[SI-LIST] Re: 100 ohm VS 85 ohm
- From: "Ye, Chunfei" <chunfei.ye@xxxxxxxxx>
- To: "Chris Padilla (cpad)" <cpad@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Jun 2011 11:29:58 -0700
Hi Chris,
Some of my colleagues at Intel and I are consistently driving for a 85ohm
interconnect for the past few years. Since I am responsible for server
high-speed IO signal integrity supporting USB/SATA/SAS and chipset package
design, I would like to share my experience and observations:
* 85Ohm seems to be with more challenges for SATA3 (6Gbps) and SAS2 (6Gbps)
because of the cables and connectors are 100Ohm in specification. Even in such
case, 85Ohm for package and board still has advantage.
* Designing connector and cable at 85Ohm will be very beneficial to further
improve SI performance.
* In the above observation, TX and RX termination is 100Ohm differential.
Under high volume manufacturing assumption
You may reference to my very recent publications for more detail if you have
interest:
"Improve storage IO performance by using 85Ohm package and motherboard
routing," Chunfei Ye Xiaoning Ye, Thanh Do-Nguyen, pp281 - 284, IEEE EPEPS
2010.
"Full Link Impedance Optimization for Serial IOs," Chunfei Ye, Xiaoning Ye,
Edgar J Vargas, Odilon Argueta, To be presented on IEEE EMC 2011
Both of the above papers talks about 85Ohm versus 100Ohm from cost and SI
performance perspectives, backed by simulation and measurement data.
Best regards
Chunfei Ye
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Chris Padilla (cpad)
Sent: Friday, June 17, 2011 10:08 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 100 ohm VS 85 ohm
Folks,
I'm wondering if some of your higher speed designs are considering
moving to a < 100 ohm differential Zo?
We know that a 50 ohm via is difficult to make and the connector vendors
have equal trouble trying to reach 100 ohm differential on their high
speed connectors. Going to < 100 should make it easier to have lower
crosstalk and matched impedance to improve return loss, possibly better
signal to noise ratio, and wider traces could yield slightly lower loss
(depends on how you adjust the PCB geometries to reach 85 ohm, of
course).
A negative is the 50 ohm test equipment environment. One will have 42.5
ohm on their board. Can this be easily dealt with? Of course, most
chips are design with 100 ohm in mind so finding chips designed at
something else could be difficult.
I just wonder if the headache of moving off-standard is worth it or not.
I'm curious what the experience of folks here have witnessed.
Thanks,
Chris Padilla
Cisco Systems
San Jose, CA
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