[SI-LIST] Re: 65 Ohm compact PCI impedance
- From: wjcsongr@xxxxxxxxxxxxxxxxxxx
- To: "Powell, Jon N" <jon.n.powell@xxxxxxxxx>
- Date: Wed, 7 Jan 2009 13:27:04 -0600
Brian,
From my recollection, the 65 (or is it 66?) Ohm impedance is so PCI can
use wimpy drivers.
SI List, is that true?
Do they also specify 10 ohm resistors in series with most of the PCI
signals at the connector boundary?
Regards,
Bill
William Csongradi
Senior Electrical Engineer
Rockwell Collins Heads Down Display Center
319-295-7884
Mailing Address
Rockwell Collins
400 Collins Road NE
MS 105-167
Cedar Rapids, Iowa 52498-0001
"Powell, Jon N" <jon.n.powell@xxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
01/07/2009 01:13 PM
To
"Sexton, Brian M. (US SSA)" <brian.m.sexton@xxxxxxxxxxxxxx>,
"si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
cc
Subject
[SI-LIST] Re: 65 Ohm compact PCI impedance
I would be concerned about the measurement being Nominal instead of Min.
Jon
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Sexton, Brian M. (US SSA)
Sent: Wednesday, January 07, 2009 11:08 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 65 Ohm compact PCI impedance
I'm designing a 5-slot, 66MHz cPCI backplane. We recently sent our
design out for fab, and the board house came back saying they were
measuring our compact PCI lines to be in the 57 ohm range. We have
.020" wide guard traces connecting the 'Z' and 'F' ground rows of
adjacent cPCI connectors. They claim this is the reason for the lower
impedance. The cPCI traces are about .005" wide and run about .026 from
the guard traces between connectors. They also run about .024" from
vias for the cPCI pins.
I'm curious if the guard traces have that much of an effect on the
impedance. I would think the via fields would play a significant role
in reducing the characteristic impedance as well. I'm wondering if the
cPCI standard assumes you take all these external factors into account,
or if they assume you shoot for 65 ohms but get slightly lower in
reality. I can't reduce the trace width since it's already at .0045 -
.005". Does anyone have any experience with this?
Also does anyone have experience running cPCI at an impedance that is
slightly below the 10% tolerance? 57 ohms is almost with in the 10%
window of [58.5 - 71.5].
Thanks for your help.
Brian
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