An Ultraminiaturized High-Selectivity LTCC On-Chip Filtering Balun With Ultrawide Stopband

In this letter, a generalized approach for on-chip filtering baluns with high selectivity and ultrawide stopband is proposed using the single-ended and differential lumped-distributed resonators (LDRs) in low-temperature cofired ceramic (LTCC) technology. The LDR with an inherent out-of-phase feature exhibits ultrawide harmonic suppression. Meanwhile, three transmission zeros (TZs) are generated by the LDRs and the input LC tank. Furthermore, the theoretical extensions of the arbitrary-order topology are easily realized. For verification, a filtering balun covering the B41 band with one of the smallest reported areas of $0.0028\lambda _{\text {g}}^{2}$ is designed, which exhibits a low insertion loss (IL) of 1.4 dB, amplitude and phase imbalances (PIs) of 0.4 dB and 1.4°, and a 20-dB upper stopband (USB) up to 16.5 GHz or $6.4f_{0}$ . The lower and upper roll-off factors (ROFs) are observed to reach 100 and 64 dB/GHz.