Three-Pole Tunable Filters With Constant Bandwidth Using Mixed Combline and Split-Ring Resonators

A novel three-pole tunable bandpass filter using mixed varactor-tuned combline and split-ring resonators is proposed in this letter. A tuning range of 1.1-1.88 GHz (70.9%) with an almost constant 3 dB absolute bandwidth of 90 ±8 MHz (7.4%-4.4% fractional bandwidth) is demonstrated. The filter shows its insertion loss varying from 6.8 dB to 4.3 dB and return loss better than 13.5 dB within the tuning range. The rejection levels at both the lower and upper stopbands are > 30 dB. The measured results show good agreement with the simulated ones.