Capacitively coupled cross resonator UWB BPF with steep slopes

This paper introduces a new ultra wideband (UWB) bandpass filter (BPF) with very sharp roll-off characteristics in stripline structure for the U.S. band (3.1-10.6 GHz). The UWB characteristic is basically obtained from capacitive coupling between a main line and a λ/2 length cross resonator. At the center of the resonator, two stubs are loaded, one is λ/8 short-circuited and the other λ/8 open-circuited. The two stubs provide two attenuation poles at lower and upper cutoff frequencies. For input and output lines, two λ/4 identical capacitively coupled lines have been installed to suppress the unwanted signals in the lower and upper stopbands. The filter has been designed with two transmission zeros at 2.4 and 11.1 GHz to provide 81 dB/GHz and 86 dB/GHz slopes in the lower and upper transition bands, respectively, and realized with low temperature core-fired ceramic (LTCC) green tape which has the dielectric constant of 7.8. Measurement results agree well with HFSS simulation results. Insertion loss less than 0.6 dB and return loss better than 14 dB in the pass band have been measured. The group delay at the center frequency is 0.26 ns and the group delay variation within pass band is less than 0.5 ns. The size of the filter is 6×18×0.6 mm³.