Compact LTCC bandpass 180° hybrid using lumped single-to-differential and single-to-common bandpass filters

A compact LTCC (low temperature co-fired ceramic) bandpass 180deg hybrid is proposed. By incorporating the properly designed lumped single-to-differential (S-D) and single-to-common (S-C) bandpass filters into a four-port device with their output ports connected back-to-back, a novel 180deg hybrid with desired bandpass characteristic may be realized. The corresponding lumped S-D and S-C filters are realized based on the symmetric coupled resonator structures. To demonstrate the design concept, the bandpass 180deg hybrid is designed to possess a 2^nd-order Butterworth response with 10% fractional bandwidth. Specifically, the implemented bandpass 180deg hybrid shows the desired performances with low insertion-loss (<3 + 2.27 dB), good amplitude-/phase-balances (Delta < 0.5 dB and thetas < 2deg), high isolation (>43 dB), and compact size (7.8 mm*4.3 mm). Good agreement between measured and simulated results is also obtained.