Broadband high frequency differential coupler

This paper describes the design methodology of a broadband, high frequency differential coupler. The output signal of this differential coupler is proportional to the difference of 2 input ports, i.e. proportional to the differential signal of the 2 inputs. This signal is made using a hybrid structure. The overall performance of this differential coupler and the hybrid structure is characterized by the differential coupling factor and by the common mode rejection ratio. A pair of broadband impedances are used to scale the 2 incoming signals with 2 complex coefficients. A symmetric structure of two identical broadband directional couplers and a symmetric power combiner are used afterwards to obtain the differential signal. The high bandwidth of both the terminations, the directional couplers and the power combiner make it possible to obtain a differential coupling factor and a common mode rejection ratio which is frequency insensitive over several decades. Measurements demonstrate a differential coupler with a bandwidth of more than 3 decades