Band-splitting diplexing-hybrid for dual-band simultaneous-observing sideband-separating heterodyne mixers

Conference item

Simultaneous multi-band observations with heterodyne receivers offer significant advantages for astronomical applications requiring broad spectral coverage or frequency phase-transfer calibration, such as very-long-baselineinterferometry (VLBI) observations conducted by the Event Horizon Telescope (EHT). However, simultaneous multi-band operation is typically achieved using free-space optical diplexers, which introduce additional loss and noise when implemented in ground-based cryogenic receiver systems.

In this work, we present a novel band-splitting diplexing-hybrid architecture that enables simultaneous dual-band observations while preserving the sideband-separating (2SB) functionality of modern superconductor–insulator–superconductor (SIS) receivers. The proposed concept extends the simultaneous-observing multiband receiver (SOMBR) approach by integrating frequency-selective diplexing functionality directly into the RF quadrature-hybrid network required by a conventional 2SB receiver. As a demonstration, a dual-band architecture covering ALMA Bands 5 and 6 (163–275 GHz) is investigated.

A broadband superconducting quadrature-hybrid chip based on a three-section branch-line coupler was designed and analysed using full-wave electromagnetic simulations. The hybrid was combined with straight rectangular waveguide sections as high-pass filters to realise an integrated diplexing-hybrid network capable of routing Band 5 and Band 6 signals to independent receiver chains while simultaneously providing the quadrature power division required for sideband separation. Simulation results demonstrate the feasibility of the proposed architecture, achieving broadband power division and frequency-selective routing across the combined Band 5+6 frequency range. Although further optimisation is required to improve amplitude balance and return-loss performance, particularly at the lower edge of Band 5, the results establish the viability of integrating simultaneous dual-band and 2SB functionality within a compact superconducting circuit. The proposed architecture provides a potential pathway towards low-loss, cryogenically compatible multi-band SIS receivers for future astronomical instrumentation.

Authors: Tan, BK

• Publication status: Accepted
• Peer review status: Peer reviewed
• Acceptance date: 2026-06-17
• Event title: International Society for Optics and Photonics (SPIE) Astronomical Telescopes + Instrumentation 2026
• Event location: Copenhagen, Denmark
• Event website: https://spie.org/conferences-and-exhibitions/astronomical-telescopes-and-instrumentation
• Event start date: 2026-07-05
• Event end date: 2026-07-10
• Language: English
• Keywords: quadrature hybrid; diplexer; superconducting chip; broadband; simultaneous observing; band-combining; multiple band operation; sideband separating; SIS mixers