From: Dmitri Kuznetsov (email@example.com)
Date: Thu Dec 16 1999 - 15:06:13 PST
I would like to comment on the time-step issue.
When simulating longer traces and cables, it is important to set .option
DELMAX to the same value as the time step. During the transient
analysis, the time step is dynamically changed depending on the
waveforms at circuit nodes. For longer tlines, the waveforms at the
terminals may not change while the signal is propagating inside the
line. The simulator then relaxes the timestep, and when the signal
finally reaches the terminals, its accuracy has deteriorated.
.option DELMAX limits the maximum timestep and eliminates this problem.
This applies not just to W element but to all tline models, including
lossless T element.
> I want to warn everyone that hspice is extremely sensitive to time step.
> The time step I used in the .sp file produced the results shown.
> If you vary time step you can get different answers. Also the
> meshing in Ansoft is sensitive to geometry. I specified frequencies
> of 100 MHz for the cable geometry and 500 MHz for the stripline
> geometry in order to avoid problems in skin depth meshing.
> Ansoft as well as any FE solver at some point may not be able to
> handle high Aspect ratios (thin wide conductors).
> Even a round conductor can start to look like a plane at sufficiently
> high a frequency in terms of skin depth.
> Jim Foppiano
> Signal Integrity
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