A novel wide-stopband dual-band bandpass filter with a modified coupled line and lowpass filters is proposed. The modified coupled line is realised with two open stubs added to the conventional coupled line's open ends. With this proposed circuit, additional transmission zeros will appear around two passbands. Moreover, these predictable transmission zeros can be utilised to tune the two passbands' bandwidths. Furthermore, with the lowpass filter embedded, located at the input/output port, a wide stopband can be obtained. Finally, agreement between measurement and the theoretical prediction validates the proposed structure.
A compact wideband fully tunable filter is designed based on a quarter-mode integrated waveguide resonator. With only two varactor diodes at the port gap, the notch frequency can be tuned continuously in the entire passband range, and the interference signal in the band can be flexibly suppressed. In addition, by controlling the on and off of two PIN diodes, switching between the two bandwidths of 1.7 GHz and 3.3 GHz is achieved without affecting the notch frequency tuning in the bandwidth. The notch frequency tuning range is 4 GHz to 6.4 GHz. The insertion loss at the notch frequency is -15dB to -30dB, which is sufficient to eliminate in-band noise.
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