Multi-band tunable bandpass filter with nested switchable inductive subcircuit for multi-decade tuning

A method is introduced for implementing tunable bandpass filters with ultra-wide tuning range. This method relies on varactors for fine tuning and switchable inductive sub-circuits for course sub-band selection and is suitable for implementing filters with constant fractional bandwidth. A two-pole tunable filter is reported that utilizes this technique to achieve tunability over the 21–460 MHz frequency range using standard diode varactors and PIN diode switches. The measured bandwidth and insertion loss of this filter prototype vary in the range of 8.6–17% and 1.2–3.5 dB, respectively, within the tuning range.