Buck Converter with Loop Gain Analysis
This demonstration shows how to obtain the loop gain of a voltage-regulated buck converter with a resistive load. This can be done by performing an AC Sweep Analysis, which inherently first executes a Steady-State Analysis, or alternatively, using the Multitone Analysis, which does not execute a Steady-State Analysis.

The AC Sweep Analysis applies a set of sinusoidal perturbations to the system under study. At each of these user-specified frequencies it then finds the periodic steady-state operating point of the perturbed system, and extracts the system response using Fourier analysis. For detailed information see this tutorial.
The Multitone Analysis is similar to an AC Analysis where the response of the system to a small perturbation signal is analysed. However, instead of multiple sinusoidal signals of different frequencies, only one multitone signal is applied. It is composed of several sinusoidal signals and therefore contains all investigated frequencies at once.
The Multitone Analysis is faster than the AC Analysis because it only needs to compute the response to one signal instead of a set of signals for each frequency. The caveat with Multitone Analysis, however, is that the value for the initial simulation period parameter should be sufficiently large enough for the system to have reached steady-state conditions from a transient simulation.
To run an analysis, choose Analysis tools... from the Simulation menu, then select an analysis from the list and click Start analysis.
A demonstration of using small-signal analyses to obtain different open-loop transfer functions for an unregulated buck converter is given in the demo model Buck converter with Analysis Tools.