A Two-Pole Two-Zero Tunable Filter With Improved Linearity

Two miniaturized tunable filters with two zeros to result in sharp skirts were developed at 1.4-2.0 GHz on epsilon(r) = 6.15 substrate. The filters were built using single and back-to-back varactor diodes and compared for linearity characteristics. The single-diode filter has a 1-dB bandwidth of 5 +/- 0.5% and an insertion loss of 2.5-1.8 dB. The back-to-back diode filter has a 1-dB bandwidth of 4.9 +/- 0.5% and an insertion loss of 2.9-1.25 dB (resonator Q of 56-125). The back-to-back diode filter was designed for improved linearity characteristics using two uniformly doped varactor diode and biased with a high impedance circuit. A detailed Volterra series analysis is done on the back-to-back diode including the effect of the bias network and diode mismatches. The measured IIP3 for the back-to-back diode tunable filter is 22-41 dBm depending on the bias voltage and is 13-15 dB better than the single-diode design. The power handling capabilities of both designs is explored using large-signal S-21 measurements. To our knowledge, these planar tunable filters represent state-of-the-art insertion loss and linearity characteristics performance with varactor diodes as the tuning elements.