Varactor Tuned Dual-Band Bandpass Filter With Independently Tunable Band Positions

A dual-band bandpass filter with different even-mode and odd-mode resonant paths is presented. The band position can be tuned independently. The basic filter structure is a stub-loaded ring resonator. A varactor diode placed in the ring controls the odd-mode resonant frequency. A second varator connects the stub to the ring and controls the even-mode resonant frequency. A transmission line-based approach is used for theoretical analysis. A prototype filter is fabricated in the microstrip technology. Measured results show that the first band is tunable over 0.69–0.88 GHz with a maximum insertion loss of 1.83 dB. The second passband can be tuned over 2.67–3.78 GHz. Maximum passband insertion loss is 2.02 dB. Use of just two tuning elements makes the structure low cost and compact.