Impedance of complanar traces on PCB surface?

A

Andy

Guest
Hello:

I want to lay out a couple of traces on the FR4 PCB, to provide a
transmission line of specific impedance for a differential-mode signal.

The impedance calculation formulas that I have found so far, for the
common-mode and differential impedance, concern the traces with the
ground plane underneath. I think I don't want the ground plane
underneath, to minimise the common-mode capacitance, and I am looking
for the formula for the coplanar traces on one side of the PCB, with no
metal (copper) in the other PCB layers in the vicinity of the trace.

Can somebody help?

Thank you.

-- Andy
 
You need to think about the common mode impeadance and the differential
impedance. Unless the ground plane is very far away, both will impact
your result.
Consider a thin twisted pair inside a sewer pipe. In most practical
PWB stackups, the common mode impedances dominate because you cannot
get the two differential conductors close enough to each other relative
to their distance from the ground planes.

Mark
 
On Tue, 08 Feb 2005 08:48:33 +0100, Andy <andy@nodomain.nod> wrote:

Hello:

I want to lay out a couple of traces on the FR4 PCB, to provide a
transmission line of specific impedance for a differential-mode signal.

The impedance calculation formulas that I have found so far, for the
common-mode and differential impedance, concern the traces with the
ground plane underneath. I think I don't want the ground plane
underneath, to minimise the common-mode capacitance, and I am looking
for the formula for the coplanar traces on one side of the PCB, with no
metal (copper) in the other PCB layers in the vicinity of the trace.

Can somebody help?

Thank you.

-- Andy
Do you have a specific reason to minimize the even-mode capacitance?
In the case of a typical geometry, deleting the bottom ground plane
won't make a lot of difference, unless the board is thin.

To reduce the even-mode capacitance and keep your target differential
impedance, you could just make the traces skinnier and closer
together.

John
 
John Larkin wrote:
...


Cranking up Txline, it recommends 0.32 mm trace widths to get 124 diff
z (62 odd mode impedance), with an opposite ground plane and your
other dims. Even mode impedance is 203 ohms. I doubt that eliminating
the ground plane would change these numbers a lot.

John
I have had a problem with my computer (motherboard failure), back
on-line only recently.

In fact, I cannot approach the required impedance with the ground plane
on the opposite PCB side removed.

A nice program, that Txline.

Thank you!

-- Andy
 

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