# RE: [SI-LIST] : PCI bus impedance

From: Patel, Bhavesh ([email protected])
Date: Tue May 15 2001 - 18:29:37 PDT

Hi! Thanks for all the feedback. From the feedback I can assume that if I do
simulations with 50ohm impedance and the settle delay, propagation delay,
flight time are within my PCI 33Mhz budget, then I can go ahead with 50ohm
target impedance.
Thanks
Bhavesh

-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Tuesday, May 15, 2001 1:37 AM
To: Patel, Bhavesh
Cc: SI_LIST (E-mail)
Subject: Re: [SI-LIST] : PCI bus impedance

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
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 ??

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.
Bhavesh Patel

**** To unsubscribe from si-list or si-list-digest: send e-mail to
[email protected] In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
<http://www.qsl.net/wb6tpu>
****

**** To unsubscribe from si-list or si-list-digest: send e-mail to
[email protected] In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****

This archive was generated by hypermail 2b29 : Thu Jun 21 2001 - 10:11:58 PDT