Compact Microstrip Filter With Third-Order Quasi-Elliptic Bandpass Response

In this paper, a novel microstrip filter is proposed, which can realize a third-order quasi-elliptic bandpass response. In order to reveal the physical mechanism of the microstrip filter, a lumped-element equivalent circuit is set up and the equivalence relations between its circuit elements and the electrical parameters of the microstrip filter are presented. According to filter specifications, the initial structural parameters of the microstrip filter can be easily calculated through these equivalence relations. As demonstration, an actual microstrip filter was designed, fabricated and measured, whose center frequency is at 2.50 GHz and fractional bandwidth is 36%. There are two transmission zeros near to the passband to improve the frequency selectivity, which makes the roll-off rate at each side of the passband as high as 125 and 50 dB/GHz, respectively. The out-of-band suppression from 3.6 to 6.1 GHz is greater than 40 dB. The size of the fabricated filter is 0.21λg × 0.18λg, which is very compact.