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

[mulfycrowh]

Hi everyone,

I am looking for articles about this topic.
Thanks for help.

[qdrives]

Something like this: https://www.signalintegrityjournal.c...o-return-plane

[mulfycrowh]

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]

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]

And the result with a clearance of 1mm is as shown on the screenshot.

[qdrives]

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]

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]

So, for you, the best option is to remove all gnd polygon pours on all signal layers?

NO. Definitely not.

https://icd.com.au/CPW.html

https://www.broadcom.com/info/wireless/appcad
https://www.cadence.com/en_US/home/tools/system-analysis/rf-microwave-design/awr-tx-line.html

https://www.polarinstruments.com/support/cits/AP8142.html

https://electronics.stackexchange.com/questions/117214/impedance-of-an-edge-coupled-coplanar-waveguide-with-ground

https://github.com/tomek-szczesny/edge_coupled_cpwg

[mulfycrowh]

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".