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Problem

How do I obtain 3dB Bandwidth of a bandpass Filter?

Solution

Use the marker() function in output equations to calculate the 3D bandwidth of a filter.  

  1. Add two Auto Search markers. Right click inside the graph > Add Auto Search Marker, select Value, and enter -3. Click somewhere on the left side of the trace to add the first 3dB point.
  2. Similarly add another -3db marker on the right side of the trace for second 3dB point
  3. In Output Equations, add the following equations to get the values of markers

    Fl=marker("Filter Response","m1")

    Fh=marker("Filter Response","m2")

    BW_3dB=(Fh[1]-Fl[1])/1e9

    BW_3dB is divided by 1e9 to get the values in GHz as the output equations returns values in base unit and for frequency it is Hz.

  4. Plot the BW_3dB output equation in a graph as shown below