Frequency-Tunable Quasi-Elliptic Filter Using Liquid Metal

A fluidically-tuned substrate integrated waveguide (SIW) quasi-elliptic bandpass filter using reconfigurable TE
₁₀₁
mode cavity resonators is presented. A quadruplet filter with a pair of finite transmission zeros is presented using cross-coupled cavities to achieve a highly selective passband. The four SIW cavities have vias near their corners that are selectively filled with eutectic gallium indium (EGaIn) to tune the resonant frequency of the cavities. A symmetric via placement is implemented to maintain the par of finite transmission zeros equally spaced on either side of the passband. Measured results indicate that the filter's center frequency can be discretely tuned between 7.96 GHz, 8.12 GHz, 8.24 GHz and 8.39 GHz, indicating a 400 MHz tuning range while maintaining a symmetric and sharp roll-off. The filter was designed and fabricated on a 1.54 mm thick Rogers 4003 substrate (ε
_r
= 3.55, tan δ = 0.0027).