Electrically varactor-tuned bandpass filter with constant bandwidth and self-adaptive transmission zeros

This paper presents a wideband frequency-agile bandpass filter (BPF) with constant absolute bandwidth (ABW) and two self-adaptive transmission zeros (TZs) at lower and higher stopbands. A frequency-dependent source-to-load (S-L) coupling is incorporated in this design so that two symmetrical TZs close to the tunable passband are self-adaptive and the separations between the passband center frequency and two TZs can almost keep the same for achieving continuous high selectivity in the frequency-tuning process. A lumped capacitor C-1 and a varactor diode are loaded to the two open ends of the half-wavelength resonator. With the aid of the employed C-1, the desired external quality factor can be obtained in a wide frequency-tuning range. Meanwhile, properly choosing the specific coupling region can make the coupling coefficient close to the desired one. As a result, the constant ABW can be obtained as the passband frequency changes. For demonstration, the proposed BPF is designed and fabricated. The simulated and measured results are presented. The results exhibit the tuning range from 430 MHz to 720 MHz with 3-dB constant ABW of around 75 MHz and the insertion loss of 1.34-2.92 dB, and the distance from TZs to center frequency is nearly constant at 150 MHz.