Design optimization and implementation of bandpass filters with normally fed microstrip resonators loaded by high-permittivity dielectric

Different approaches for designing bandpass filters with transmission zeroes are investigated by using the transmission-line theory and the finite-difference time-domain method. The filters are composed of capacitively coupled uniform impedance microstrip resonators, stepped-impedance microstrip resonators, and tapped feed lines. Different design modifications are discussed and, as a result, a design with double-coupled resonators is proposed. Based on this structure, a miniaturized filter capacitively loaded by high-permittivity dielectric inclusions and fabricated by using low-temperature co-fired ceramic (LTCC) technology is presented. The measured and simulated S-parameter spectra of the LTCC-filter are in good agreement.