New Elliptic Function Type UWB BPF Based on Capacitively Coupled $\lambda/4$ Open T Resonator

This paper introduces two new circuit structures for European and U.S. ultra-wideband (UWB) bandpass filters (BPFs) with sharp roll-off characteristics. The circuit structures consist of capacitively coupled three parts in stripline structure, a λ/4 open T resonator, a λ/4 short-circuited line, and input and output lines. We show first that ultra wide bandpass property is basically obtained from the T resonator with a capacitively coupled λ/4 short-circuited line, which provides two attenuation poles at lower and upper cutoff frequencies. Two identical capacitively coupled input/output lines, which can be λ/4 length open ends or λ/2 length short ends, with the T resonator are adopted to suppress lower and higher frequency components outside of the passband. There is coupling between the input and output lines providing two additional transmission zeros in the lower and upper transition bands of the filter. Since the coupling between the T resonator with the λ/4 short-circuited line and the input/output lines limits the bandwidth of the filter to the European UWB band, both the λ/4 short-circuited line and the input/output lines are inserted between the two stacked T resonators for the U.S. UWB band. The filter structures are simulated with ADS and HFSS and realized with low-temperature co-fired ceramic (LTCC) green tape which has the dielectric constant of 7.8. Measurement results agree well with HFSS simulation results.