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

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From: Ken Cantrell (Ken.Cantrell@srccomp.com)
Date: Wed May 23 2001 - 09:27:10 PDT


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
> >
> > __________________________________________________
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