Advice regarding scope wave form

Hi ,

I have just got hold of a 2 channel pico scope and was doing some tests to see how the scope is.
I have attached a screen shot of the waveform ,it's a primary ignition current (waste spark coil ) can anyone tell me why the middle of the ramp curves up then down ? car runs fine ..

Thanks


Also have done a camshaft sensor test ...why does the square wave have the curves at the end .are these a ground issue ?

Anyone have any suggestions about the wave forms ..just trying to make sure the scope is fine and not a problem.

Hi, did you have a battery charger connected at the time of taking these captures or your laptop plugged into the mains?
It could also be an issue with your USB connection / cable.
What does the waveform look like if you measure battery voltage at the battery?
HTH
Doc...

Hi thanks for reply.No mains connected only laptop battery i used and no battery charger.I did an injector voltage and current wave form and they were fine.Just the cam and coil waveform don't look good to me.Will try a different usb cable and see if that makes a difference (will any usb cable be fine to use ? ) need to find out the issue as if it is the scope it will be going back.

There are faults that can cause this on the vehicle, I assume the vehicle has no issues and you were just testing?
Routing of your test leads across high current cables can cause issues as can a bad scope GND
Sample rate can also do this - 1MS sample rate is more than good enough for a 500mS capture

Hi i was just testing the scope out on my car.Car running fine .I also tested injector voltage and current wave form and they looked ok .So not sure with the cam and coil wave forms .have included injector and coil wave forms.

For some reason cannot post the original pico file so had to convert them to show here.

What kind of car is it? Did you try to compare your captures to known good waveforms on a similar vehicle on IATN? I think PICO has a data base for automotive users as well.

Doc wrote: Hi, did you have a battery charger connected at the time of taking these captures or your laptop plugged into the mains?
It could also be an issue with your USB connection / cable.
What does the waveform look like if you measure battery voltage at the battery?
HTH
Doc...

Hi Doc ,

Think you might have hit the nail on the head regarding usb lead..i used one from my computer and retested on coil current ramp and cam sensor and the wave forms look good.see below (cam sensor is with filter applied and removed) ..need to do some more tests on secondary wave forms to see how it performs.








@ Noah , I was mainly trying to find out if the scope i just got was working correctly .The car is a vauxhall astra 1.6 z16se 2003.I know what the correct waveforms should look like but was baffled by the first images i uploaded as i knew they should not look like that.Thanks.

I replaced the USB & BNC connectors in my scope some time ago.
Constant connect / disconnect takes its toll on the leads and connectors.
I try to leave all my leads on my tools connected these days as they get so expensive to replace.
When I was mobile it was too risky as it's all too easy to snap them off but now I'm static thats what I prefer
Glad you got it sorted
Cheers

Yeah thanks Doc .Will be trying some further testing so will see how things go.

Thats cool, I wouldn't have thought that a connection issue would have put that characteristic "hump" in the middle of that current ramp.

Looks to me as if you are picking up some cross interference from some other timed electrical event on the engine it could be a cable shielding issue but try just routing the cable to the scope away from other cables.

For BNC cables old style computer ethernet cables or CCTV cables work fine, you can also buy couplers (gender changers) off ebay very cheaply. Another cheap alternative is to use converters from BNC to standard RCA plugs used on home audio equipment, then you can use normal audio cables.

Thanks Andy,

Done some more testing today .Seems that some interference was caused by the cables being to close to other components as well as the usb cable being intermittent as well .Tried the old cable and wiggled it and it was causing issues with some signals but as soon as i swapped back to the other usb lead the signals were fine.Just glad it was not the scope Just need to be able to capture some good secondary images but not having much luck using the HT pick up probe at the moment.Can get primary voltage no problem at the coil just not with HT probe as the image is all over the place even with correct trigger.

Primary should give you most of the info you need mate
I rarely use secondary apart from a quick cylinder sync

Good video on scoping ignition primary and analysing secondary issues

Thanks Doc, Will refer back to the video .I am also a premium member so have loads to go back through.Will mainly stick with primary voltage at coil than secondary as the HT probe i have seems very hit and miss with getting a waveform.

Bulldog clip style HT capacitive probes were fine with old style ignitions, I still use one on some models I work on which have a shared plug top coil with wasted spark. Getting secondary from plug top coils is hit or miss and you have to know what a good waveform looks like for that particular system. Back probing the primary is very useful and relatively easy but won't work on 3 and 4 wire coils.
As I normally work on straight fours that are easy to pull the coil packs on stress testing the coil against a 25mm/1" spark gap with current clamp on the 12v power feed is a frequent plan of attack.