General Form:

fourier fundamental_frequency [expression ...]

Fourier is used to analyze the output vector(s) of a preceding transient analysis (see 17.5.84). It does a Fourier analysis of each of the given values, using the first 10 multiples of the fundamental frequency (or the first nfreqs multiples, if that variable is set - see 17.7). The printed output is like that of the .four ngspice line (Chapt. 15.6.4). The expressions may be any valid expression (see 17.2), e.g. v(2). The evaluated expression values are interpolated onto a fixed-space grid with the number of points given by the fourgridsize variable, or 200 if it is not set. The interpolation is of degree polydegree if that variable is set, or 1. If polydegree is 0, then no interpolation is done. This is likely to give erroneous results if the time scale is not monotonic, though.

The fourier command not only issues a printout, but also generates vectors, one per expression. The size of the vector is 3 x nfreqs (per default 3 x 10). The name of the new vector is fouriermn, where m is set by the mth call to the fourier command, n is the nth expression given in the actual fourier command. fouriermn[0] is the vector of the 10 (nfreqs) frequency values, fouriermn[1] contains the 10 (nfreqs) magnitude values, fouriermn[2] the 10 (nfreqs) phase values of the result.

Example:

* do the transient analysis
tran 1n 1m
* do the fourier analysis
fourier 3.34e6 v(2) v(3) $ first call
fourier 100e6 v(2) v(3)  $ second call
* get individual values
let newt1 = fourier11[0][1] 
let newt2 = fourier11[1][1] 
let newt3 = fourier11[2][1]
let newt4 = fourier12[0][4]
let newt5 = fourier12[1][4]
let newt6 = fourier12[2][4]
* plot magnitude of second expression (v(3))
* from first call versus frequency
plot fourier12[1] vs fourier12[0]

The plot command from the example plots the vector of the magnitude values, obtained by the first call to fourier and evaluating the first expression in this call, against the vector of the frequency values.