User`s guide

6
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42 Analo
g
Behavioral Modelin
g
(2π) = 159 Hz. At 159 Hz, the response is down to .001 (down
by 60 db). Since some 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 A/D 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 A/D does not have a problem
calculating the transient (or DC) response. However, you need
to be aware that such poles will make the actual device oscillate.
Non-causalit
y
and Laplace transforms
PSpice A/D 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 must be applied to both past and
future samples of the input in order to properly represent the
inverse of the Laplace expression.