A tunable LTCC fourth‐order bandpass filter with high selectivity for L‐band satellite applications

This Letter presents a varactor‐loaded double‐layer coupled stripline resonator to implement compact fourth‐order tunable bandpass filter. The whole structure of the proposed resonator consists of two strongly coupled striplines, a metal post and a varactor with bias circuit. Strong capacitive coupling between the striplines at different layers is introduced to significantly reduce the resonator size. A detailed investigation on the relationship between the transmission line length and loading capacitance is presented. The stripline is connected to the varactor by the metal post so that the resonant frequency can be tuned. Through cross‐coupled technology, transmission zeros are introduced in the upper and lower out‐of‐bands of the filter using combline structure for high selectivity. One prototype bandpass filter has been designed to validate the principle, and the measured results show good agreement with the simulated ones. The designed fourth‐order filter has a size of <0.1 λg × 0.1 λg and a steep frequency response with the shape factor <1.7.