Lumped-Element Realization of Absorptive Bandstop Filter With Anomalously High Spectral Isolation

In this paper, we show a new absorptive banstop filter topology that is capable of creating large attenuation using low-Q small-size resonators. In addition, the implementation of a lumped-element absorptive bandstop filter is shown for the first time. Compared with the conventional absorptive filter structure, the new absorptive filter structure is smaller in size because there is no quarter-wavelength transmission line between two resonators and the resonators are lumped elements. For verification of the new topology, a lumped-element low-temperature co-fired ceramic (LTCC) bandstop filter with low-Q resonators has been designed and measured. Theory, simulation, and measurement showed good agreement between them, and the measurement showed 60-dB attenuation level at the center frequency. This attenuation level of the absorptive bandstop filter is 50 dB larger than the one obtained from the reflective bandstop filter with the same Q-factor and bandwidth. The small size and absorptive nature of the filter allow us to cascade the filters to create many different filter responses. It is shown that the lumped-element implementation makes the filter very amenable to realization of higher order responses in small form factors.