A Quasi Elliptic Function 1.75-2.25 GHz 3-Pole Bandpass Filter With Bandwidth Control

A three-pole microstrip combline filter with a center frequency of 1.75-2.25 GHz and a tunable bandwidth of 70-100 MHz is presented. In this design, an extra transmission zero is created using a non-adjacent resonator coupling, and is placed on the lower side of the passband to improve the filter selectivity. The filter bandwidth is also tuned by controlling the non-adjacent resonator coupling. The measured S-parameters, third-order intercept point and power handling, for different center frequencies and bandwidths, are presented and with good agreement with simulations. This topology is simple to implement and requires only one additional tunable component between the first and third resonator for bandwidth control.