Wide-frequency tunable bandpass filter with high-frequency selectivity

Based on the coupling-compensating technologies, a wide-frequency tunable bandpass filter with high-frequency selectivity has been presented in this paper. The coupling coefficient between a pair of resonator and the external quality can be easily controlled, the constant absolute bandwidth (ABW) and the characteristic of tunable wide-frequency can be achieved by changing the position of varactors. By introducing the source-load cross-coupling circuit, the bandpass filter has two transmission zeros, resulting in high-frequency selectivity for the passband. Moreover, only two varactor diodes have been used in the presented tunable bandpass filter. The measured results show that the tunable filter has an almost constant 3 dB absolute bandwidth of 90 +/- 5 MHz from 1.55 GHz to 2.00 GHz. The measured insertion loss of the tunable bandpass filter is between 2.2 dB and 3.3 dB, while the return loss is greater than 10 dB within the tunable center frequencies. The measured results of the fabricated tunable bandpass filter show good agreement with the simulated ones.