Tunable Microstrip Combline L-Band Filter Design, Fabrication and Characterization

A third order tunable microstrip combline bandpass filter is designed, simulated, fabricated and measured. Varactor diodes provides the electrical tuning, resulting in almost 1 GHz sweep range near the center of the L band. An alternative varactor feeding setup is proposed, which simplifies the fabrication and allows to increase the filter efficiency. A bandwidth of nearly 40 MHz, resulting in less than 3 % FBW is obtained. Insertion loss results in excess of 10 dB over the full sweep range. Filter performance hindrance is evaluated by adequately modeling varactors Q variation and its series resistance nonlinear behavior with frequency. Proper simulation through the use of the Spice model in co-simulation between 3D EM and nonlinear environment resulted of outmost need.