Step 1: Calculate order of filter

The first step in designing a simple Butterworth filter is to determine the order (N) of the filter. The factors that affect the order of your filter are:

• Out of band attenuation - A (expressed in dB)
• Amount of ripple allowed in-band - R (expressed in dB)
• The frequencies (Fr and Fx, respectively) at which these specifications must be met, and their relation to the 3dB cutoff frequency.

The applet below allows you to enter values in the text boxes for each of the paramters mentioned above. If you are not concerned with passband ripple, you can leave those values unchanged. Once the values are entered, the applet automatically calculates the order of the filter based on the equations for passband ripple with respect to cutoff frequency:

and attenuation with respect to an arbitrary frequency Fx:

Note: if you wish to enter frequency values in GHz, click on the "GHz" button in the applet.

Use your own design values, or try the following values:

• Fc = 100 MHz
• Fx = 500 MHz
• AdB = 48
• Leave Fr and RdB as they are.

Click on the "Next >" button in the applet to continue with your design.