Simulation Types and How They Work

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In LTSpice there are multiple types of simulation, aside from the obvious one - transient analysis, where it's much like an oscilloscope screen.

Transient Analysis:

This simulation graphs magnitude vs. time and displays like an oscilloscope (however you can selectively view any portion of the simulation, as if the whole simulation was just captured by a sampling scope).

The X-Axis is time, Y-Axis is magnitude.

AC Analysis:

This simulation shows frequency vs. magnitude and displays like a spectrum analyzer. To get any meaningful output you must add an AC source to the circuit somewhere; the difference in magnitude and phase from this original source will be displayed.

The phase ruler is on the right and magnitude on the left. Phase is the dotted line. You can turn off phase through the left-click menu on the right side ruler.

To set up the simulation, go to [simulate->simulation cmd->AC Analysis]. There are different sweep types; choosing this will determine the law of your x-axis. Decade and Octave are logarithmic laws.

As an example, what if you wanted to measure the frequency response of an amplifier (we get asked this a lot)? Well, right-click on your input source. Enter "AC 1" into the box that says "AC Amplitude". Run the simulation with the following command:

.ac dec 10 20 20k

The magnitude is always shown in db, but you can change this by left-clicking on the Y-Axis (the cursor will become a ruler) and selecting different type.

DC Operating point:

syntax: .op

see Vbe Multiplier and Temp Compensation for an example.