Miniaturized Microstrip Quasi-Elliptical Bandpass Filters Using Slotted Resonators

Novel two-pole microstrip bandpass filters consist of two slotted resonators with elliptical frequency responses are proposed. Due to two coupling paths that are equal-magnitude and nearly 180deg out-of-phase at certain frequencies, the filter has several transmission zeros in the stopband, resulting in excellent out-of-band response. The elliptical frequency response can be achieved without increasing the circuit area comparing to traditional two-pole open-loop resonator filters. In addition, some of the transmission zeros can be tuned to locate at certain frequencies to reject possible interference by adjusting the structural parameters of the slotted resonator, which hardly affects the passband performance. Furthermore, a tunable transmission zero can be moved to suppress the spurious passband. Consequently, the proposed filters not only have compact size but also have pretty good out-of-band response