From: Chandan ([email protected])
Date: Wed May 23 2001 - 02:00:07 PDT
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?
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