Direct Synthesis of Quad-Band Band-Pass Filter by Frequency Transformation Methods

This article presents a method to design a quad-band band-pass filter. It is based on a frequency transformation function allowing the direct transformation of a low-pass prototype in a quad-band band-pass filter. By defining the low and high frequencies of each pass-band, all the parameters of the frequency transformation (resonant angular frequencies and susceptance slope parameters) can be retrieved analytically. To validate the method, a third-order Chebyshev filter is designed and fabricated using microstrip technologies. Very good agreements are achieved between theoretical responses and measurements.