Waveform differences with attenuators

Chad Technician Winter park, Florida Posted   Latest  
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1998 BMW 328i 2.8L (M52) 5-spd (S5D310Z) — WBACD4321WAV60521

Hello everyone hope you had a great weekend​.​Im doing a little practicing with the modis. I have 10:1 attenuators now. So I start using it. Well I noticed that with injectors and ignition coils the waveform changes. Well mainly the dwell time but overall the waveform will have slight differences. Can anyone help me interpret what is going on? These pictures are of injectors but the dwell did the same on the ignition coil. The dwell is flat without the attenuators and curves downward with it. Thanks much 

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Caleb Diagnostician
Mishawaka, Indiana
Caleb Default
   

So i have used my Modis Ultra for a long time and never used attenuators for injector voltage or amperage. Not sure why you would need to honestly. The Modis can more than handle the voltage spike from the normal solenoid port fuel injector you are dealing with here. I do not have a scientific term for why the voltage trace looks like that with attenuator. However I can tell you the correct…

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Olle Instructor
Albert Lea, Minnesota
Olle Default
 

Looks like your coupling is set to AC. That will insert a capacitor in the signal chain and the attenuator significantly reduces the scope impedance.

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Caleb Diagnostician
Mishawaka, Indiana
Caleb Default
 

The coupling AC is off in both of his photos Olle. When you have the AC coupling feature on there is a little sine wave in place of the dashes.

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Dan Educator
Bellingham, Washington
Dan Default
 

His scope is not AC coupled in either of these photos. It would be indicated on the bottom of the screen.

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Chad Mobile Technician
Winter park, Florida
Chad Default
 

olle I have the current wave form inverted because the clamp was backwards. Thats the symbol that you see

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Olle Instructor
Albert Lea, Minnesota
Olle Default
 

Been a while since I used that type of scope. I was clearly mistaken in interpreting the "Coupling AC" verbiage as saying "AC coupled". Did you use one of those blue Fluke attenuators which are also a low pass filter? Your signal certainly look AC coupled.

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Chad Mobile Technician
Winter park, Florida
Chad Default
 

Hey Caleb thanks for the reply. I know it can handle 400v and I know it does Kv for ignition probes. I just don’t like the snap on flag pick ups and that’s the only ones I have at the time. So I have just been back probing and using the attenuators as insurance. This was something that I noticed and yes I agree all the literature I find shows without the attenuator being the correct waveform

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Dan Educator
Bellingham, Washington
Dan Default
 

How do you have the attenuator hooked up? It is clearly working as a capacitance to your signal. The Modis uses banana connections but are you using a BNC style attenuator with an adapter? Is your attenuator grounded properly? I would take the amp out of the picture and out of Ch1 and put the attenuator by itself in Ch1 since the Modis doesn't use individual grounds for each channel. I suspect…

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Chad Mobile Technician
Winter park, Florida
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Yes I am using a bnc attenuator with an adapter for banana jacks. The ground is good and did have it hooked to an ignition coil without the amp clamp. Same dwell waveform.

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Dan Educator
Bellingham, Washington
Dan Default
 

I grabbed my Verus to try this out because I was curious. First I grabbed a 20:1 Pico attenuator I had lying around. I had to Frankenstein it to convert BNC to banana and change genders but it seems to work as expected. Then I grabbed an OLD Fluke 10:1 I had lying around and it did not work as expected. It gave a similar signal to yours. Not sure if the Fluke one I have is more than a simple…

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
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Hi Dan: Your "attenuator" is also a low pass filter. It removes signals above 4khz. Take a look at page 145. Do the patterns look familiar? (I understand that the section is for smoothing. The illustration is meant to show what filtering can do. assets​.​fluke​.​com/manuals/98…​.​pdf If Chad is using the same attenuator, that would explain a lot. I used them primarily when looking at O2…

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Chad Mobile Technician
Winter park, Florida
Chad Default
 

Yes it seems You did have a similar result. Your fluke Hardware specs are much different than mine. I am using a pico. Which is most likely the issue I’m thinking. I got this set up from aeswave​.​com 

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default
 

Ahem, for TA010, have 14V got translated into 1.2V? That’s not what is expected of 20:1 attenuators! (Though at least the shape is OK...)

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Stephen Technician
Gallatin, Tennessee
Stephen Default
 

I’m guessin that the TAO10 is a 20db attenuator vs 20:1. But why did the top of that waveform’s flyback section round over? I also wonder why Chad’s waveform, with attenuator, is rounded off at the pull down and at the relaxation but the flyback spike is sharp at both ends and about the correct voltage? Why aren’t all parts of the wave treated the same?

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default
   

TA010 is 20:1, and works as such with older automotive Picoscopes: interworldna​.​com/pico/automotiv…​.​php Simply, when the attenuator does not match the scope input characteristics, all bets are off.

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Chad Mobile Technician
Winter park, Florida
Chad Default
 

Hey Stephen. check out Justin’s post. A little below this one. He works at aeswave which is where I purchased my attenuator. Smart guy. It’s wrong hardware. That’s why the waveform is so wacky, voltages changed not in relation of 10:1 and the waveform rounds instead of being flat.

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Justin Technical Support Specialist
Fresno, California
Justin Default
 

Thanks for the kind words Chad, but I can assure you I have learned much from Jorge, and as the rest of the community here at DN I have much to continue to learn.

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default
 

“These are not the attenuators you are looking for”. A proper attenuator should be matched to the scope’s input characteristics (resistance and capacitance). Otherwise you see what you see...

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Chad Mobile Technician
Winter park, Florida
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I believe that’s it Dmitriy. With Dans last post I learned that there are different attenuators (duh) so what’s going on is a hardware issue. It’s just not compatible.

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Justin Technical Support Specialist
Fresno, California
Justin Default
   

When dealing with specialized scopes (Snap On, Pico auto, Fluke) you need a probe specifically designed for these types of scopes. Example: a Fluke 10:1 attenuator doesn't display correctly on a Pico scope. When you use an incompatible attenuator or probe you will get unintended waveform smoothing, waveform distortion, or spiking on sharp edges. This is somewhat common when swapping attenuators…

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Chad Mobile Technician
Winter park, Florida
Chad Default
 

Thank you Justin. I keep hearing To make sure to use the attenuators to protect against voltage spikes. This is just part of the learning curve

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Justin Technical Support Specialist
Fresno, California
Justin Default
   

In most cases using the Snap On you are safe. They were designed for exactly what you are using it for. I am glad to see you are on a quest to learn thru experimenting. That is one of the most powerful traits of the top techs in the trade. Now, if for some reason you really want or need a properly tuned attenuator for the snap on, contact Jorge at …, as I am sure he can build one tuned…

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Robert Diagnostician
Fair Oaks, California
Robert Default
 

attenuators reduce the voltage signal by 10 to 1 or 20 to 1. You had about 85 volt spike in first one and 8.5 volt spike in second one. Dropped the voltage by 10 to 1. if you multiply the voltage by 10 you have the same reading. On the pico scope you can set the scope to adjust for the attenuator and the readings look normal on the screen.

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