Compact LTCC balun filter using folded half-wavelength resonators

This paper presents the design of compact balanced-to-unbalanced (balun) bandpass filters (BPFs) using the low temperature co-fired ceramic (LTCC) technology. Three coupled half wavelength transmission-line are used to form the proposed circuit. The balun filter is designed to work at the center frequency of 5.5GHz with the fractional bandwidth of 9.8%. The size of the circuit is only 2.2mm*2.7mm*1.2mm due to the LTCC multilayer structures. Good balun and filter responses are obtained in this design which verifies the theoretical predications.