[SI-LIST] Re: Touchstone for transmission lines (was Re: touchstone models)
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 27 Oct 2011 12:33:55 -0400
Conrad
There is no inherent problem with using S-parameter models for long
transmission line segments. The trick is using a solver that can create
accurate causal models (Ansoft 2D, HFSS, Simberian Simbeor, ... etc),
with resolution that is appropriate for the maximum length of entire
interconnect system. The disadvantage in using them is in the lack of
scalability. However this can be resolved by some solvers, such as
Simbeor, with length scaling at the time of touchstone file generation,
or by creating shorter length touchstone files that can be concatenated,
either in a network simulator, or with additional custom code. I
routinely create sweeps with touchstone interconnect models swept from 1
mm to multiple meters in length, in increments of 1 mm, with no loss of
accuracy. I've published multiple papers that show our modeling match
to measurements with extremely low error from DC to 50 GHz.
Other transmission line models, such as the W-element, can be
attractive, since they are inherently scalable. However,
implementations from tool to tool may be different. If you use them, I
would definitely advise correlation of your solver and your simulator
with physical measurements.
best regards,
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
On 10/27/2011 11:42 AM, Conrad Herse wrote:
> This thread reminded me of a discussion we had awhile back with a CAD
> vendor (who shall remain nameless). They discouraged us from using
> s-parameter models for long segments of traces, stating that s-parameter
> models were better suited for small geometry discontinuities such as
> vias, connectors, and packages. For long PCB traces they recommended use
> of transmission line models rather than an s-parameter model.
>
> We had been using s-parameter models for sections of a channel when
> modeling a full system, the s-parameter model included sections of PCB
> routing along with vias, connectors, etc. Their recommendation was to
> model the trace portions of the channel with transmission line models,
> and use s-parameter models for just the vias, connectors, etc. I can't
> remember the issue which triggered our original discussion with them but
> I believe it was related to flight time delays (or lack there of) when
> including a long transmission line in an s-parameter model.
>
> I'd be interested in hearing other's thoughts on this topic, is it bad
> practice in general to include long transmission line segments within an
> s-parameter model?
>
> Thanks,
>
> Conrad Herse
>
>
> Carrier, Patrick wrote:
>> Hi Megan--
>> Others have forwarded links to some other great resources, but I wanted to
>> let you know that there are a number of videos which discuss S-parameters up
>> at mentor.com, the most relevant being:
>> http://www.mentor.com/products/pcb-system-design/multimedia/s-parameters
>>
>> Many connector models are delivered in S-parameter format.
>> S-parameters are also useful for comparing your total interconnect loss
>> (including traces, vias, and connectors) against a specified loss budget.
>> For AC power analysis, Z-parameters are most often used, which are similar.
>> The HyperLynx Touchstone Viewer will let you convert between S-, Z-, and
>> Y-parameters.
>>
>> Enjoy!
>> --Pat
>>
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