Buck Converter with Analysis Tools
This demonstration shows how to perform a Steady-State Analysis and use small-signal analysis to obtain different open-loop transfer functions for an unregulated buck converter with a resistive load. This can be done by performing an AC Sweep or Impulse Response Analysis, which both inherently first execute 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. One advantage is that the user can choose specific frequencies of interest when investigating a transfer function.
The Impulse Response Analysis provides an alternative and faster method to determine the open-loop transfer function of a system. Instead of perturbing a system with sinusoidal stimuli of different frequencies, one at a time, a single impulse is applied when the system is in steady state. The system transfer function can then be calculated very efficiently over a wide frequency range by computing the Laplace transform of the transient impulse response.
For detailed information on AC Sweep and Impulse Response Analysis see this tutorial.
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. As long as this is true, Multitone Analysis has the advantage that it still works in cases where the Steady-State Analysis fails.
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 a Loop Gain Analysis is given in the demo model Buck converter with Loop Gain Analysis.