A miniaturized open-loop resonator filter constructed with floating plate overlays

This work presents a new technique that uses floating plate overlays to create an open-loop resonator bandpass filter characterized by a compact size and four controllable transmission zeros to achieve multispurious suppression. Three floating plate overlays are used to cover parts of the open-loop resonator filter to increase the coupling between resonators and move the transmission zeros to the desired frequencies to enhance harmonic suppression. A design procedure developed based on an equivalent circuit model of such new type of filter is proposed. Two experimental prototypes are designed and fabricated to verify the proposed design method. The measured results agree well with the simulations. The measured insertion losses of the prototypes in the passband are all less than 2 dB. One prototype is designed to suppress the third, fourth, and fifth harmonics with suppression greater than 30 dBc. Another prototype can suppress second, third, and fourth order harmonics below 20 dBc. In addition, the prototype circuit areas are only about 50% of the conventional open-loop filter.