Creating a Nyquist Plot
A Nyquist plot typically is used to assess the stability of a closed-loop feedback system by plotting the gain and phase of a system’s frequency response using polar coordinates.
An AC analysis setup must be defined before attempting to create a Nyquist plot.
Follow this procedure to create a Nyquist plot:
- Select Twin Builder > Results > Create Standard Report > Nyquist Plot. The Report dialog box appears.
- In the Context section, make selections from the following field or fields, depending on the design and solution type:
- Solution – Lists the available solutions. For Nyquist Plots, select AC:AC.
- Domain – Select Sweep.
- Optimetrics setup – Lists all defined Optimetrics analysis setups.
- Select Quantities – Click to open the Select Quantities dialog box in which you can select additional quantities for plotting.
- On the Trace tab, specify the information to plot:
- For the Primary Sweep field, the default sweep is F (frequency).
- In the Category list, select the type of information to plot.
- In the Quantity list, select the value to plot. Typically this is the output signal you want to investigate.
- Optionally, in the Function list, select the mathematical function to apply to the quantity to plot.
The Signal field displays the currently specified Quantity and Function that will be plotted along the Y-axis. You can edit this field directly.
The text color shows whether the expression is valid (blue for valid expressions; red for invalid).
- Optionally, click Range Function to open the Set Range Function dialog box, in which you can select a range function to apply to the currently specified quantity and function.
- For the Base field, choose the desired quantity to plot from the Select Base Component dialog box.
- Optionally, on the Families tab, confirm or modify the sweep variables that will be plotted.
- Click New Report.
This creates a new Nyquist plot in the Project tree, displays the plot with the defined trace, and enables the Add Trace button on the Report dialog box.
The function of the selected quantity is plotted against the swept variable values or quantities you specified on a polar Nyquist graph. The plot is listed under Results in the Project tree and the traces appear under the plot. When you select the traces or plots, their properties display in the Properties dialog box. You can edit these properties directly to modify the plot.
- Optionally, add another trace to the plot by following the procedure above, using Add Trace rather than New Report.