A 640–1030 MHz four-pole tunable filter with improved stopband rejection and controllable bandwidth and transmission zeros

A compact 4-pole tunable filter with improved stopband rejection is presented. Three combline resonators are employed, and the coupling between the adjacent and non-adjacent resonators are achieved by tuning varactors. It is shown that with the non-adjacent coupling path, two transmission zeroes at both sides of the passband (totally four) are present, and therefore the stopband rejection is significantly improved. This non-adjacent coupling also contributes one additional pole to the passband response. The positions of the transmission zeroes and passband bandwidth are controllable by changing the coupling values. A tunable filter at 640-1030 MHz is demonstrated with a controllable bandwidth of 8-12% and a wide stopband region. The application areas are in modern multi-standard communication systems.