Four-port microwave networks with intrinsic broad-band suppression of common-mode signals

Two classes of four-port microwave networks are defined with discriminative even- and odd-mode transmission characteristics. Design of the networks is based on filter prototypes with predefined sets of transmission zeros and, consequently, a desired response can be synthesized. A configuration consisting of a pseudodifferential active circuit in cascade with either class results in a sub-system with high common-mode rejection ratio over a broad frequency range. A pseudodifferential circuit implementation demonstrated low common-mode gain over a 3:1 operating bandwidth ranging from 250 to 750 MHz.