Stepped-impedance resonators with diodes loaded at one end are used to develop switchable bandpass filters in this paper. The loaded diodes are used to switch the resonance conditions of the stepped-impedance resonators. The equations for resonance conditions of the stepped-impedance resonators with different loads at one end are derived and discussed. With these derived equations, the switchable filters can be easily designed and synthesized using the coupled-resonator filter theory. When the switchable filter is switched on, a bandpass filter response with a wide stopband rejection is achieved by making the on-state coupled resonators have the same fundamental resonant frequency, but different higher order resonant frequencies. When switched off, a high and wideband isolation is obtained by properly misaligning the resonant modes of the off-state resonators. The design concept is demonstrated by two single-pole-single-throw fourth-order Butterworth-type switchable microstrip bandpass filters, which utilize two and three switched stepped-impedance resonators, respectively. Finally, a compact single-pole-double-throw switchable microstrip bandpass filter using common resonators is demonstrated for wireless communication applications.Index Terms-Coupled-resonator bandpass filter, steppedimpedance resonator, switch, switchable filter.

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A compact coupled-line bandpass filter with multiple capacitive cross-couplings to create four transmission zeros is proposed. The locations of transmission zeros can be adjusted by varying the values of cross-coupled capacitances so as to improve the filter selectivity. The proposed filter is based on one-eighthwavelength ( 8) coupled-line sections and quarter-wavelength ( 4) resonators, thus it occupies only half of the circuit area comparing with the conventional coupled-line bandpass filters. Specifically, a fourth-order microstrip coupled-line bandpass filter centered at 2.46 GHz with a 3-dB bandwidth of 10.2% and four transmission zeros is implemented and examined.

A compact coupled-line bandpass filter with two capacitively cross-coupled paths to create multiple transmission zeros is proposed. The locations of transmission zeros can be adjusted by varying the values of cross-coupled capacitances so as to improve the filter selectivity. Specifically, a 4th-order microstrip coupled-line bandpass filter centered at 4.9 GHz with a 3-dB bandwidth of 6% and six transmission zeros is implemented and examined.