User Guide

Chapter 6 Analog behavioral modeling
190
Chebyshev filters
All of the considerations given above for Laplace parts
also apply to Chebyshev filter parts. However, PSpice also
attempts to deal directly with inaccuracies due to
sampling by applying Nyquist criteria based on the
highest filter cutoff frequency. This is done by checking
the value of TMAX. If TMAX is not specified it is assigned
a value, or if it is specified, it may be reduced.
For low pass and band pass filters, TMAX is set to
(0.5/FS), where FS is the stop band cutoff in the case of a
low pass filter, or the upper stop band cutoff in the case of
a band pass filter.
For high pass and band reject filters, there is no clear way
to apply the Nyquist criterion directly, so an additional
factor of two is thrown in as a safety margin. Thus, TMAX
is set to (0.25/FP), where FP is the pass band cutoff for the
high pass case or the upper pass band cutoff for the band
reject case. It may be necessary to set TMAX to something
smaller if the filter input has significant frequency content
above these limits.
Frequency tables
For frequency response tables, the maximum frequency is
twice the highest value. It will be reduced to
10/(RELTOL⋅TSTOP) or 8192 times the frequency
resolution if either value is smaller.
The frequency resolution for frequency response tables is
taken to be either the smallest frequency increment in the
table or the fastest rate of phase change, whichever is least.
PSpice then checks to see if it can be loosened without
inducing sampling errors.
Pspug.book Page 190 Wednesday, November 11, 1998 1:14 PM