We report the design and cryogenic characterisation of a broadband superconducting 90◦ intermediate-frequency (IF) hybrid covering 4–20 GHz for sideband-separating (2SB) heterodyne receivers. The hybrid is implemented in thin-film niobium using capacitively loaded edge coupled transmission lines, avoiding the air-bridge interconnections required in Lange-coupler layouts while retaining a compact superconducting implementation with low loss and good quadrature balance. Cryogenic measurements using Vector Network Analyser (VNA) show amplitude and phase imbalance better than 0.6 dB and 6◦, respectively, across the full band. When used in a 2SB receiver, this level of IF balance is consistent with the observed image-rejection performance in the 10–20 dB range, for the full receiver. The design supports the development of wide-instantaneous-bandwidth 2SB receivers for millimeter and submillimeter observatories, improving spectral grasp for broadband line surveys and high-redshift line searches.
Joint et al. (Thu,) studied this question.