise Levels between Scope Models 3243 vs 4425

Postby Alan » Thu Oct 20, 2016 4:43 pm

Hi Richard,

You ask for suggestions, a bit of a long shot but if the noise is still there when the vehicle goes to sleep then we are already in the twilight zone of diagnostics.

How about making sure the noise is still there on the scope screen then disconnecting the main battery. If it goes away then we have something not going properly to sleep.

If its still there then either the signal is still coming from within the vehicle so must be something with its own battery (tracker unit etc?) or its somehow the length of can bus wiring is acting as an antenna picking up a signal from the surrounding area. No idea what the latter might be so try the rest first!

Regards
Alan
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Re: Noise Levels between Scope Models 3243 vs 4425

Postby FioranoCars » Thu Oct 20, 2016 7:27 pm

Thanks Alan
All makes sense, and I'll run tests tomorrow, was a little tied up today!

Thanks
Richard
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