Dual-Band and Wide-Stopband Single-Band Balanced Bandpass Filters With High Selectivity and Common-Mode Suppression

Novel dual-band and wide-stopband single-band balanced bandpass filters with high selectivity and common-mode suppression are presented in this paper. Stepped-impedance resonators (SIRs) are usually used for designing dual-band bandpass filters; however, they have a strong common-mode response when designing a balanced filter. To suppress the common-mode signal, a half-wavelength SIR loaded by a capacitor or a resistor has been introduced and theoretically analyzed. It is found that the capacitor can minimize the common-mode external quality factor, and the resistor can reduce the common-mode unloaded quality factor. With the use of this property, the common-mode response can be suppressed, whereas the differential-mode response is almost unaffected. This property can be easily verified by comparing the results of the balanced bandpass filters with and without loaded elements. To demonstrate the design idea, one balanced dual-band bandpass filter operating at 2.4 and 5 GHz and another balanced single-band bandpass filter with a wide stopband are designed. It was found that the common-mode suppression level of both filters can be greatly improved, and high selectivity is obtained by giving two differential-mode coupling paths.