2.18 FOURIER command
2.18.1 Syntax
fourier start stop stepsize {options ...}
2.18.2 Purpose
Performs a nonlinear time domain (transient) analysis, but displays the
results in the frequency domain.
Start, stop, and stepsize are frequencies.
2.18.3 Comments
This command is slightly different and more flexible than the SPICE
counterpart. SPICE always gives you the fundamental and 9 harmonics.
Gnucap will do the same if you only specify one frequency. SPICE has
the probes on the same line. Gnucap requires you to specify the probes
with the print command.
SPICE uses the last piece of a transient that was already done. Gnucap
does its own transient analysis, continuing from where the most recent
one left off, and choosing the step size to match the Fourier
Transform to be done. Because of this the Gnucap Fourier analysis is
much more accurate than SPICE.
The nodes to look at must have been previously selected by the print or plot command.
Three parameters are normally needed for a Fourier analysis: start
frequency, stop frequency and step size, in this order.
If the start frequency is omitted it is assumed to be 0. The two
remaining parameters are stop and step, such that stop > step.
If only one frequency is specified, it is assumed to be step size,
which is equivalent to the fundamental frequency. The start
frequency is zero and the stop frequency is set according the harmonics option (from the options command. The default is
9 harmonics.
If two frequencies are specified, they are stop and step. The
order doesn't matter since stop is always larger than step.
Actually, this command does a nonlinear time domain analysis, then
performs a Fourier transform on the data to get the frequency data.
The transient analysis parameters (start, stop, step) are determined
by the program as necessary to produce the desired spectral results.
The internal time steps are selected to match the Fourier points,
so there is no interpolation done.
The underlying transient analysis begins where the previous one
left off. If you specify the "cold" option, it begins at time =
0. Often repeating a run will improve the accuracy by giving more
time for initial transients to settle out.
See also: Transient command.
2.18.4 Options
 > file

Send results of analysis to file.
 >> file

Append results to file.
 cold

Zero initial conditions. Cold start from powerup.
 dtemp degrees

Temperature offset, degrees C. Add this number to the temperature
from the options command.
 dtmin time

The minimum internal time step, as a time. (Default = option
dtmin Time cannot be resolved closer than this.
 dtratio number

The minimum internal time step, as a ratio. (Default = option
dtratio This is the maximum number of internal time steps for every
requested step.
 quiet

Suppress console output.
 skip count

Force at least count internal transient time steps for each one
used.
 temperature degrees

Temperature, degrees C.
 trace n

Show extended information during solution.
Must be followed by one of the following:

off
 No extended trace information (default, override .opt)
 warnings
 Show extended warnings
 alltime
 Show all accepted internal time steps.
 rejected
 Show all internal time steps including rejected steps.
 iterations
 Show every iteration.
 verbose
 Show extended diagnostics.
2.18.5 Examples
 fourier 1Meg
 Analyze the spectrum assuming a fundamental
frequency of 1 mHz. Use the harmonics option to determine
how many harmonics (usually 9) to display.
 fourier 40 20k 20
 Analyze the spectrum from 40 Hz to 20 kHz in
20 Hz steps. This will result in a transient analysis with 25 microsecond
steps. (1 / 40k). It will run for .05 second. (1 / 20).
 fourier 0 20k 20
 Similar to the previous example, but show the
DC and 20 Hz terms, also.
 fourier
 No parameters mean use the same ones as the last time.
In this case: from 0 to 20 kHz, in 20 Hz steps.
 fourier Skip 10
 Do 10 transient steps internally for every step
that is used. In this case it means to internally step at 2.5 microsecond,
or 10 steps for every one actually used.
 fourier Cold
 Restart at time = 0. This will show the
spectrum of the poweron transient.