You are now ready to analyze the circuit. The first type of analysis to be done is ac analysis. This allows us to see the behavior of the circuit when steady state ac signals are applied. It allows us to plot gain, phase, and other circuit variables as a functions of frequency. To start an AC Analysis go to Analysis in the menu at the top of the screen then select AC Analysis from the drop down menu, or press Alt+2

This will bring up the following menu.

This is the AC analysis limits window which allows you to specify what you want to look at in the analysis. The example above contains the default values that the program inserts as a starting point. The frequency is set for a range from 10⁸ Hz down to 1 MegaHz.  You must change this to the frequency range you wish to use.  The format is Upper limit, Lower limit.  The other values in the middle section can be left alone at this time, but you might want to come back and change the Maximum Change % after looking at the graph produced.  If the graph has sharp changes in the curve, then lowering the value of the Max. % change form point to point will cause the program to calculate more intermediate points and give a smoother curve.

The section at the bottom of the screen controls what variables or functions you will observe. The default is to plot a graph of the voltage at node 1 converted to dB and the phase of v(1) verses the frequency using a log frequency scale.  The entries in the table are column 1 Plot or graph number in the default example above both the gain (dB) and the phase are to be plotted on graph 1, which is the only graph requested here.  The X expression for both is, F, for frequency.  The Y Expression is db(v(1)) for the first line this tells the program to calculate the values of the voltage at circuit node 1 and convert them to dB and plot them.  The second Y Expression is Ph(v(1)), which tells the program to calculate the phase angle of the voltage at node one and plot it as the second curve.  The push buttons at the left indicate other how the data is to be plotted an it the values should be saved to a file.  The left most button is set showing vertical grid lines indicating the x axis and the lines have logarithmic spacing indicating the x axis is to use a log scale.  The second button shows horizontal grid lines indicating the y axis. These lines are evenly spaced indicating that the y axis should use a linear scale.  Clicking on these buttons toggles between log and linear plotting scales.

The third button shows the color that is to be used to plot that curve.  Clicking on this button brings up a palette of colors from which you can choose if you do not like the default color.  The fourth button is the numeric output button.  It shows a simulated table of values.  clicking on this button will cause its appearance to toggle between a button that is out and a button that is pushed in.  When it is pushed in it causes the data from this line to be written to an output file.  The file name will be the same as the name of the circuit file, but the file extension will be .ANO for Ac Numeric Output file instead of .CIR for the circuit file.  This output file may be printed, displayed, or imported into another program such as a spread sheet or word processor.

Below is a sample of a modified analysis limits window, with the two curves plotted on different graphs.  The Y Expressions have been changed to indicate the values to be plotted for a particular circuit.  In this case the voltage gain in dB between node 1 and node 2 of the circuit and the phase of the voltage at node 2.  This assumes that the phase of the voltage at node 1 is zero.  If since a sine source with a zero phase angle is connected to node 1 in this circuit.  If this were not the case hen the phase difference between the signals at the two nodes could be obtained using the expression,  Ph(V(2)) - Ph(V(1)), as the Y Expression.

Note, that the X Range and Y Range columns have upper and lower limit values instead of the AUTO that was in the previous AC Analysis Limits window.  This is because the simulation has been run and the AUTO specifier caused the program to calculate the upper and lower limits and insert them and adjust the graph accordingly.  The second Y range shows no lower limit since the lower limit was 0.  The Fmt column specifies the numeric format to be used for displayed values.  The two check boxes above the X and Y Ranges add additional control over the simulation.  the Operating Point box selects pre calculation of the DC operating point or Q-Point.  This should usually be used for all AC analysis runs.  The Auto Scale Ranges box should be selected if you have changed something in your circuit or in the limits that will effect the range of values in the graph.  You could also replace the numeric values in the Range columns with the word auto.  The difference is that the Auto Scale Ranges box will cause the ranges to be auto scaled for every simulation that you run until you uncheck it.  The word auto typed in a specific range box will cause that particular range to be auto scaled on the next RUN at which time the numeric values will replace the word auto and no auto and be used on following runs.

After completing an analysis run you can return to the circuit window by pressing the F3 key.  at this point you can observe the DC node voltages, which had to be calculated in order to do the analysis.  Use the icon next to the one that turns the node numbers on and off.  to turn node voltage display on and off.