AC Sweep Frequency Analysis

With AC Sweep Frequency Analysis, the frequency response of a circuit or a control loop can be obtained. A key feature of the AC Sweep Frequency Analysis in DSIM is that, if a circuit is switch-mode, the frequency response can be acquired in its original switch-mode form, rather than relying on an average model.

The principle of the AC Sweep Frequency Analysis is that a small AC excitation signal is injected into the system as the perturbation, and the output signal at the same frequency is then extracted. To achieve accurate results, it’s crucial to set the excitation source amplitude appropriately. It should be small enough to keep the measured system within the linear region, yet large enough to avoid interference from numerical errors or other noise.

Typically, physical systems exhibit low attenuation in the low-frequency range and high attenuation in the high-frequency range. For optimal results, consider using a relatively small excitation source amplitude at the start frequency and gradually increasing it to a larger amplitude at the end frequency.

The Steps to Set up the AC Sweep Frequency Analysis:

For various circuits, the AC Sweep element allows you to configure different excitation source frequency ranges, specify the number of calculation points, and include additional measurement points of excitation frequencies. In this case, we utilize the AC Sweep block to analyze the frequency characteristics of a Buck circuit, resulting in the generation of a Bode diagram. Firstly, an excitation source is added to the duty cycle of the Buck circuit, and its frequency range is set to 200~20kHz, the number of calculation points is 10. Four additional calculation points are added, namely excitation frequency 2.1kHz, 2.2kHz, 2.3kHz, and 2.4kHz (near the inflection point of the amplitude-frequency curve). Secondly, an AC Sweep probes is added to the load circuit. The amplitude-frequency characteristic of Buck circuit can be calculated by the response of load voltage under different frequency excitation.

The following figure shows the amplitude-frequency response curve and phase-frequency response curve of Buck circuit respectively.

● Identify a sinusoidal voltage source as the excitation source.

● Place the AC Sweep probes at the desired output location.

● Place the AC Sweep block on the schematic, and set the parameters.

● Run simulation.

A Case of AC Sweep Frequency Analysis in DSIM