A compact tune-all bandpass filter based on coupled slow-wave resonators

A compact hybrid tune-all bandpass filter based on coupled slow-wave resonators is demonstrated. The performance of such a filter electronically tuned with commercially available low-cost semiconductor varactors is promising in terms of center-frequency (f_c) and bandwidth wide continuous tunings. Indeed, the -3 dB bandwidth of this filter can be tuned between ∼50 MHz and ∼78 MHz for a ± 18%-center-frequency tuning around 0.7 GHz, an insertion loss smaller than 5 dB and a return loss higher than 13 dB at the center frequency. Moreover, for a ∼50 MHz fixed bandwidth, the center frequency can be tuned between 0.51 GHz and 0.81 GHz leading to a relative ± 24%-center-frequency tuning. Finally, the total physical length of the prototype filter is about 0.27λ_c for a 0.7 GHz center frequency.