Tunable 4-Pole Dual-Notch Filters for Cognitive Radios and Carrier Aggregation Systems

This paper presents 4-pole tunable dual-notch filters with independent frequency tuning for each notch response based on a stub-loaded resonator with asymmetrical loading capacitances. The resonator is designed based on lumped notch filters, and is folded on a three-layer ε_r = 3.55 board for compact filter size. Frequency tuning is achieved using silicon varactor diodes. The first filter (Filter A) results in measured rejection of 47-53/49-52 dB at 0.71-1.01/1.10-1.45 GHz. The second filter (Filter B) shows rejection levels of 42-48/34-39 dB at 0.76-1.05/1.80-2.12 GHz. Also, tuning in one band has virtually no effect on the other band, showing high isolation between the two notches. Application areas are in cognitive radios and carrier aggregation systems with two transmit frequencies.