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About the influence of a ground plane on the differential pair impedance

mulfycrowh , 07-17-2023, 12:49 PM
Hi everyone,

I am looking for articles about this topic.
Thanks for help.
qdrives , 07-17-2023, 05:18 PM
mulfycrowh , 07-18-2023, 03:59 PM
@qdrivesI don’t think that it is what I was waiting for.
I didn’t explain very well.
I meant the gnd plane being on the same layer than the differential pair. Please refer to screenshot.
mulfycrowh , 07-19-2023, 07:05 AM
I didn't find any article about the topic.
Now the idea could be to set up a new rule for polygon clearance around differential pairs in order to enlarge it, something between 1 and 2mm instead 0.2mm.
What do you think?
mulfycrowh , 07-19-2023, 10:47 AM
And the result with a clearance of 1mm is as shown on the screenshot.
qdrives , 07-19-2023, 02:53 PM
Seeing I got the first part wrong, let me ask what it is you are thinking of.
1) Do you have a Gnd return (plane/polygon) below/above the tracks?
2) I do not think you would design this on a single layer board, what is the distance from this signal layer to the next?
​3) Are you asking because of the other discussion on co-planar waveguides? The one where I mentioned that the 'Gnd' on the same layer still had an influence > 2mm according to most calculators.
4) Are you thinking here as to have the two signals couple into each other and not a return plane? Thus making them more true differential pairs.

If it is point 3, then I found it very interesting and frightening at the same time. However, I think most calculators are wrong as otherwise you could never calculate impedance correctly in a 'normal' situation.
mulfycrowh , 07-19-2023, 03:20 PM
@qdrives 1/ Yes, we have
2/ This is a 16 layers stackup
3/ No

So, for you, the best option is to remove all gnd polygon pours on all signal layers?
Is there any calculator for differential pairs guarded by gnd polygon pour?

qdrives , 07-19-2023, 04:22 PM
So, for you, the best option is to remove all gnd polygon pours on all signal layers?
NO. Definitely not.

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How can I compute the differential impedance of an Edge-Coupled Coplanar Waveguide With Ground?I couldn't find any free calculator online, so I wrote a small program which computes the the impeda...

mulfycrowh , 07-20-2023, 12:18 AM
@qdrivesThank you so much for all this information.
I checked everything with different calculators, there is a slight difference of impedance between with or without coplanar waveguide for a same width and gap.
The tolerance about the impedance is generally 10% and 20% for LAN.
The calculation shows we are always within this range if we consider "differential" in AD instead "differential coplanar".
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