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

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From: Chandan (chandan_career@yahoo.com)
Date: Wed May 23 2001 - 02:00:07 PDT


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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