A typical example script named spectrum is delivered with the ngspice distribution. Even if it is made obsolete by the internal spec command (see 17.5.76), and especially by the much faster fft command (see 17.5.27), it is a good example for getting acquainted with the ngspice (or nutmeg) post-processor language.

As a suitable input for spectrum you may run a ring-oscillator, delivered with ngspice in e.g. test/bsim3soi/ring51_41.cir. For an adequate resolution a simulation time of 1(\mu)s is needed. A small control script starts ngspice by loading the R.O. simulation data and executing spectrum.

Small script to start ngspice, read the simulation data and start spectrum:

* test for script 'spectrum'
.control
load ring51_41.out
spectrum 10MEG 2500MEG 1MEG v(out25) v(out50)
.endc