Design and Fabrication of Three-Bit Reconfigurable Bandpass Filter Using Branch-Line Type Variable Resonator

Takumi KATO
Kazuhiro AZUMA
Yasushi YAMAO

IEICE TRANSACTIONS on Electronics   Vol.E98-C    No.7    pp.636-643
Publication Date: 2015/07/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E98.C.636
Type of Manuscript: Special Section PAPER (Special Section on Microwave and Millimeter-Wave Technology)
Category: Microwaves, Millimeter-Waves
bandpass filter,  transmission line,  reconfigurable circuit,  quality factor,  coupling coefficient,  

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Current mobile communication terminals are equipped with multiple RF circuits that cover all frequency bands assigned for the communication. In order to make efficient use of frequency spectrum and to reduce circuits in a terminal, a low-loss reconfigurable RF filter is necessary to flexibly change RF frequencies. In this paper, a new reconfigurable bandpass filter (BPF) having eight-frequency (three-bit) selection capability is proposed. It employs branch-line switched type variable resonators that provide low insertion loss. One of the design issues is how to control pass bandwidths among selectable frequencies. In order to analyze the bandwidth variation of the reconfigurable BPF, we calculate the changes of external Q and coupling coefficients. It is shown that the inductive coupling design can achieve less variation of bandwidth for the reconfigurable BPF, compared with commonly used capacitive coupling design. A prototype BPF on a printed circuit board with high dielectric constant substrate has been fabricated and evaluated in 2 GHz bands. It presents performance very close to the design results with respect to insertion loss, center frequency and passband bandwidth. Low insertion loss of less than 1 dB is achieved among the eight frequencies.