[SI-LIST] Re: 50 ohms cable model

> Does anyone have an explanation for why the 50 ohms cable induces
> overshoot?

The generic answer: a cable with any characteristic impedance induces
overshoot if it is not terminated properly.  A simulation of any cable
induces overshoot if not modeled appropriately.

I agree with Steve's and Ed's replies.  It is a little hard to tell, but you
implied you modeled the transmission line using one or more lumped L/C
elements.

If you did this, understand that the lumped or L/C model is only an
approximation with known shortcomings, and that it does exhibit ringing
because of that approximation.  The approximation gets better and the model
becomes valid only in the limit as the number of L/C sections approaches
infinite.

For lossless lines, SPICE's ideal transmission line model (T-element)
performs correctly and is what you want.  It behaves like the L/C model with
an infinite number of infinitesimally small elements (or actually,
vice-versa).

For almost all practical purposes, there is little difference whether your
transmission line is a coax cable or a trace.  Both of them are transmission
lines, and can be modeled pretty much the same in circuit simulations (if
you're not concerned about things like loss or higher propagation modes).

Also, you said you ran the simulation with a "buffer that has a 50 ohm
impedance", but how accurate is that impedance?  If you used an ideal
Thevenin source with a resistive 50 ohms, OK.  If you used a model of an
actual buffer designed to drive 50 ohm lines, its impedance might differ
significantly (unless it is designed for video or instrumentation).  Even if
it is actually supposed to be 50 ohms, the impedance of an active circuit is
somewhat nonlinear and has tolerances, and the package adds reactance.

Finally, there is no magic to 50 ohms.

Regards,
Andy

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