Too much noise on the 'scope traces

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  • Last Post 4 weeks ago
cd_sharp posted this 4 weeks ago

Hey, guys,

A new video describing a problem that I've been struggling with for a lot of time:

Regarding the problem of amp auto-shutdown, I figured it out. It's because I have the blue trace on the amp input and when I connect another probe to measure the input current I'm making a ground loop. The amp is a good device with a good protection.

I would be glad to read your opinions on how to solve that noise. The probes are compensated well, the scope is calibrated by the book, so I can only imagine it's because of the mains connection.

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

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baerndorfer posted this 4 weeks ago

hi cd_sharp,

vidura gave me the tip to use larger resistors for measuring the voltage drop on shunt when the current is low. for me it works - i have a nice signal now for the current.

regards

cd_sharp posted this 4 weeks ago

Hey, Baern

Thanks for the advice, but I cannot afford using larger resistors. They would introduce too much impedance and change the behavior of the D.U.T. .

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

Chris posted this 4 weeks ago

Hey CD,

From what I can see, youre quite low in the Divisions, about 200mV or so, some may be lower?

Very low voltages are very hard to accurately measure for this reason!

 

Please do not worry to much, the Average Scopes are noisy down about 50mV and Scopes are measured at this standard, called the Noise Floor:

 

In the video, you can see there are things you can do to reduce it, but eliminating it, I would say impossible. 

Also you can turn down your screen illumination some, this will show less Bright Screen Reflection and give you better signal quality on Screen.

Best Wishes My Friend,

   Chris

Jagau posted this 4 weeks ago

Hello CD

I watched your video and sometimes it might be interesting to know more about your scope shot.
Can you try to increase the frequency of your time base of your scope to try to read at wich is the frequency this noise occurs?
I ask you the question because in the search for the small lamp this is the kind of image that I am looking for. Maybe a harmonic of your base frequency, I don't know it's up to you to investigate.


Jagau

cd_sharp posted this 4 weeks ago

Hey, Jagau

I investigated the noise a little. Here is the video;

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

Jagau posted this 4 weeks ago

Ok Cd thanks for the test.
At 27 seconds I saw that your ground probe is very very long
It could be a source of noise and the problem you encountered.
I am attaching you a very informative video on this and it is very important.


Jagau

cd_sharp posted this 4 weeks ago

Hey, Jagau

The ground probe is standard. I understand it introduces parasitic inductance. I'll see what I can do about it.

A question: is it possible that if I power my 'scope through an isolation transformer I could partially get rid of that noise?

What I have in mind is this isolation transformer.

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

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raivope posted this 4 weeks ago

Hi,

You can build the best current shunt (0.1R) - very clean signal:
https://www.aboveunity.com/thread/raivope-s-poc-replication-attempt/?order=all#comment-379aa42b-37aa-4387-8957-ac7f015d917f

Comparing current shunt versus clamp (noisy):
https://www.aboveunity.com/thread/measurements/?order=all#comment-26cc6bc5-ff04-4fa3-bb19-ac7f01596f0e

You can build one under 1h, if you buy all the right components.

Best,

Raivo

 

cd_sharp posted this 4 weeks ago

Hey, raivope

It's hard to tell in the scope shots you took, what's the volts/div value?

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

Chris posted this 4 weeks ago

Hey CD,

It depends where the Noise is comming from. Whats causing the noise.

Because VDrop = IR, we may be able to either:

  1. Turn your Input up more to Increase the Currents being measured
  2. Change the CSR for a better Resistance Value, but change the Circuit behaviour.

 

At 0.1 Ohms we see around 200mV, but if we change the Currrent Sense Resistor the Circuit will exhibit different behaviour.

I would say, best thing to try first is turn your Input up a bit more, so you measure greater Voltages, remember 50mV the Noise Floor and youre close to it, and then see if the noise is reduced.

Best Wishes,

   Chris

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cd_sharp posted this 4 weeks ago

Hey, guys

Chris, yes, I know it, it does not bother me to increase the input and I did that while experimenting. But I need to make current measurements to see where the peak values are. That noise is fooling the 'scope.

I think this is proof enough the 'scope is the source of the noise:

I need to isolate it from the mains line.

Any other opinions, please let me know!

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

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Chris posted this 4 weeks ago

Hey CD,

EMI Noise in your environment may be something to consider, a video here may help:

 

Environmental Noise is very common, I hope this helps some?

Best Wishes,

   Chris

cd_sharp posted this 4 weeks ago

Hey, buddy

Yes, I went over everything. I reduced the noise to 120mV PP.

I post here all the issues I found:

  1. Limit every channel's bandwidth to 20M.
  2. Don't set every channel's probe to 10x when you need to measure the current. Do the measurement over the 0.1 ohms resistor and then compute separately.
  3. Check the compensation of the probes on 10x attenuation.

Thanks guys!

Stay strong!

"It's just the knowledge of the coils and how they interact with each other" (Steven Mark)

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