Miniaturized tunable bandstop filter using multilayer technology

A new type of miniaturized tunable bandstop filter (BSF) based on multilayer technology is presented in this letter. One of the transmission zero could be individually tuned by replacing an outer replaceable element, which can be considered as part of the inner resonant cell of the bandstop filter. Thus, a tradeoff between the stopband rejection depth and bandwidth can be conveniently realized according to different applications. A bandstop filter works from 3.1 to 4.2 GHz has been designed and fabricated in a low temperature cofired ceramic (LTCC) substrate to verify the proposed concept. By virtue of the LTCC multilayer technology, the filter occupies a very compact volume of 2.0 mm × 1.2 mm × 0.8 mm.