RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES

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From: Ken Cantrell (Ken.Cantrell@srccomp.com)
Date: Thu May 24 2001 - 08:19:44 PDT


Dave,
Absolutely right, or at least that's my understanding. Nice condensation of
the material. Then, next, we get into Mike's concerns of package effects,
and whether you need to account for die to die (or will pin to pin be OK),
stubs, input protection diodes, ESR, ESL, mode effects, or my favorite, what
to do when package effects are important and there's no ebd file, etc. It's
kind of an endless story, isn't it.
Ken
-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of David Instone
Sent: Thursday, May 24, 2001 1:56 AM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES

Well if Chandan isn't confused I am!
A simple experiment with a TDR shows that if the line is long enough for
a pulse edge to have left the driver and not yet reached the capacitor
the line looks like a TL of Z0 (ignoring the cap) and the capacitor acts
as a capacitive load. In this case the series resistor+Rdriver should
match Z0. If the line is short enough or the rise time long enough that
the pulse has not completely left the driver before it hits the
capacitor then it should probably be considered as a distributed
capacitance when calculating the series resistor. The problem comes
when the range of risetimes possible spans both conditions!
Or am I being over simplistic?
Now on the same theme, I believe that if lumped caps are distributed
along a TL and the distance between them is greater than a rise time
then the TL should be treated as a TL of unloaded Z0 with capacitive
discontinuities and the terminators should match unloaded Z0 (unless
they are closer than a risetime to a cap), only if the caps are closer
together than a risetime should Z0 be considered to have been lowered
and the terminators matched to loaded Z0 (unless they are further from
the last/first cap than a risetime).
Again am I right or over simplistic/just plain wrong.

Dave

Degerstrom, Michael J. wrote:
>
> Ken,
>
> No problem - I was trying to do the same thing for Chandan
> as my feeling was that he was trying to apply this
> Zoprime formula when it wasn't apparent that he understood
> its utility. I'm sure many of us have difficulty interpreting
> the original posts to this bulletin board. Hopefully one
> or both of our posts were beneficial.
>
> Mike
>
> _______________________________________________________________
> Mike Degerstrom Email: degerstrom.michael@mayo.edu
> Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
> Phone: (507) 538-5462 FAX: (507) 284-9171
> WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
> _______________________________________________________________
>
> > -----Original Message-----
> > From: Ken Cantrell [mailto:Ken.Cantrell@srccomp.com]
> > Sent: Wednesday, May 23, 2001 12:02 PM
> > To: Degerstrom, Michael J.
> > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES
> >
> >

