General form:

.ac dec nd fstart fstop
.ac oct no fstart fstop
.ac lin np fstart fstop

Examples:

.ac dec 10 1 10K
.ac dec 10 1K 100MEG
.ac lin 100 1 100HZ

dec stands for decade variation, and nd is the number of points per decade. oct stands for octave variation, and no is the number of points per octave. lin stands for linear variation, and np is the number of points. fstart is the starting frequency, and fstop is the final frequency. If this line is included in the input file, Ngspice performs an AC analysis of the circuit over the specified frequency range. Note that in order for this analysis to be meaningful, at least one independent source must have been specified with an ac value. Typically it does not make much sense to specify more than one ac source. If you do, the result will be a superposition of all sources and difficult to interpret.

Example:

Basic RC circuit
r 1 2 1.0
c 2 0 1.0
vin 1 0 dc 0 ac 1  $ <--- the ac source
.options noacct
.ac dec 10 .01 10
.plot ac  vdb(2) xlog
.end

In this AC (or 'small signal') analysis all non-linear devices are linearized around their actual DC operating point. All L and C devices get their imaginary value that depends on the actual frequency step. Each output vector will be calculated relative to the input voltage (current) given by the AC value ((V_{in}) equals 1 in the example above). The resulting node voltages (and branch currents) are complex vectors. Therefore one has to be careful using the plot command, specifically, one may use the variants of vxx(node) described in Chapt. 15.6.2 like vdb(2) (see also the above example).