User`s guide

Cautions and Recommendations for Simulation and Analysis 6
-41
Frequenc
y
-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 A/D 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 A/D uses the
Transient Analysis Print Step, TSTEP, instead.
Note
TSTOP, TMAX, and TSTEP values are confi
g
ured
usin
g
Transient on the Setup menu. The RELTOL
attribute is set usin
g
Options on the Setup menu.
PSpice A/D 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/