Re: [SI-LIST] : Decoupling Capacitors and SSN

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From: Scott McMorrow ([email protected])
Date: Mon Aug 28 2000 - 10:11:27 PDT

Josip Popovic wrote:

> wouldn't Zo od a transmission line loaded by a capacitor (Cd) change and
> be different then Zo of a line with a resistive load:
> Zo'=Zo/(SQRT(1+Cd/Co))
> where:
> Zo' is new characteristic impedance of the loaded transmission line,
> Cd is a capacitive load (per the transmission length).
> Co intristic capacitance (per length)
> I do not think that an ideal transmission line model (as is one that
> comes with HSPICE for example) would show this change in Zo.


The answer to your question is no. It is a misnomer to believe that the
characteristic impedance of a transmission line changes due to loading.
The transmission line waveguide structure before the capacitive load
has not changed. The total system, however, has changed. An electromagnetic
wave does not "see" the capacitive load until it propogates down the
waveguide to the capacitive load. The effect of this capacitive load
is then reflected back down the waveguide to the driver. Again, the
waveguide itself has not changed. It still has it's characteristic impedance
and velocity of propogation. What has changed is the reflection at
the discontinuity ( the capacitor). A driver will not see the effect of
the capacitor until one round trip across the transmission line.

A Spice ideal transmission line terminated by a capacitive load
is a very good model of what actually happens in the real circuit
(excluding transmission line losses).

What has been regarded as a reduction in the characteristic impedance of
the line is really a reduction in the average characteristic impedance
of the circuit, which is a different thing. The unstated assumption in all
of these "formulas" is that the rise/fall time of the driver is slow enough that a lumped
circuit approximation is valid. For a lumped circuit approximation of a transmission
line to be reasonably valid, the delay of each LC section ( sqrt(LC) ) must be less
than 1/20 of the minimum of the rise/fall time of the signal passing through the



Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231

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