This is a frequency domain analysis that computes the noise due to Johnson, shot and flicker noise. The output data is noise spectral density per unit square root bandwidth.
Syntax: .noise V(<out>[,<ref>]) <src> <oct, dec, lin> <Nsteps> <StartFreq> <EndFreq>
.noise V(<out>[,<ref>]) <src> list <FirstFreq>[ <NextFreq> [<NextFreq> ...]]
V(<out>[,<ref>]) is the node at which the total output noise is calculated. It can be expressed as V(n1, n2) to represent the voltage between two nodes. <src> is the name of an independent source to which input noise is referred. <src> is the noiseless input signal. The parameters <oct, dec, lin>, <Nsteps>, <StartFreq>, and <EndFreq> define the frequency range of interest and resolution in the manner used in the .ac directive.
Output data trace V(onoise) is the noise spectral voltage density referenced to the node(s) specified as the output in the above syntax. If the input signal is given as a voltage source, then data trace V(inoise) is the input-referred noise voltage density. If the input is specified as a current source, then the data trace inoise is the noise referred to the input current source signal. The noise contribution of each component can be plotted. These contributions are referenced to the output. You can reference them to the input by dividing by the data trace "gain".
The waveform viewer can integrate noise over a bandwidth by <Ctrl-Key> + left mouse button clicking on the corresponding data trace label.
The syntax ".noise V(<out>[,<ref>]) <src> list <Freq>" with a single analysis frequency is useful in combination with .step. It allows you to plot noise densities as a function of a stepped parameter as shown in this exammple.