Spurious free independently controllable compact quad-band filter using folded T-shaped stub loaded modified ring resonator.

A compact spurious free, quad-band, band-pass filter (BPF) having independently controllable and widely spaced frequency bands is proposed. The design consists of a basic BPF to which the authors introduce transmission zeros (TZs) in the frequency response using T-shaped stubs loaded with a modified ring resonator (MRR) thereby creating the four passbands. This design results in very high band-to-band isolation. The TZs are obtained by transversal signal path cancellation and also due to the loaded T-shaped stubs. The BPF consists of a stepped impedance low-pass filter which provides spurious free out of band characteristics up to 20 GHz. By changing the tapping position of the MRR, the frequencies of the TZs can be adjusted to get the desired pass band characteristics. The proposed quad-band filter provides low-insertion loss (<1 dB) at the four passbands centred at 2.4, 5.1, 7.7 and 10.1 GHz. The simulation results of the filter are verified experimentally. [ABSTRACT FROM AUTHOR]