A compact tunable microstrip diplexer using varactor-tuned dual-mode stub-loaded resonators

A new compact tunable microstrip diplexer based on varactor-tuned dual-mode stub-loaded resonators is investigated. By using the dual coupling paths, the two channel filters can then be designed independently. In addition, by properly varying the dc bias voltages applied to the varactors loaded onto the dual-mode resonators, the passband frequencies of the diplexer can be changed over a wide tuning range while still keeping a good in-band performance. For the demonstration, a compact tunable second-order bandpass diplexer with a size of 0.13 λ_g by 0.21 λ_g has been designed and fabricated. The measured results show that a wide frequency tuning range of about 25% is achieved, while the isolation is better than 22 dB for the two channels over the entire tuning range.