# 06_passive_ac: freqency domain simulation: ac

## Scope

In this simulation we are going to look at how a multi-stage RC filter behaves when stimulated with sine waves of various frequencies.

## The schematics

The single-sheet schematic contains the filter, directy usable in the PCB workflow.

Click the image to get the sch-rnd sheet; also requires this project.lht in the same directory

## SPICE: what an ac simulation is

In the ac analysis simulation a DC solution is calculated first and then a small signal sinusoidal stimulus is applied over a range of frequencies. The result is typically a graph with frequency on the X axis and gain or phase on the Y axis. The AC analysis is often used to get a transfer function.

## Preparing for simulation

The process is largely the same as in the base example of dc op point. The simulation setup in this example is called "freq domain characteristics". Below are the summary of differences compared to the trans analysis of a very similar circuit

### Modifications

Our input source is not a stable DC voltage source anymore, but an AC source. It's still a 'V' for voltage source, connected to CN1-2, but the AC field is set to 1 (volt). Because of the AC field set, a spice AC analysis will automatically use this voltage source to feed in various frequencies.

### Sim setup: output config

This simulation has two output configs, one for displaying the transfer (in decibel) and one for the phase (in radian). The reason for specify them in two separate output is the largely different y scale and unit.

The first output uses ac (dec) for analysis. This will feed in 10 different frequencies per decade and capture the output. This also means the X axis, frequency, is logarithmic (common for frequency domain analysis).

The property to plot is vdb(out), which is the "voltage decibel" of the network called out. Instead of the net name a component-port could be specified within vdb().

The second output uses anlaysis previous, which means no new simulation is ran, but the data of the previous simulation is used. The presentation is also a plot of "out", but using the cph() function, which is the phase in radian.

Simulation setup dialog, third tab, after execution