The Four (or Fourier) statement controls whether DR. SPICE performs a Fourier analysis immediately after performing a Transient analysis. The syntax of the Four statement is
.FOUR FREQ <NUM> OV1 <OV2 OV3...>
FREQ is the fundamental frequency and OV1, ..., are the output variables for which the analysis is desired. The Fourier analysis is performed over the interval TFINAL-period to TFINAL, where TFINAL is the final time specified for the transient analysis and period is one period of the fundamental frequency. The DC component and the first NUM (default is 9) harmonics are determined. For maximum accuracy, TMAX (see Transient Analysis ) should be set to period/100. (or less for very high-Q circuits).
Examples:
.FOUR 10MEG V(2) .FOUR 150K 12 I(R1) V(OUT)