Compact LTCC bandpass filters using vertically folded half wavelength resonator

In this paper, a pair of novel compact low temperature co-fired ceramic (LTCC) bandpass filters (BPFs) based on transmission line theory are proposed. The employed half wavelength (λ_g/2) resonator has been vertically folded to form a multi-layer transmission line resonator for minimizing the circuit size significantly. Since there is a virtual ground in the symmetrical plane of the multi-layer resonator, the characteristic of the resonator keeps almost unchanged. Transmission zeros have been realized near to the passband and in the stopband, which improve the selectivity and harmonic rejection of the BPFs. For demonstration, two BPFs centered at 3.1GHz and 10GHz are designed using the proposed resonator, and the circuit sizes are 5.1×3.9×1.8 mm³ and 4.0×3.9×1.5 mm³.