A two-pole tunable filter with constant fractional-bandwidth characteristics

In this paper, a tunable filter with constant fractional bandwidth is designed and fabricated on er = 2.2, h = 0.508 mm duroid substrates. A new mixed electrical and magnetic coupling scheme is introduced by this structure to achieve an overall constant coupling coefficient during the tuning range, which can meet the requirement of constant fractional bandwidth. A detailed analysis on the external quality (Qext) and coupling coefficient is presented. Two-chip capacitors (CM = 1 pF), using radio frequency (RF) and microwave capacitors, were used to enhance Qext. The Qext (22 ± 3) shows a relatively small variation over the entire tuning range. The tunable constant fractional-bandwidth filter shows a tuning range of 1.28–1.86 GHz, a 3-dB bandwidth of 5.3 ± 0.6% and an insertion loss of 5.11–2.69 dB. The measured and simulation results are in a good agreement.