.FRA — Perform a Time-Domain Frequency Response Analysis

Perform a specialized transient simulation to analyze the frequency response of a feedback loop. The circuit must contain at least one FRA device instance (prefix "@"), which controls the detailed analysis parameters. The Frequency Response Analysis is ideal for for analyzing the stability of Switched Mode Power Supplies.

Syntax:

.fra [Tstart=<val>] [dTmax=<val>] [Tstep=<val>] [Tstop=<val>] [uic] [startup]

Example:

.fra

The.FRA command parameters and modifiers are all optional, specified by keyword, and may be included in any order.

The UIC and startup modifiers can be included with the .fra directive.
UIC: Skip the D.C. operating solution and use user-specified initial conditions.
startup: Solve the initial operating point with independent voltage and current sources turned off. Then start the transient analysis and turn these sources on in the first 20 us of the simulation.
If Tstart is specified, the waveform data between time zero and Tstart is not saved, in order to reduce the size of waveform files. Regardless of the value of Tstart, simulation always starts at time equal to zero.
Tstop is an optional argument to allow early termination of the simulation, e.g. for debug. If Tstop is not specified, the FRA automatically stops when all of the FRA devices in the schematic have completed their analyses.
dTmax is the maximum time step to take while integrating the circuit equations.
dTstep is the plotting increment for the waveforms but is also used as an initial step-size guess. LTspice uses waveform compression, so this parameter is of little value and can be omitted or set to zero.

See the examples that are included with LTspice by choosing File > Open Examples... and navigating to subdirectory Educational\FRA\