[SI-LIST] Re: Touchstone for transmission lines (was Re: touchstone models)
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: "'Conrad Herse'" <conrad.herse@xxxxxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 31 Oct 2011 10:33:48 -0700
A longer trace can have lower frequency resonances. To see those you need
tighter frequency spacing on your s-parameters.
Tom Dagostino
Teraspeed Labs
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USA
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tom@xxxxxxxxxxxxx
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Teraspeed Consulting Group LLC
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Conrad Herse
Sent: Monday, October 31, 2011 6:54 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Touchstone for transmission lines (was Re: touchstone
models)
Thanks Aubrey, and to everyone else who replied. Apparently the issue was
not with the s-parameter models themselves but rather with the process/tools
I was using to generate them (I've since moved to using scalable
transmission line models for the PCB routes themselves). I find it a bit
counter intuitive that I need to decrease the frequency step interval as the
transmission line length increases, I'll need to ponder that a bit more.
Thanks,
Conrad
Aubrey K Sparkman wrote:
> Conrad,
>
> Just to pick on Scott... :-) Hi, Scott! The "inherent problem with
> using S-parameter models for long transmission line segments" is that
> people forget to use sufficiently small frequency step intervals.
>
> The thing that can get you is file/model size if you remember to
> include fine enough resolution or if you don't remember, you will have
accuracy
> issues. As the transmission line length (or end to end delay) gets
> longer, you need smaller and smaller frequency step intervals to get
> the correct response.
>
> Easy to verify. Just setup a simple transmission line, pick a line
> length
> (ex: 1" long) sweep frequency (1GHz to 10GHz) and frequency step (0.1GHz).
> S11 looks like you might expect, right? Then increase the line
> length to 10"and take another look at S11. Looks weird, right Now
> change the frequency step to 0.01 GHz and take another look at S11. Back
to what you
> might expect? As the length gets longer the file size explodes. A good
> "per unit length" (accurate, passive, causal) model will save time and
> disk space.
>
> Good Luck,
> Aubrey Sparkman
> SparkRight Solutions
>
> On 10/27/11 11:33 AM, "Scott McMorrow" <scott@xxxxxxxxxxxxx> wrote:
>
>> 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
>>
>> TeraspeedR 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-param
>>>> eters
>>>>
>>>> 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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