AC Frequency Sweep Test


The AC test is used to view circuit behavior at different input frequencies. For example, a low-pass filter circuit could be analyzed to determine the cut off frequency for the filter, that is, the frequency above which the magnitude of the output is negligible.

Before running the analysis, B2 Spice shorts out all of the non-constant voltage and current sources that have zero for their AC magnitudes. The sources with non-zero AC values are treated as sinusoidal sources, with their specified peak to peak voltages/currents and phases, and zero offsets. (You can view/edit the AC values of voltage and current sources by double clicking on the components.)

With these voltage and current source values, the DC operating point is calculated, and all non-linear elements are replaced with their small signal models. If the DC operating point of the circuit with the sources shorted out is not where you want it, you may have to place some constant voltage sources in the circuit to compensate. Also, make sure that the voltage and current sources used as inputs have non-zero AC voltage/current values.

AC Parameter Sweep

The parameterized AC sweep runs a set of AC sweep analyses with a parameter changing its value for each AC sweep. In the example below, the capacitance of C3 is being swept from the set value of 1n to 5n at 1n increments.  The setup parameters and resulting graph are shown.

param_ac_graph.gif (7487 bytes)

param_ac_ckt.gif (2685 bytes)

param_ac_setup.gif (7533 bytes)