Novel elliptic function type ultra wideband bandpass filter based on λ/4 open stripline T section

This paper introduces a novel ultra wideband bandpass filter with sharp roll-off characteristics. Basic filter characteristic is obtained from a lambda/4 open T section with capacitively coupled lambda/4 short-circuited line in stripline structure, which provides two attenuation poles at lower and upper cutoff frequencies. For the elliptic function type slope, two transmission zeros at 2.9 GHz and 9.0 GHz have been realized for the European UWB band (3.4-8.5 GHz). For input and output lines, two identical capacitively coupled lines with the T section are also used. These two lines can be lambda/4 length open ends or lambda/2 length short ends. The filter has been realized with low temperature core-fired ceramic (LTCC) green tape which has the dielectric constant of 7.8. Measurement results are well agreed with HFSS simulation results. Insertion loss less than 0.9 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 0.2 ns. The size of the filter is 10times10times0.5 mm³.