For people still reading the thread, you're not going to regret it
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My scope has a fancy "DPO" capability. So I thought, why not use it ? It may help see more things.
Setup :
- Positive side LISN on Ch1
- Negative side LISN on Ch2
I reproduced the current pulses from my radio transmitter using my active load.
You can see them here, because they generate more noise on the power supply :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180152;image)
Zoomed in :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180154;image)
As you can see, the traces are so noisy that it's very hard to see what's going on.
Yea we see there is some DM noise outside the pulses, but inside the pulses it's just too noisy to see anything.
And then I had an idea...
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Perfect differential mode noise means that if Ch1 goes high, Ch2 should go low. If Ch2 goes low, Ch1 should go high.
So if I set the scope to XY mode, I should see a perfect diagonal line, if the noise was 100% differential.
If the line is not diagonal, it means some noise current does not flow normally, and that's probably common mode noise.
So...
Here is the control trace, with the UUT unpowered :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180162;image)
Here is the UUT powered but idle :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180156;image)
Here is the UUT powered and the active load simulating the pulses from the radio transmitter :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180158;image)
See that weird looking spot at the top-left of the center ?
I was able to make the phenomenon clearer by setting the active load to draw current permanently :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180160;image)
This is clearly not a straight line.
So, unless I'm mistaken, it means I have some non-differential noise.
The "line" is also a bit slanted and offset but it may come from the oscilloscope not being calibrated (cal due 2010) so I'm not drawing any conclusion from this, especially because we're dealing with milliVolts so that'd be sensitive to calibration being off. It could also be the LISNs being slightly different. Green would be the correct line, red is the actual line :
![](https://www.eevblog.com/forum/projects/common-mode-noise-mystery/?action=dlattach;attach=180164;image)