Broadband Quadrature Hybrid for Image Rejection in Millimeter-wave Receivers

This paper presents the design architecture of millimeter-wave receiver in which the un-desired image frequency band can be rejected at the IF stage using broadband quadrature hybrid. This scheme requires quadrature de-modulation/ down-conversion of the received signal at the front end; followed by quadrature hybrid at IF stage. The image rejection ratio (IRR) depends on the amplitude imbalance and phase imbalance of this quadrature hybrid. The design of C-Band broadband quadrature hybrid; based on 3-stage micro-strip cascaded branch-line topology; has been proposed for this receiver. The proposed broadband quadrature hybrid exhibits 66 % fractional bandwidth corresponding to measured amplitude balance of better than ±0.85 dB, phase imbalance better than 3 degrees, return loss and isolation better than 15 dB, in the frequency range (4-8 GHz). This quadrature hybrid shows tight 3-dB quadrature coupling response in much wider band; as compared to earlier published design schemes, to the best of our knowledge. This quadrature hybrid integrated in the proposed millimeter wave receiver (32-36 GHz) achieved IRR better than 20 dB in the whole frequency band.