This design uses both TDFS and standard AC Analysis methods to assess the stability of a closed-loop system. The design is the "Buck DC to DC Converter", using a state-average model for the modulator function.
The use of state-average models supports AC Analysis, but TDFS (Time Domain Frequency Sweep) is also used to show the equivalency of the methods. Note that the TDFS approach can be used for systems that do not support AC Analysis, such as switching circuits or systems that contain sampling or digital control aspects. This particular example is directly comparable to the design "TDFS Loop Stability for Buck DC to DC Converter - Switching". In that design, the TDFS method is used to measure the open loop transfer function of an operational switching circuit.
Note that the approach used to characterize the loop stability, by injecting a small sinusoidal stimulus signal in series with the loop and then measuring the complex ratio of the ground referenced return signal to the injected signal, is described in:
D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries