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.