Using Goal Functions to Find the Upper 3 dB Frequency

Using Goal Functions to Find the Upper 3 dB Frequency 

A second method can be used to find the upper 3 dB frequency found in the previous section. We will continue from the end of the previous section. First, hide the cursors by selecting Trace, Cursor, and then Display from the Probe menus. Next, select Window and then New Window to open a new Probe window  

We will now evaluate the goal function. In Probe, select Trace and then Eval Goal Function  

The right window pane lists the available goal functions. Use this list to locate the Upper3dB goal function. We would like to find the upper 3 dB frequency of the voltage trace V(VO), so enter the following Trace Expression, upper3dB(V(VO))  

Probe plots the trace V(VO), locates the 3dB point, and then marks the point on the plot. Note that the plot of V(VO) is not in decibels. The plot is generated only if you have Probe set to display the plot used for the evaluation. If this option is not set, a small dialog box will appear and tell you the result of the goal function. To check the setting, select Tools and then Options from the Probe menus  

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