Very Compact and Low-Profile LTCC Unbalanced-to-Balanced Filters With Hybrid Resonators

In this paper, two design approaches are presented for realizing very compact and low-profile unbalanced-balanced filters with special configuration called hybrid resonators. One of the design concepts for the unbalanced-balanced filter is based on the folded hybrid resonators. This filter consists of two hybrid resonators, which are folded face to face symmetrically. The capacitance between two resonators is generated by folding the resonators. This approach makes design of compact and low-profile filters possible. The other design concept for the unbalanced-balanced filter consists of resonators that have capacitive and inductive inverters. The feature of this filter is that phase-lag is expressed by inductive coupling. This filter also achieves very low profile and compact size. The dimensions of the two filters fabricated by low temperature co-fired ceramic are 1.6 mm × 1.05 mm × 0.67 mm and 1.6 mm × 0.8 mm × 0.56 mm, respectively. For the simulation results, the maximum insertion losses for the first and second filters are 2.2 and 2.3 dB in the 2.4-GHz band, respectively. Common mode rejection for both is more than 22 dB in the same band. The amplitude imbalance and phase imbalance for the first filter are less than 1.8 dB and 6.8° in the passband, respectively. The amplitude imbalance and phase imbalance for the second filter are less than 1.9 dB and 1.8° in the passband, respectively. For both design methods, good agreement between measured and computed results is obtained.