Balanced Bandpass Filter With Common-Mode Reflectionless Feature by Terminated Coupled Lines
T. C. Lin, Tzong‐Lin Wu
Abstract
Based on the terminated coupled-line pairs, this article reports a novel design method to synthesize a common-mode (CM) reflectionless all-stop filter with differential mode (DM) bandpass characteristic for balanced radio-frequency (RF) transmitter. With the adoption of symmetric property, the equivalent circuit models for both DM and CM are developed for the design purpose. The main contribution of this article is the development of a systematic impedance matching method to CM noise. Compared with conventional approach like adjusting the transmission zeros, the CM noise power would be dissipated on the terminators (resistors) rather than reflected. For practical validation, two butterworth microstrip prototypes (2nd-order/4th-order) were manufactured and measured in the 0-5.0 GHz range. It would be found that the measured CM absorption rate is over 88% from 0 to 5 GHz for both samples. Besides, the DM center frequency locates at 1.0 GHz with the measured 3-dB fractional bandwidth of 56% and 65%, respectively. To author's best knowledge, this work is the first RF balanced bandpass filter that is capable of absorbing the CM noise at all frequencies.