[SI-LIST] Re: Choosing extraction frequency for package RLGC
- From: "Michael Khusid" <mkhusid@xxxxxxxxxx>
- To: <fdunlap@xxxxxxxxxxxxxxxxx>
- Date: Fri, 2 Jan 2004 17:22:00 -0500
Frank,
You are right -- the RLGC data changes with frequency. When an extractor
produces an RLGC matrix, it will do it at a certain frequency.
Hspice manual is also right -- if you assume Resistance to be constant over
the broad range of frequencies, and then pick a resistance value at a very
high frequency, you will greatly overestimate the loss at low frequencies.
So, the answer to your first question is "it depends". In principle, an
approximation of resistance constant with frequency is a poor approximation
if resistance matters. I cannot recommend any rule of thumb for frequency
selection because it primarily depends on the problem you are solving and
whether the constant R(f) assumption is valid.
I'll take another prospective for your second question. If your timing
budget is tight, then you may need to accurately simulate frequency
dependence of the package parasitics. That frees you from the assumption of
a single frequency of extraction. The frequency-dependant models may not be
as simple as RLGC models, but they typically run well in Spice.
Mike
----------------------------------------------------------------
Michael Khusid
Ansoft Corporation
SI/HF Application Engineer
25 Burlington Mall Road, 5th floor
Burlington, MA 01803-4100
Tel 781-229-8900 Ext. 134
Fax 781-229-8624
---------------------http://www.ansoft.com ---------------------
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Frank Dunlap
> Sent: Friday, January 02, 2004 4:08 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Choosing extraction frequency for package RLGC
>
> I apologize if my question has been answered in previous postings. I
> searched the archives, yet did not find an answer.
>
> From my understanding, an "extraction" frequency must be specified for
> tools that generate RLGC from package data. My question is, "how does
> one choose an appropriate frequency?"
>
> Note the excerpt below and take note of the implications of the last
> sentence:
>
> ******************************************
> Pages_5-21_5-22_from hspice_signal_integrity_guide.pdf
>
> Some designers use 0.35/trise to estimate the skin effect frequency.
> This estimate is good for the bandwidth occupied by a transient, but
> not for the clock frequency, at which most of the energy is
> transferred. In fact, a frequency of 0.35/trise is far too high and
> results in excessive loss for almost all applications. Simulation
> computes the skin effect frequency from 1/(15*trise). If you use
> precomputed model parameters (ELEV =3D 2), compute the resistance
> matrix at the skin effect frequency.
>
> ******************************************
>
> I've heard various opinions regarding frequency selection, yet wanted to
> gather more opinions from you SI folks.
>
> One other question: when working with a limited budget while dealing
> with a package that supports multiple interfaces that operate at
> different frequencies, are there recommendations as to how to determine
> the best frequency for RLGC extraction that yields reasonable results
> for all interfaces? Given only one frequency to use for the entire RLGC
> extraction, my choice would be to use an extraction frequency based upon
> the requirements of the highest-frequency interface.
>
> Thanks,
>
> Frank
>
> Some more search terms to aid others in finding this e-mail later:
>
> RLC LCR RLGC=20
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