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

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From: Degerstrom, Michael J. (degerstrom.michael@mayo.edu)
Date: Thu May 24 2001 - 07:10:14 PDT


Dave,

My original posting tried to cover the lumped case and Ken's
posting covered the distributed case. You covered both cases.
I believe Chandan's message indicated that it was a
source with one destination and that the interconnect between
them was long enough to be considered a transmission line.
Normally, as you said below, you match the sum of source
resistance and series resistance to the transmission line
impedance so that the capacitive reflection is absorbed.

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: david_instone@uk.xyratex.com
> [mailto:david_instone@uk.xyratex.com]
> Sent: Thursday, May 24, 2001 2:56 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
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