> > Mike,
> > What I was getting at, if you re-examine at Chandan's
> > message, is that he
> > doesn't understand the basic material yet. Note his formula
> > for Zoprime. I
> > thought your answer was more advanced than the question that
> > he asked, and I
> > didn't want him to miss the fundamentals.
> > Ken
> >
> > -----Original Message-----
> > From: owner-si-list@silab.eng.sun.com
> > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
> > Degerstrom, Michael
> > J.
> > Sent: Wednesday, May 23, 2001 9:37 AM
> > To: Ken.Cantrell@srccomp.com; si-list@silab.eng.sun.com
> > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES
> >
> >
> > Ken,
> >
> > I'm not sure why you would recommend to use an approach that
> > takes more effort to implement and then provides less accurate
> > SI results. You can use this technique where lumped load
> > capacitance is included into the transmission line capacitance
> > only for certain net topologies. But using this approach will
> > not allow you to see the capacitive reflections and any stubbing
> > effects from package leads. Also, you may not be predicting
> > the delay from your source to your load accurately.
> >
> > Mike
> >
> > _______________________________________________________________
> > Mike Degerstrom Email: degerstrom.michael@mayo.edu
> > Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
> > Phone: (507) 538-5462 FAX: (507) 284-9171
> > WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
> > _______________________________________________________________
> >
> >
> > > -----Original Message-----
> > > From: Ken Cantrell [mailto:Ken.Cantrell@srccomp.com]
> > > Sent: Wednesday, May 23, 2001 9:55 AM
> > > To: Degerstrom, Michael J.; chandan_career@yahoo.com;
> > > si-list@silab.eng.sun.com
> > > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES
> > >
> > >
> > > Chandan,
> > > The receiver capacitance is modeled as part of the toal distributed
> > > capacitance where the new capacitance (Cprime) is equal to the [line
> > > capacitance plus (the load capacitance * the number of loads
> > > divided by the
> > > length of the transmission line)]. Zo prime is then equal to sqrt
> > > (L/Cprime). It's effect will be to lower Zo. I refer you to Howard
> > > Johnson's book, page 173, section 4.4.3.1. The only time I
> > treat the
> > > receiver capacitance as lumped is in a bi-directional mode where the
> > > receiver capacitance introduces a group delay equal to 2*Zo*Cl.
> > > Ken
> > >
> > > -----Original Message-----
> > > From: owner-si-list@silab.eng.sun.com
> > > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
> > > Degerstrom, Michael
> > > J.
> > > Sent: Wednesday, May 23, 2001 7:39 AM
> > > To: chandan_career@yahoo.com; si-list@silab.eng.sun.com
> > > Subject: RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES
> > >
> > >
> > > Model the transmission line as a transmission line and the
> > > load should be modeled as a lumped capacitance. You
> > > may also want to model the package inductance and capacitance
> > > depending on your application. Some of the load capacitance
> > > may have series resistance due to ESD filter circuitry but
> > > I rarely see vendors supplying this information unless it
> > > it is factored into the IBIS model package resistance.
> > >
> > > Mike
> > >
> > > _______________________________________________________________
> > > Mike Degerstrom Email: degerstrom.michael@mayo.edu
> > > Mayo Clinic; 200 1st Street SW ; Rochester, MN 55905
> > > Phone: (507) 538-5462 FAX: (507) 284-9171
> > > WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html
> > > _______________________________________________________________
> > >
> > >
> > > > -----Original Message-----
> > > > From: Chandan [mailto:chandan_career@yahoo.com]
> > > > Sent: Wednesday, May 23, 2001 4:00 AM
> > > > To: SI FORUM
> > > > Subject: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES
> > > >
> > > >
> > > > Hi,
> > > >
> > > > Consider a long trace - long enough for it to be
> > > > considered as a transmission line. Let this trace be
> > > > connected between a CMOS driver & a CMOS receiver. The
> > > > receiver can therefore be modeled as a capacitive
> > > > load at the end of the line.
> > > >
> > > > 1. Should this capacitor be considered as a lumped
> > > > element or distributed?
> > > > This question assumes importance when point-to-point
> > > > clock traces are routed between a synthesizer & a
> > > > receiver. Ideally, the value of series (or source)
> > > > termination resistance that must be used is the
> > > > difference between the characteristic impedance and
> > > > the driver's internal resistance. I was wondering if I
> > > > should use the loaded characteristic impedance or
> > > > unloaded characteristic impedance. I then began to
> > > > wonder if I should consider the load as distributed or
> > > > lumped. Kindly comment.
> > > >
> > > > 2. Does the lumped capacitance affect the properties
> > > > of a transmission line? If so, how?
> > > >
> > > > 3. The following approximation describes the
> > > > relationship between the loaded characteristic
> > > > impedance, unloaded characteristic impedance and
> > > > the value of DISTRIBUTED capacitance:
> > > >
> > > > Z0' = Z0/ROOT OF (1 + CL/CO)
> > > >
> > > > where Z0' = Loaded characteristic impedance
> > > > Z0 = Unloaded (Intrinsic) characteristic impedance
> > > > CL = Load capacitance/unit length
> > > > C0 = Intrinsic capacitance/unit length
> > > >
> > > > Is there any relationship between loaded
> > > > characteristic impedance, unloaded characteristic
> > > > impedance and the value of LUMPED capacitance?
> > > >
> > > > Thanks,
> > > > Chandan
> > > >
> > > > __________________________________________________
> > > > Do You Yahoo!?
> > > > Yahoo! Auctions - buy the things you want at great prices
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--
Regards

Dave Instone. Compliance Engineer Storage Systems Development, MP24/22 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK. Tel: +44 (0)23-92-496862 (direct line) Fax: +44 (0)23-92-496014 http://www.xyratex.com Tel: +44 (0)23-92-496000

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