Switchable Triple-Band Filter with Common Resonators and Hybrid Coupling Scheme

This work presents a compact tri-band microstrip coupled filter by using common resonators and hybrid coupling scheme. The proposed filter utilizes common stepped-impedance resonators shared by two of the three passbands to reduce the required resonators and circuit size. Although common resonators are used, the proposed filter utilizes hybrid coupling scheme such that the coupling paths of the three passbands are independent, except the external quality factors of the lower band and middle band. Therefore, the central frequencies and bandwidths of the three passbands can be designed nearly individually. The concurrent tri-band filter is then modified to obtain a switchable tri-band filter by adding suitable diode switching circuits. The three passbands are controlled by different switching mechanisms and each passband can be switched on or off independently.