All the sources with non-zero AC values are treated as sinusoidal sources with the specified peak voltage/current amplitudes and phases and default zero offsets. You can view or edit the AC values of voltage and current sources by double clicking on the parts and selecting the "Small-Signal AC and Distortion" tab of their property dialog. Make sure to check the "Use" checkbox in the section titled "AC Properties for Small-Signal AC Properties only". Given the specified voltage and current source values, the DC operating point of your circuit is calculated, and all nonlinear elements are replaced with their small-signal circuit models. If the DC operating point of your circuit with the sources shorted out is not where you want it, you may have to place some constant (DC) voltage sources in the circuit to compensate. Alternatively, you can set a nonzero value for the "Small-Signal Offset" parameter of an AC source as shown in the figure below.
Like the DC analysis, you must specify the [[parameters]] of the AC Sweep Test before you can execute it. The Start frequency and Stop frequency specify the range of frequencies for the analysis. The analysis starts at start frequency, stops at stop frequency, and takes place at a number of frequency samples in between. There are three Interval Types: Linear, Decade and Octave. The default scale is "Decade". In a linear scale, you have to specify the "Step Size", which determines the frequency increment. Starting from the Start Frequency, the frequency of the AC source is consecutively incremented by the Step Size until it exceeds the Stop Frequency and the sweep process is terminated. In the case of Decade or Octave interval types, you need to specify the Number of Steps per Interval. A decade is a range from one power of 10 to the next, for example, from 100 to 1000. The points in this case are equally spaced on a log scale. An octave I a range form one power of 2 to the next, for example, from 1 to 2 or from 2 to 4. To actually run the AC analysis, you must click the Run button in the Test Setup for the type of test (Single, Sweep, or Monte Carlo) after specifying the AC Sweep options as described above.
The default results of an AC frequency sweep test consist of plots of the magnitude and phase of the voltage across each voltmeter in the circuit and the magnitude and phase of the current through each ammeter as a function of frequency. The magnitude can be displayed in either decibels or simply magnitude (on a linear scale). A decibel is 20*log10(abs(x)), where x is a real or complex value. For instance, 1000 corresponds to 60 decibels (db), 100 corresponds to 40 db, and 0.1 corresponds to -20db. You can customize the results by choosing any node voltage or device current using "Preset Graph Plots" and "Preset Table Plots". To learn how you can define output voltages or currents anywhere in your circuit, refer to the section on [[B2_output#Setting_Up_Circuit_Observables | Setting Up Circuit Observables]]. You may also place three types of AC voltage probe: voltage probe (dB), voltage probe (Mag) and voltage probe (Ph-Deg), or three types of AC current probe: current probe (dB), current probe (Mag) and current probe (Ph-Deg) in your circuit to plot their signal as a function of frequency. To learn more about probes, see the section on [[B2_output#Using_Probes_or_Meters_as_Observables | Using Probes or Meters as Observables]]. The default scale used for the plotted graphs at the end of an AC frequency sweep test is base-10-log along the x-axis. If you prefer a linearly scaled frequency axis, change the "Bottom Axis" settings of the graph using the "Edit Axes" tab of the Toolbox in the graph view.
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