Let's try using LTspice - Dot Commands (1): AC Analysis (.ac)

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This article is a series of articles about dot commands available in SPICE. In this first article, we will explain AC analysis (.ac command), which is often used in analysis.

If you answer the last questionnaire, you can download the supplementary material for the AC analysis (.ac) command in LTspice Help. The Help material is a netlist-based material, but if you would like to obtain an explanatory material that actually performs a simulation with the GUI like this article, please help us by answering the questionnaire.

AC analysis ( .ac command)

AC analysis with the .ac command is useful for filters, networks, stability analysis, and noise studies.

Normally, when performing AC analysis, you would set the analysis mode on LTspice using the GUI, but here we will explain how to write the netlist after setting it up. There are two types of LTspice syntax. < > is a required item when setting it up. [ ] is optional.

① .ac <oct, dec, lin> <Nsteps> <StartFreq> <EndFreq>

The frequency is set by StartFreq, the analysis start frequency, and EndFreq, the analysis end frequency. The number of analysis steps is defined by "oct", "dec", "lin" and Nsteps.

Type of sweep	Nsteps
Oct	Number of analysis points per octave (2x frequency) (usually 20-40 points)
Dec	Number of analysis points per decade (usually 30-100 points)
Lin	Number of analysis points between StartFreq and EndFreq

② .ac list <FirstFreq> [<NextFreq> [<NextFreq> ...]]

If you set Type of sweep to list, you can specify a single frequency for analysis. This is useful for use with the .step command. This analysis using a list is a syntax that is only added to LTspice and is not supported by general SPICE.

Please refer to the explanatory materials for usage and sample circuits. If you would like to receive the explanatory materials, please fill out the questionnaire.

Performing an AC Analysis

Let's check the .ac command using an RC filter circuit. Create the following RC filter circuit on LTspice.

Configure the .ac command. ClickClick to open the Configure Analysis menu. Open the AC Analysis tab and set the analysis commands on the GUI as follows:

The schematic with the .ac command set looks like this:Click RUN to run the simulation.

Figure 3: Circuit diagram and AC commands

Checking the node voltage between R1 and C1 gives the following result, which confirms the frequency response of the RC filter.

Download supplementary materials for AC analysis (.ac) commands

The explanation of the AC analysis (.ac) command in the LTspice Help is based on a netlist, but if you fill out the questionnaire, you will be able to download the explanation document modified to a GUI-based format like this article, as well as the LTspice sample circuit used in the downloadable material. After filling out the questionnaire, we will send you a URL for downloading the supplementary material by email.

AC Analysis (.ac) Command Supplementary Material

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