[SI-LIST] Re: 100 ohm VS 85 ohm
- From: "Ye, Xiaoning" <xiaoning.ye@xxxxxxxxx>
- To: "Chris Padilla (cpad)" <cpad@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 17 Jun 2011 15:46:32 -0700
Hi Chris,
Intel server platforms started 85ohm design for high speed serial link since
PCIe first generation work started about 10 years ago, so we have been enjoying
the benefits you mentioned for a long time, plus some other benefits such as
easy manufacturing and stackup planning, etc. :)
As to your concern about 50 ohm test environment, I think it is not necessary.
Here I am assuming VNA is the equipment used to characterize the channel:
* Let's say you measure the DUT with 50ohm environment, and get S parameter
matrix of [S1]. Then you measure the same DUT with 42.5ohm environment, and
get [S2]. If you compare values of [S1] to [S2], they are going to be quite
different. However, both of them are representing the exact same interconnect.
The key is the reference impedance. Giving out S parameters without mentioning
ref impedance is asking for trouble.
* Meanwhile, if you want to run simulation with your DUT, you can plug [S1]
into any (good) simulator, and tell the simulator that your [S1] is based on
50ohm ref impedance. Alternatively, you can plug in [S2], but you must tell the
simulator that your S parameter is based on 42.5ohm ref impedance. Both
approaches should give you the exact same answer if the simulator is good.
* One analogy to this is to measure temperature: it does not matter whether
the temperature is 32F or 0C, it is the same. However, if I tell someone the
temperature is 32, without mentioning the unit, people may get lost.
* BTW, I have seen some tools ignore the reference impedance associated with
the S parameter, and treat all S parameters as if they are based on 50ohm. If I
plug-in the simulator with [S2] (based on 42.5ohm ref impedance), and the
simulator treats it as with ref impedance of 50ohm, then I will definitely get
wrong results! To avoid the trap (by some bad simulator), my advice is: always
ship someone else S parameters with 50 ohm reference impedance.
Just my 2 cents.
Xiaoning 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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
