A Novel Analytical Technique to design a Tunable Bandpass Filter with Constant Bandwidth

In this paper, a new tunable bandpass filter with constant absolute bandwidth and ultra wide tuning range is proposed. First, the bandwidth of two conventional circuits is theoretically approximated in a new and simple way. Then one of these two is selected since it does not have any spurious bandwidth in a very wide range of frequencies and its bandwidth is half of the other, which causes a smaller capacitance and less circuit dimensions. Then a tuning lumped-based circuit is designed with bandwidth equal to 0.528 GHz, which covers all ultra wide band (UWB) of frequency tuning range, namely 3.1-10.6 GHz as an example. Finally, it is assembled by transmission line elements and HFSS software tools simulate it. The center frequency tuning range of the circuit is 137% between 3.85 and 9 GHz with constant absolute bandwidth equal to 0.52 ± 0.06 GHz. The compact size of the filter is 5 mm × 3 mm and its insertion loss is smaller than 3.5 dB across the whole tuning range. Simulations show that its behavior agrees well with the ideal equivalent lumped-based circuit over a very wide range of frequencies.