User Guide

Cautions and recommendations for simulation and analysis
187
Frequency-domain parts
Some caution is in order when moving between frequency
and time domains. This section discusses several points
that are involved in the implementation of
frequency-domain parts. These discussions all involve the
transient analysis, since both the DC and AC analyses are
straightforward.
The first point is that there are limits on the maximum
values and on the resolution of both time and frequency.
These are related: the frequency resolution is the inverse
of the maximum time and vice versa. The maximum time
is the length of the transient analysis, TSTOP. Therefore,
the frequency resolution is 1/TSTOP.
Laplace transforms
For Laplace transforms, PSpice starts off with initial
bounds on the frequency resolution and the maximum
frequency determined by the transient analysis
parameters as follows. The frequency resolution is
initially set below the theoretical limit to (.25/TSTOP) and
is then made as large as possible without inducing
sampling errors. The maximum frequency has an initial
upper bound of (1/(RELTOL*TMAX)), where TMAX is
the transient analysis Step Ceiling value, and RELTOL is
the relative accuracy of all calculated voltages and
currents. If a Step Ceiling value is not specified, PSpice
uses the Transient Analysis Print Step, TSTEP, instead.
PSpice then attempts to reduce the maximum frequency
by searching for the frequency at which the response has
fallen to RELTOL times the maximum response. For
instance, for the transform:
1/(1+s)
the maximum response, 1.0, is at s = j·
ω
= 0 (DC). The
cutoff frequency used when RELTOL=.001, is
approximately 1000/(2π) = 159 Hz. At 159 Hz, the
response is down to .001 (down by 60 db). Since some
Note
TSTOP, TMAX, an
d
TSTEP va
l
ues are
configured using Transient on the Setup
menu. The RELTOL property is set using
Options on the Setup menu.
Pspug.book Page 187 Wednesday, November 11, 1998 1:14 PM