Re: [IBIS] Re: hspice vs IBIS model correlation at different capacitive loads

On Wednesday 04 January 2006 12:11 pm, Helliwell, Kim wrote:
> The numeric oscillations that can be cured by METHOD=GEAR are
> very distinctive: they look like a sawtooth wave in which the
> timepoints oscillate between two values (this behavior
> sometimes will damp out over time, but usually will continue
> between exactly the same two values). The key characteristic
> is the
> sawtooth wave pattern with only two timepoints per period. If
> you have an oscillation that looks like a sawtooth (or just a
> high frequency ringing) but with more than two timepoints
> defining one period of the ringing, this won't be helped by
> changing the integration method.
>
> By the way, this sawtooth pattern is called "Trap ringing"
> because it's a characteristic of the trapezoidal integration
> method SPICE uses by default.

This is true of the internal data, but SPICE re-interpolates the 
output data before showing it to the user.  This 
re-interpolation will mask this distinctive appearance.

Also.....

In some models, the specified tables are contaminated by 
trap-ringing.  Then when you simulate with similar improper 
settings, you get trap ringing again, asynchronous with the 
original.  You get the sum of two sawtooths, but probably don't 
recognize it as such, especially when it is further 
contaminated by re-interpolation, which may happen twice.

There's another one that happens as an artifact of de-embedding 
c_comp and c_fixture.  The de-embedding can suffer from 
trap-ringing.  Even if it doesn't, the artifacts of re-sampling 
can have sort of a sawtooth appearance.
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