Impedance of complanar traces on PCB surface?
in [Design]
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From: Andy on 8 Feb 2005 02:48 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
From: Don Pearce on 8 Feb 2005 03:56 On Tue, 08 Feb 2005 08:48:33 +0100, Andy <andy(a)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 This is quite normal as there is usually a ground plane somewhere about - if just the screening box. Make the calculation with the ground plane at an appropriate distance for this, and all should be well. I don't know how tight your coupling is, and the calculations really aren't terribly good much tighter than about 8dB, but you will probably need a couple of iterations or a bit of scalpel scraping to get it right. d Pearce Consulting http://www.pearce.uk.com
From: Chris Carlen on 8 Feb 2005 10:49 Don Pearce wrote: > On Tue, 08 Feb 2005 08:48:33 +0100, Andy <andy(a)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 > > > This is quite normal as there is usually a ground plane somewhere > about - if just the screening box. Make the calculation with the > ground plane at an appropriate distance for this, and all should be > well. But some formulas, being approximate models that fit fairly well over a limited geometrical domain, might deviate too much from meaningful results when taken far from their intended domain of usefulness. Might it be better to find expressions for the transmission line parameters based solely on the adjacent rectangular cross section conductors that the OP is working with, considering no ground plane? > I don't know how tight your coupling is, and the calculations really > aren't terribly good much tighter than about 8dB, but you will > probably need a couple of iterations or a bit of scalpel scraping to > get it right. > > d > > Pearce Consulting > http://www.pearce.uk.com -- _______________________________________________________________________ Christopher R. Carlen Principal Laser/Optical Technologist Sandia National Laboratories CA USA crcarle(a)sandia.gov -- NOTE: Remove "BOGUS" from email address to reply.
From: Mark on 8 Feb 2005 12:06 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
From: Don Pearce on 8 Feb 2005 12:07
On Tue, 08 Feb 2005 07:49:25 -0800, Chris Carlen <crcarle(a)BOGUS.sandia.gov> wrote: >> This is quite normal as there is usually a ground plane somewhere >> about - if just the screening box. Make the calculation with the >> ground plane at an appropriate distance for this, and all should be >> well. > >But some formulas, being approximate models that fit fairly well over a >limited geometrical domain, might deviate too much from meaningful >results when taken far from their intended domain of usefulness. > >Might it be better to find expressions for the transmission line >parameters based solely on the adjacent rectangular cross section >conductors that the OP is working with, considering no ground plane? > As I remember it, a distant ground plane was not one of the limit conditions that gave rise to large errors. Areas of importance are close traces, and finite thicknesses of traces. He could always do a finite element analysis based on Laplace's equation. Most CAD packages offer some sort of field evaluator that works that way. Mostly they are a bit clunky, and you need to calculate odd and even modes separately, but they do give the right answers. d Pearce Consulting http://www.pearce.uk.com |