Re: [SI-LIST] : Oscillation in lumped circuits and

Douglas McKean (dmckean@corp.auspex.com)
Tue, 02 Mar 1999 17:51:20 -0800

I hesitated responding to this, but have spent a good portion
of time reading all the responses that have come in.

IMO, there's two mechanisms going on here -
the ability to sustain electrical energy,
and
the ability to reflect electrical energy.

In an electrical system, both feed on one another.

First the former. In the derivation of characteristic impedance,
let's not forget that nearly all the books that I've read use
the model of a completely lossless transmission line in order
to derive the Z0 = (L/C)^0.5 Be that as it may, since the
variables L and C are in fact variables of inductance per unit
length and capacitance per length respectively,
"the per length lengths" cancel and one is left with a
characteristic impedance no matter what the length
*in a perfectly lossless system*. In the real world,
it's something different.

With regard to the latter, the amplitude of reflection is the
issue dependent upon the similarity or dissimilarity between
the impedance of the line and the impedance of the termination.

In other words, the oscillatory response of the line is specific
with regard to frequency. The reflection response of a line
and its termination is not.

In some designs, the last thing you may want is reflection.
In others, such as PCI, you do. Anyway, that's only my two
and a half cents worth and sorry if I'm beating a dead horse ...

Regards, Doug McKean

At 12:01 AM 2/5/99 -0800, Arani Sinha wrote:
>Hi,
>
>I have the following question.
>
>We can model an interconnect as either a lumped circuit or a
>transmission line. By means of lumped modeling, we can say that
>it has an oscillatory response if its damping factor is less
>than 1. By means of transmission line modeling, we can say that
>it has an oscillatory response if the signal reflection
>co-efficients at source and load satisfy certain conditions.
>
>My question is whether oscillation in a lumped circuit and
>signal reflection in a transmission line are actually the same
>phenomenon. If so, there should be a correlation between
>conditions for oscillation in a lumped circuit and those for
>oscillation in a transmission line.
>
>After many discussions and much thought, I have not been able
>to determine a correlation. I am also ambivalent about whether
>they are the same phenomenon.
>
>I understand that the damping factor in a lumped circuit is
>equivalent to the attenuation constant in a transmission line
>and that condition of no reflection is equivalent to the
>maximum power transfer theorem.
>
>I will really appreciate help in this regard.
>
>Thanks,

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