General Form:

fft vector1 [vector2] ...

This analysis provides a fast Fourier transform of the input vector(s) in forward direction. fft is much faster than spec (17.5.76) (about a factor of 50 to 100 for larger vectors).

The fft command will create a new plot consisting of the Fourier transforms of the vectors given on the command line. Each vector given should be a transient analysis result, i.e. it should have time as a scale. You will have got these vectors by the tran Tstep Tstop Tstart command.

The vector should have a linear equidistant time scale. Therefore linearization using the linearize command is recommended before running fft. Be careful selecting a Tstep value small enough for good interpolation, e.g. much smaller than any signal period to be resolved by fft (see linearize command). The Fast Fourier Transform will be computed using a window function as given with the specwindow variable. A new plot named specx will be generated with a new vector (having the same name as the input vector, see command above) containing the transformed data.

Ngspice has two FFT implementations:

1. Standard code is based on the FFT function provided by John Green `FFTs for RISC 2.0`, downloaded 2012, now to be found here. These are a power-of-two routines for fft and ifft. If the input size doesn't fit this requirement the remaining data will be zero padded up to the next 2(N) field size. You have to take care of the correlated change in the scale vector.
2. If available on the operating system (see Chapter 32) ngspice can be linked to the famous FFTW-3 package, found here. This high performance package has advantages in speed and accuracy compared to most of the freely available FFT libraries. It makes arbitrary size transforms for even and odd data.

How to compute the fft from a transient simulation output:

ngspice 8 -> setplot tran1
ngspice 9 -> linearize V(2)
ngspice 9 -> set specwindow=blackman
ngspice 10 -> fft V(2)
ngspice 11 -> plot mag(V(2))

Linearize will create a new vector V(2) in a new plot tran2. The command fft V(2) will create a new plot spec1 with vector V(2) holding the resulting data.

The variables listed in the following table control operation of the fft command. Each can be set with the set command before calling** **fft.