**From:** Ken Cantrell (*Ken.Cantrell@srccomp.com*)

**Date:** Wed May 23 2001 - 09:27:10 PDT

**Next message:**Khanh Le: "RE: [SI-LIST] : Antenna Problem on the Board"**Previous message:**Issa, Elie: "RE: [SI-LIST] : Antenna Problem on the Board"**In reply to:**Degerstrom, Michael J.: "RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES"**Next in thread:**Degerstrom, Michael J.: "RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES"

Mike,

The point was to indicate a correct set of calculations for capacitive

loading effects on a transmission line, not simulation. I additionally

referenced a text source, which will indicate the topology dependencies,

capacitive reflection, and risetime degradation. The basic effect of

capacitive loading on a TL is to reduce Zo, and has to be taken into account

as a distributed effect. I wanted to clarify that to Chandan. If this

isn't done, you're in for a big surprise when the board comes in. You are

correct that there are many factors involved, and simulation will reveal

those issues.

Ken

-----Original Message-----

From: Degerstrom, Michael J. [mailto:degerstrom.michael@mayo.edu]

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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*> >
*

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**Next message:**Khanh Le: "RE: [SI-LIST] : Antenna Problem on the Board"**Previous message:**Issa, Elie: "RE: [SI-LIST] : Antenna Problem on the Board"**In reply to:**Degerstrom, Michael J.: "RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES"**Next in thread:**Degerstrom, Michael J.: "RE: [SI-LIST] : EFFECT OF LUMPED LOAD ON TRANSMISSION LINES"

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