Synthesis of extremely flat broadband multi-section quadrature coupler

Abstract:
Future broadband communication systems can be realized by employing broadband-matched modulator and amplifier architectures, which offer potential for broadband firststage IF signals at lower RF frequency ranges. Quadrature couplers are adapted for broadband power division and 90deg phase shifting, e.g. for image rejection mixers. We present the synthesis equations and a broadband quadrature hybrid with an extremely flat pass band characteristic, based on a novel approach of stub-line-matched multi-section branch-line couplers. The presented hybrid has been simulated and experimentally demonstrated in entirely planar microstrip technology on a 35 µm copper-cladded FR4 material. The structure is tuned towards broad bandwidth with respect to a maximal flat phase response and transmission behaviour. We achieve more than an octave bandwidth in a frequency range of 2.5-5.8 GHz.