Re: [SI-LIST] : PCI bus impedance

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From: Georg Ramsch (Georg.Ramsch@t-online.de)
Date: Tue May 15 2001 - 01:37:29 PDT


Hello Bhavesh !

For bussed systems it is always to put into question, why using 50 ohm
references.
The advice from your vendor refers to the impact of via- and driver capacitance
on the impedance of bussed lines.
Including the capacitive impact of vias, a 65 ohm transmissionline will yield
50 ohms.

As example:

given a 50-Ohm asymetric stripline FR4 (4.5) with 310/420 um dielectric and
250um width / 35um thickness of copper,
following results will be the outcome:

Target impedance is 50 ohm. Production tolerances of pcb fabrication will lead
to a mean variation of about +/-15%,
so the range of the impedance will be from 43 - 60 ohm.

Calculating the capacitive impact of the vias, impedance will drop to 40 - 45
ohm (ref: 50 ohm line), dependant on the geometry of the vias.
Adding the capacitve effect of the drivers (f.e. 8 pF), impedance will reach 25
ohm. So drivers at the ends of the line will "see" a 25 ohm line;
drivers in between 12.5 ohm.

Question to all others: as driver output resistance is less than 10 ohm, the
line is "overdriven". Any rule, at which impedance limit the transmission line
impedance affects the output waveform of the driver ??

In general: the higher the target impedance and the capacitive load is, the
wider is the drop of the impedance.
From my experinece of designing bussed systems, it is not a big difference
whether you try to achieve a target impedance of 50 ohm
or 40 ohm. Lower impedance targets also decreases the effect of
crosstalk-phenomenons.

Any comments from other side ??

Hope I could help you with my 0.02.

CU

Georg

"Patel, Bhavesh" wrote:

> Hi! Gurus, I was simulating a PCI bus with 50ohm trace impedance because we
> generally try to keep on all our boards 50ohm single ended & 100ohm
> differential(easy for the fab house). Now, the vendor says use 65ohm trace
> impedance and I did simulations with 65ohm but I don't see much difference
> i.e. not significant where I would say'YES' let's go to 65ohm. Also, if I
> have to do 65ohm then it will eating up my routing layers because this bus
> is confined to one section of my board.
> I needed some feedback regarding this and whether it will really make or
> break the PCI bus by going from 50ohm to 65ohm.
> Thanks in advance
> Bhavesh Patel
>
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