From: Ray Anderson (Raymond.Anderson@eng.sun.com)
Date: Tue Nov 28 2000 - 09:23:13 PST
Recently there has been an ongoing discussion on si-list
regarding the accuracy of the Hspice W element. I am forwarding
the most recent message posted by Avanti's developer currently in charge
of the W element (the 3rd or 4th guy in the past 3 years) in which
he addresses 4 of the open issues known to exist with the W element.
ECIT SI Group
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Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Subject: RE: [SI-LIST] : Accuracy of HSPICE W-element transmission lines
Date: Mon, 27 Nov 2000 17:58:30 -0800
From: Ted Mido <email@example.com>
X-Dispatcher: imput version 990905(IM130)
I am a developper of hspice and currently taking in charge of W-element.
You may have seen following information on W element feature from
the person who had been taking in charge of W element. But for those who
haven't read seen it I would like to resend this by adding up to date
information. Please take a look at this.
I would like to clarify the four Hspice issues which have been actively discussed:
1) Accuracy of the field solver for computing the skin-effect matrix Rs. 2) Discussion of the inclusion of the imaginary term for Rs: SQRT(f)*Rs [eq1] vs. SQRT(f)*Rs*(1+j) [eq2]. 3) Discrepancy between the transient and ac analysis results for the cases with non-zero Gd values. 4) Discrepancy between the transient analysis results using a single long line versus several short lines.
(1) It is clearly stated in the Hspice manual that the computation of Rs does NOT account for any proximity or edge effects; hence, the resulting value is significantly smaller than the actual value. This is consistent with the computed data by Richard Mellitz. We are currently working on implementing a new solver which accurately models the skin, proximity and edge effects. For 2000.4 release, we have implemented a new field solver which is for skin effect resistance and inductance which have following features.
1. Effects due to magnetic coupling such as skin effect and proximity effect can be modeled. 2. Resistive ground plane is taken into account (but not in C and G).
Side Effect: (*important*)
1. Rs has off diagonal components due to magnetic coupling 2. Lo becomes bigger than previous extraction because of proximity effect especially under the condition of high resistive ground plane.
1. due to the limitation of "RLGC" model input (only real value), imaginary part of Rs is ignored. this may cause error in high frequency R,L value. In fact Rs is computed to fit the formula [eq.2] which models skin effect resistance. (in future release, this will be solved by changing the output to "frequency tabular RLGC model" which is implemented in 2000.4 described below.)
(2) The main issue for this second problem is that while including the imaginary term [eq2] models the skin effect (marginally) better at high frequencies, it may introduce a significant error at low frequencies. Some people, especially those working on high frequency applications, are accustomed to use [eq2] while others prefer [eq1].
I prefer to use [eq2] since it is mathematically more valid solution at the high frequency range where the SQRT(f) dependency is valid, and furthermore, both equations are not accurate at low frequencies anyway. I ran some test cases using both equations and observed no significant differences in ac analysis. However, the accuracy of the transient analysis is slightly degraded when [eq1] is used.
Nonetheless, we will change the implementation so that you can optionally exclude the imaginary term.
(The ultimate solution to this Rs problem, as it has been pointed out by Michael Tsuk, could be to use a better modeling equation or even tabular data.)
(3) The difference in attenuation between transient and ac analyses with non-zero Gd values is a rather subtle problem. The cause of this problem is mainly due to the limitation of the dielectric model equation based on f*Gd as has been pointed out by Dmitri.
(4) The discrepancy between the transient results using a single line versus several cascaded short lines is mainly due to the limitations on the difference approximation routine used in W-element for the frequency-dependent cases. Recently, we have significantly improved the difference approximation routine, and and this discrepancy has been eliminated. (No risetime parameter setting is necessary anymore.)
All of these improvements will be available for the next release of Hspice (version 99.4). We will soon make a beta version available (approximately 1 month); however, the new solver mentioned in (1) won't be implemented in this beta version.
I hope this note clarifies all the issues related to the W-element and field solver in Hspice. I apologize for a somewhat late response.
: Ted Mido : Avant! Corporation (Oregon R&D) : 9205 SW Gemini Drive : Beaverton, Oregon 97008
From: David Kaiser <firstname.lastname@example.org> Subject: RE: [SI-LIST] : Accuracy of HSPICE W-element transmission lines Date: Mon, 27 Nov 2000 17:28:12 -0700
> Nope. This is conjecture based on my expectations and experience. But > when I confronted a support person at Avanti, he addmitted that there > may be limitations above 1GHz. > > David > > -----Original Message----- > From: Larry Miller [mailto:email@example.com] > Sent: Monday, November 27, 2000 11:53 AM > To: David Kaiser; Kai Keskinen; firstname.lastname@example.org > Subject: RE: [SI-LIST] : Accuracy of HSPICE W-element transmission lines > > > > Yes, but have you compared your results (either) with real hardware? > > Inquiring minds want to know.... > > Thanks, > > Larry Miller > > -----Original Message----- > From: David Kaiser [SMTP:email@example.com] > Sent: Monday, November 27, 2000 10:17 AM > To: Keskinen, Kai [KAN:0G15:EXCH]; firstname.lastname@example.org > Subject: RE: [SI-LIST] : Accuracy of HSPICE W-element > transmission lines > > I have compared the W-element with the RLGC matrix from running > APSIM on 100 Ohm diff'l pairs). With a 30ps pulse and 10ps rise times, > the reflected pulse was very symetrical for the W-element, but showed > and RC roll off with APSIM. The APSIM seemed to shown a more realistic > reflection pulse. > > > David Kaiser > McDATA Corp. > 310 Interlocken Pkwy. > Broomfield, CO 80021 > (303) 460-4431 > email@example.com > > > > -----Original Message----- > From: Kai Keskinen [ mailto:firstname.lastname@example.org > <mailto:email@example.com> ] > Sent: Thursday, November 23, 2000 11:19 AM > To: firstname.lastname@example.org > Subject: [SI-LIST] : Accuracy of HSPICE W-element transmission lines > > > > Hello SI-People: > > We have now several times encountered differences in results > from other modelling packages and results from HSPICE w-element models > for applications involving fairly long striplines > 30cm on FR4 and > GETEK with 2.5Gbps signals with ~100ps edge rates. The W-element model > appears to give lower loss than the other techniques. Typical track > widths are 6-8 mils with 100Ohm loosely coupled differential pairs. > > Avant! claims the W-element method is accurate without providing > a paper showing how it was validated. > > How does the rest of the SI community feel about the accuracy of > the W-element model? > > Thanks in advance, > > Kai Keskinen > Equipment and Network Interconnect > Nortel Subsystems and Performance Networks (NSPaN) > (613)-765-3506 (ESN 395) > email@example.com > >
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