High‐power VHF tunable filters with high selectivity

Abstract

A tunable helical bandpass filter with high selectivity designed for 30-88 MHz operation is presented. The two-pole tunable filter is discretely tuned by connecting PIN switched capacitor banks at the terminal ends of the helix. The filter schematic provides two finite transmission zeros located in the lower and upper stopbands, respectively. The experimental filter has an insertion loss between 1.65 and 2.75 dB with its return loss better than 13 dB. The 3-dB fractional bandwidth of the filter is 3.3%-3.7% and the shape factor (BW 30 dB /BW 3 dB ) is smaller than 5.5 over the entire tuning range. The input power handling capability is greater than 45 dBm. The measured results show good agreement with the simulated ones.Introduction: Tunable bandpass filters (BPFs) with high power, high selectivity, low loss and wide tuning range are significantly required in modern multifunction and multiband frequency-hopping transceivers to improve the anti-jamming ability of the communication system. They are especially needed in crowded high frequency (HF) and very high frequency (VHF) bands with dense electromagnetic interference from nearby transceivers. In [1,2], parallel LC tunable resonators are realised by a lumped inductor and radio frequency microelectromechanical system (RF MEMS) switched capacitor banks. However, the RF MEMS switch is not the optimum choice at the VHF band due to high cost, low reliability and high switch series resistance. To overcome the low Q of the lumped elements, helical resonator filters have been extensively utilised due to their advantages of high Q and compact size [3][4][5]. As can be seen in [3,4], however, it is difficult to realise widely tunable filters for parallel coupled helical resonators. In addition, most of the reported two-pole tunable helical filters only have transmission zeros (Tzs) in the lower or higher stopband and this leads to poor selectivity [3,5].

Single‐/dual‐band triple‐mode cavity filters with wide tunable frequency range

“…Conventional cavity filters suffer from the problem of failing to provide wide tuning ranges or multiple passbands. In recent years, the increasing demands from wideband or multiband applications have initiated research activities on cavity filters with various tunable features to solve the problem above . In the study of Kwak et al, filters characterized by slot irises for coupling between cavities are presented as a solution, but they can only offer limited passband ranging from 19.7 to 20.3 GHz.…”

“…In the study of Kwak et al, filters characterized by slot irises for coupling between cavities are presented as a solution, but they can only offer limited passband ranging from 19.7 to 20.3 GHz. In the study of Zhao et al, PIN diodes were employed to switch between passbands and to conduct tuning for the location of the desired passband, in which a wide adjustable frequency range from 38 to 88 MHz can be obtained, but it is not yet a continuously tunable bandpass filter. In Nocella et al study, a novel design using dielectric‐loaded cavities produced a good dual‐band filter.…”

“…In recent years, the increasing demands from wideband or multiband applications have initiated research activities on cavity filters with various tunable features to solve the problem above. [4][5][6][7] In the study of Kwak et al, 4 filters characterized by slot irises for coupling between cavities are presented as a solution, but they can only offer limited passband ranging from 19.7 to 20.3 GHz. In the study of Zhao et al, 5 PIN diodes were employed to switch between passbands and to conduct tuning for the location of the desired passband, in which a wide adjustable frequency range from 38 to 88 MHz can be obtained, but it is not yet a continuously tunable bandpass filter.…”