[SI-LIST] Re: 100 ohm VS 85 ohm
- From: Hermann Ruckerbauer <hermann.ruckerbauer@xxxxxxxxxxxxx>
- To: "Ye, Xiaoning" <xiaoning.ye@xxxxxxxxx>
- Date: Sat, 18 Jun 2011 11:27:18 +0200
Hello,
I have asked myself the same question too, especially considering
platforms with board to baord connectors, where you do not know the
impedance of the other board.
My main concern was High volume manufacturing: If my 85Ohm board is at
the -15% edge (around 72 ohm) and my customers board 100 Ohm and +15%
(115 ohm) we will get a jump in impedance > 40 ohm.
Having one via in both bords the worst case routing will be:
- Driver output impedance (calibrated 100 Ohm ? )
- 85Ohm + 15% = 97 ohm
- Via (e. g. optimized to 85 Ohm, not sure what the variation will be)
- 85 Ohm - 15% = 72 Ohm
- Connector (100 Ohm)
- 100 Ohm + 15% = 115 Ohm
- Via (e. g. optimized to 100 Ohm, not sure what the variation will be)
- 100 Ohm + 15% = 85 Ohm
- Terminiation Impedance (calibrated 100 Ohm ?)
I have to admit, that just looking to this numbers I can not really
imagine that this is anything valid for 5Gb or above. Just imagine the
impedance profile this does not look nice ...
So far I have not checked the papers mentioned in this thread and I
hope to find some words there on HVM (High Volume Manufacturing).
Thanks and regards
Hermann
EKH - EyeKnowHow
Hermann Ruckerbauer
www.EyeKnowHow.de
Hermann.Ruckerbauer@xxxxxxxxxxxxx
Veilchenstrasse 1
94554 Moos
Tel.: +49 (0)9938 / 902 083
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schrieb Ye, Xiaoning:
> 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
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