High Performance Single-Ended Wideband and Balanced Bandpass Filters Loaded With Stepped-Impedance Stubs

High-performance single-ended wideband and balanced bandpass filters loaded with stepped-impedance stubs are proposed in this paper. For the proposed wideband filter and the differential mode of the proposed balanced filter, two transmission zeros near the passband are realized by using simple stepped-impedance stubs, which enhances the passband selectivity of these new filters. The analytical and accurate design equations for transmission poles and zeros of these single-ended and balanced filters are provided based on the scattering-parameters theory. The single-ended bandpass filter, employing a T-shaped structure to produce a wide passband, is achieved with 3-dB fractional bandwidth of 79.33%, 19-dB passband return loss, and 35-dB harmonic suppression (<inline-formula> <tex-math notation="LaTeX">$1.56f_{\mathrm { {0}}}$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$2.51f_{\mathrm { {0}}}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$f_{\mathrm { {0}}}$ </tex-math></inline-formula> is the center frequency of the wide passband). The 3-dB differential-mode fractional bandwidth of 33% with 22-dB passband return loss and the 30-dB harmonic suppression (<inline-formula> <tex-math notation="LaTeX">$1.29f_{\mathrm { {0}}}$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$2.67f_{\mathrm { {0}}})$ </tex-math></inline-formula> are obtained in the proposed balanced bandpass filter. In addition, by inserting open-circuit stubs into the proposed balanced bandpass filter, over 25-dB common-mode suppression is obtained from 0 to <inline-formula> <tex-math notation="LaTeX">$2.67f_{\mathrm { {0}}}$ </tex-math></inline-formula>. Good agreement is observed between the simulation and measurement results.