Planar Wideband High-Selectivity Impedance-Transforming Differential Bandpass Filter With Deep Common-Mode Suppression

• Published in: IEEE transactions on circuits and systems. II, Express briefs Vol. 67; no. 10; pp. 1914 - 1918
• Main Authors: Chen, Weijuan, Wu, Yongle, Wang, Weimin
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.10.2020; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Band-pass filters; Bandpass filter (BPF); Bandpass filters; Bandwidths; Broadband; Chebyshev approximation; common-mode suppression; differential; Equivalent circuits; Excitation; Filtering theory; Impedance; impedance transformation; Selectivity; Transfer functions; Transmission lines; Wideband
• ISSN: 1549-7747; 1558-3791
• DOI: 10.1109/TCSII.2019.2959814

In this brief, a novel balanced filter named impedance-transforming differential bandpass filter (ITDBPF) is proposed, which is the first differential device to realize both wideband impedance transformation and bandpass filtering simultaneously. A core block consisting of cascaded transmission lines and parallel stubs forms this presented circuit with two additional open stubs. Under differential-mode excitation, the transfer function of the equivalent circuit is the same as the one of Chebyshev, and the filtering characteristics are obtained corresponding to the specified filtering parameters. The open stubs increase multiple transmission zeros under common-mode excitation to greatly broaden the bandwidth of common-mode rejection. To verify the proposed theory, a wideband balanced filter centered at 2 GHz with a theoretical 3-dB bandwidth of 72% is designed and measured to prove the correctness of the design theory.