User`s guide
PSpice A/D-Equivalent Parts 6
-35
Frequenc
y
-Domain Device
Models
Frequency-domain models (ELAPLACE, GLAPLACE,
EFREQ, and GFREQ) are characterized by output that depends
on the current input as well as the input history. The relationship
is therefore non-instantaneous. For example, the output may be
equal to the integral of the input over time. In other words, the
response depends upon frequency.
During AC analysis, the frequency response determines the
complex gain at each frequency. During DC analysis and bias
point calculation, the gain is the zero-frequency response.
During transient analysis, the output of the device is the
convolution of the input and the impulse response of the device.
Laplace Transforms (LAPLACE)
The ELAPLACE and GLAPLACE parts allow a transfer
function to be described by a Laplace transform function. The
ELAPLACE and GLAPLACE symbols are defined, in part, by
the following attributes (default values are shown):
ELAPLACE
EXPR V(%IN+, %IN-)
XFORM 1/s
GLAPLACE
EXPR V(%IN+, %IN-)
XFORM 1/s
The LAPLACE parts use a Laplace transform description. The
input to the transform is the value of EXPR, where EXPR
follows the same rules as for VALUE expressions (see
EVALUE and GVALUE parts
on page 6-30). XFORM is an
expression in the Laplace variable, s. It follows the rules for
standard expressions as described for VALUE expressions with
the addition of the s variable.
The output of the device depends on the type of analysis being
done. For DC and bias point, the output is simply the zero
Movin
g
back and forth between
the time and frequency-domains
can cause surprisin
g
results.
Often the results are quite
different than what one would
intuitively expect. For this
reason, we stron
g
ly recommend
familiarity with a reference on
Fourier and Laplace transforms.
A
g
ood one is:
1
R. Bracewell,
The Fourier
Transform and Its
Applications
, McGraw-Hill,
Revised Second Edition
(1986)
We also recommend familiarity
with the use of transforms in
analyzin
g
linear systems. Some
references on this subject:
2
W. H. Chen,
The Analysis of
Linear Systems
, McGraw-Hill
(1962)
3
J. A. Aseltine,
Transform
Method in Linear System
Analysis
, McGraw-Hill (1958)
4
G. R. Cooper and C. D.
McGillen,
Methods of Si
g
nal
Volta
g
es, currents, and TIME
cannot appear in a Laplace
transform.