User Guide

Chapter 6 Analog behavioral modeling
188
transforms do not have such a limit, there is also a limit of
10/RELTOL times the frequency resolution, or
10/(RELTOL·TSTOP). For example, consider the
transform:
e
-0.001·s
This is an ideal delay of 1 millisecond and has no
frequency cutoff. If TSTOP = 10 milliseconds and
RELTOL=.001, then PSpice imposes a frequency cutoff of
10 MHz. Since the time resolution is the inverse of the
maximum frequency, this is equivalent to saying that the
delay cannot resolve changes in the input at a rate faster
than .1 microseconds. In general, the time resolution will
be limited to RELTOL·TSTOP/10.
A final computational consideration for Laplace parts is
that the impulse response is determined by means of an
FFT on the Laplace expression. The FFT is limited to 8192
points to keep it tractable, and this places an additional
limit on the maximum frequency, which may not be
greater than 8192 times the frequency resolution.
If your circuit contains many Laplace parts which can be
combined into a more complex single device, it is
generally preferable to do this. This saves computation
and memory since there are fewer impulse responses. It
also reduces the number of opportunities for numerical
artifacts that might reduce the accuracy of your transient
analyses.
Laplace transforms can contain poles in the left half-plane.
Such poles will cause an impulse response that increases
with time instead of decaying. Since the transient analysis
is always for a finite time span, PSpice does not have a
problem calculating the transient (or DC) response.
However, such poles will make the actual device oscillate.
Non-causality and Laplace transforms
PSpice applies an inverse FFT to the Laplace expression to
obtain an impulse response, and then convolves the
impulse response with the dependent source input to
obtain the output. Some common impulse responses are
inherently non-causal. This means that the convolution
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