LTspice IV is a powerful free analog and mixed signal circuit simulation and schematic capture tool offering unmatched performance, speed and ease of use.

1. Use the program as a general-purpose schematic capture program with an integrated simulator. Menu commands File=>New, and File=>Open(file type .asc)

2. Feed the simulator with a handcrafted netlist or a foreign netlist generated with a different schematic capture tool. Menu command File=>Open(file type .cir)

The schematic is ultimately converted to a textual SPICE netlist that is passed to the simulator. While the netlist is usually extracted from a graphical schematic drafted in LTspice, an imported netlist can be run directly without having a schematic. This has several uses: (i) Linear Technology’s filter synthesis program, FilterCAD, can synthesize a netlist for LTspice to simulate the time domain or frequency response of a filter. (ii) it simplifies benchmarking LTspice against other SPICE programs (iii) professionals historically experienced with SPICE circuit simulators are familiar with working directly with the textual netlists because schematic capture was not integrated with SPICE simulators in older systems.