RE: [SI-LIST] : Diff clocks length matching

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From: Larry Miller ([email protected])
Date: Tue Apr 10 2001 - 19:06:10 PDT

2 mils (0.002") is a ridiculous spec unless you are operating at 200 GHz.

0.1" is close enough for 2 GHz at anything like normal propagation
velocities (in the vicinity of 5.6" per ns).

Search on to get tutorials on differential pairs or search on Eric
Bogatin or just search on differential pairs. National Semi has quite a
tutorial in their LVDS literature, You have the right idea, but you are not
looking at the correct parameters. When you find it, show it to whomever
asked for 0.002" phase matching!

Larry Miller

-----Original Message-----
From: AA [mailto:[email protected]]
Sent: Tuesday, April 10, 2001 6:34 PM
To: Todd Westerhoff; Kim Helliwell; Anthony Davidson
Cc: 'Dunbar, Tony'; [email protected]
Subject: [SI-LIST] : Diff clocks length matching

This questions concern routing fast differential
clocks pairs from a clock driver to chipset. The pair
is series terminated and the termination resistor set
near the clock driver. I learned that the pair needs
to be length matched to within 2 mills.
- First questions is which trace do we need to length
match is the one between the chipset and the
termination resistor or is the entire trace length
(between clock drive and chipset) and why?
- Second it was suggested that the spacing between
the pair should be set to 12 mills ( a multiplier of
the distance to the GND plane). This was based on
simulation. How does one come up with the ideal
spacing and what factors impact this? I thought
separating the differential pair to far would be
counter intuitive since it impacts their common mode
noise rejections? But then getting them to close may
present cross talk issues!!

Your input is very much appreciating it.


--- Todd Westerhoff <[email protected]> wrote:
> "So I think the accuracy issue is illusory."
> I like that. Truer words were never typed ;-).
> The argument of IBIS vs. HSPICE is a recurring one.
> While I think there is
> a lot of substance to it, I also think the whole
> issue is incredibly
> over-hyped. We're talking about analog analysis
> after all; error is
> inherent. It *cannot* be avoided, and therefore,
> the real issue is keeping
> the "accuracy" of the analysis in perspective. It
> doesn't do you much good
> to go after the last 1% of accuracy with your
> simulator algorithms when your
> models are only +/- 5% to start with. However,
> understanding how models
> correlate back to reality is difficult, at best.
> The real problem, I suspect, is that "HSPICE is more
> accurate than IBIS"
> makes for a good sound bite, and "you really have to
> understand what you're
> modeling, and how" doesn't. After all, modeling
> isn't fun. Right?
> I think there are lots of places where the "SPICE or
> not to SPICE" arguments
> have merit. But I've also spent enough time
> trudging through models and
> data where the most basic things were wrong, to know
> that until we have a
> firm, common foundation, arguing about details
> doesn't make much sense. And
> guess what - we're not there yet!
> The disguised blessing with IBIS is that by
> standardizing the model format,
> it made it easier for users to find problems with
> the models they use, and
> to correlate those models against test load
> conditions and datasheets.
> HSPICE models, in contrast, are often encrypted and
> have unique interface
> requirements (control pins, voltages and slew
> rates). Bottom line, an IBIS
> model is a lot easier to check and use than a
> corresponding HSPICE model.
> If you're analyzing phenomena that only a SPICE
> model can represent, then
> there's no choice. But I'd use IBIS as the "first
> line of defense" in any
> situation where I could, and only back that analysis
> up with SPICE when
> needed.
> My $0.01 (only worth half of a typical opinion).
> Todd.
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of
> Kim Helliwell
> Sent: Tuesday, April 10, 2001 4:19 PM
> To: Anthony Davidson
> Cc: 'Dunbar, Tony'; '[email protected]'
> Subject: Re: [SI-LIST] : Hspice: Windows vs Unix
> Actually, accuracy isn't really the issue, or at
> least not in the
> sense your management probably means it, Anthony.
> SpecctraQuest uses a spice-like simulator, TLSIM, to
> do the work.
> This simulator is a derivative of SPICE (I don't
> know which flavor),
> and therefore has all the usual accuracy plusses and
> minuses of
> any SPICE, including HSPICE.
> In addition, TLSIM has a coupled transmission line
> model, and the
> diode and transistor models have been removed, and
> the whole simulator
> has been optimized for the problem it's intended to
> solve.
> The question of whether TLSIM's coupled transmission
> line is more
> or less accurate than HSPICE's W-element is one of
> the issues, and
> I cannot quantify it, except that I've never seen
> any reason to
> distrust either one. From that I conclude that they
> are probably
> equally good.
> The real issue you face is the classical conundrum
> of SPICE: that accuracy
> of results depends on accuracy of the models. So
> the issue is: what's
> more accurate: the manufacturer's original BSIM
> models of their buffers,
> or their IBIS models? The answer is obvious, since
> presumably the IBIS
> models derive from the SPICE buffer models (almost
> no one creates IBIS
> models from lab measurements, you see). But IBIS
> models can be very
> close to the buffer models they derive from, and
> it's possible to lose
> very little accuracy in using them. Whereas you
> might not be able to even
> get the buffer model. And then you are forced to
> use HSPICE's IBIS buffer
> model, at which point the accuracy of the two is on
> an even footing, and
> it's *MUCH* harder to use HSPICE in this way than to
> use SpecctraQuest.
> So I think the accuracy issue is illusory. If your
> management has enough
> confidence in you, you have a chance to educate
> him/her/them as to the
> realities of the situation.
> Personally, I've used SpecctraQuest a lot in the
> last 2 years, and HSPICE
> only
> occasionally. I use it for 2 things: as a field
> solver when the problem is
> not easily expressible in terms that SQ understands,
> and perhaps to create
> an IBIS model when the vendor provides an HSPICE
> buffer model but no IBIS
> model.
> A third possibility is when IBIS doesn't accommodate
> a particular type of
> buffer.
> So I think Tony has a good question, and it's also
> not clear to me what
> value-added HSPICE provides in your management's
> view. There is some, but
> perhaps not where they are looking for it.
> Kim
> Anthony Davidson wrote:
> >
> > Perhaps that's where I have seen your name.
> >
> > I am a new user to Hspice, and SpecctraQuest for
> that matter. But the
> > opinions of my team leaders is that the tools that
> are able to do analysis
> > on complete boards and board-to-board
> interconnects are not as accurate as
> > Hspice. And Hspice is more accurate, however, the
> analysis of complex
> (many
> > connections) boards is very difficult.
> >
> > Note that the "less accurate" and "more accurate"
> statements are the
> > opinions of others and not necessarily
> quantifiable.
> >
> > Anthony Davidson
> >
> > -----Original Message-----
> > From: Dunbar, Tony
> [mailto:[email protected]]
> > Sent: Tuesday, April 10, 2001 11:18 AM
> > To: 'Anthony Davidson'
> > Subject: RE: [SI-LIST] : Hspice: Windows vs Unix
> >
> > Hi Anthony,
> >
> > No, neither Nortel, nor Univ of Western Ontario.
> Maybe you've seen my name
> > on the list or something.
> >
> > The reasoning behind my question is that the
> platform on which you're
> > running the other tools might give some pointers
> to the H-SPICE platform
> > choice. Actually, since you're going with
> SPECCTRAQuest, I don't really
> see
> > the need for H-SPICE for the so-called "fewer,
> critical nets". On these
> > nets, what is it you're looking for SPICE to
> provide that SPECCTRAQuest
> > can't? I'm not saying there is never room for
> co-existance,
=== message truncated ===

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