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 - 07:54:59 PDT


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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