Steeply Sloped UWB Bandpass Filter Based on Stub-Loaded Resonator

This letter introduces an ultrawideband (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 lambda/2-length stub-loaded resonator. The stub-loaded resonator consists of lambda/8 short-circuited and open-circuited stubs at the center of the lambda/2 line. The two stubs provide two attenuation poles at lower and upper cutoff frequencies. For input and output lines, two lambda/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- and 86-dB/GHz slopes in the lower and upper transition bands, respectively, and realized with low-temperature co-fired ceramic (LTCC) green tape that has the dielectric constant of 7.8. Measurement results agree well with HFSS simulation results. Insertion loss less than 1.1 dB and return loss better than 14 dB in the whole passband have been measured. The group delay at the center frequency is 0.26 ns, and the group delay variation within the passband is less than 0.5 ns. The size of the filter is 6 x 18 x 0.6 mm(3).