Lithium Niobate mmWave Bandpass Filter With a Minimum IL of 1.12 dB

A millimeter-wave (mmWave) bandpass filter (BPF) based on single-crystal lithium niobate (LN) with multiple transmission zeros (TZs) is proposed in this work, which consists of four coplanar waveguide (CPW) resonators embedded with T-shaped slots and two T-shaped stubs for miniaturization and high stopband suppression. By leveraging the LN’s small loss tangent and large permittivity, the filter achieves exceptional performance in terms of selectivity, miniaturization, and cost efficiency. The LN thickness is optimized to balance the tradeoff between compact size and the cutoff frequency of the first high-order mode. Each T-shaped slot creates parallel inductance and capacitance, reducing the resonator size while generating a TZ in the upper stopband. Simultaneously, the dual-mode T-shaped stub can create two TZs in the lower stopband, further enhancing selectivity. The fabricated mmWave BPF has a measured center frequency of 30.26 GHz, a fractional bandwidth of 17%, a minimum insertion loss of 1.12 dB, and a compact size of 2 mm x 1.7 mm x 0.16 mm. These features demonstrate the LN-based mmWave BPF as a promising solution with high selectivity, compact size, and low cost for 5G/6G mmWave terminal applications.