Compact LTCC balun filter with high level harmonic suppression using discriminating coupling

This paper presents a compact low temperature co-fired ceramic (LTCC) balun filter with the second harmonic suppression. It is composed of two half-wavelength resonators and three feeding lines. The 180° phase difference between two output ports is obtained owing to the out-of-phase characteristic at the two open ends of the half-wavelength resonator. By using discriminating coupling between feeding lines and resonators, the second harmonic is rejected. The coupling coefficient in the passband can be tuned to the desired value through the gap. A LTCC balun filter centered at 4.6 GHz is implemented. Good agreement between the simulated and measured results is observed. The stopband rejection below 34 dB is from 1.2f₀ to 2.6f₀ and the core circuit size is 3.3 mm × 2.7 mm × 1.6 mm or 0.122λ_g × 0.1λ_g× 0.059λ_g.