Low-loss tunable filters with three different fractional-bandwidth variations were designed and fabricated on $epsilon _{r} = 2.2, 0.787~ {hbox {mm}}$ Duroid substrates for 850–1400-MHz applications. A detailed analysis for realizing predefined bandwidth characteristics is presented, and a design technique to take into account the source and load impedance loading is discussed. It is found that independent electric and magnetic coupling makes it possible to realize three different coupling coefficient variations with the same filter structure. The proposed topology is different from the comb-line design in that all three filters have identical electrical lengths, the same varactors, and the same filter $Q$ values. Three different filters are built using Schottky varactor diodes with a tuning range from $sim{hbox {850}}$ to $sim {hbox {1400 MHz}}$. The constant fraction-bandwidth filter has a 1-dB bandwidth of ${hbox {5.4}}%pm {hbox {0.3}}%$ and an insertion loss of 2.88–1.04 dB. The decreasing fractional-bandwidth filter shows a 1-dB bandwidth decrease from 5.2% to 2.9% with an insertion loss of 2.89–1.93 dB (this is effectively a 40–45-MHz constant absolute-bandwidth filter). The increasing fractional-bandwidth filter shows a 1-dB bandwidth increase from 4.3% to 6.5% with an insertion loss of 3.47–1.18 dB. The measured ${ Q}$ of the filters are between 53–152 from $sim{hbox {850}}$ to $sim {hbox {1400 MHz}}$. The measured third-order intermodulation i-ntercept point ranges from 11.3 to 20.1 dBm depending on the bias voltage. To our knowledge, these planar tunable filters represent state-of-the art insertion-loss performance at this frequency range.
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