Architecture of highly integrated cryogenic active planar OrthoMode transducer for the 3-mm band

We describe the design concepts of two highly integrated cryogenic receiver modules based on "active" planar Orthomode Transducers (OMTs) for dual-polarization radio astronomy observations across 75-116 GHz (3-mm band). Both OMTs share common features and comprise active parts with MMIC (Monolithic Microwave Integrated Circuit) Low Noise Amplifier (LNA) chips, each with ≈20 dB gain, and DC power supply boards located inside a mechanical assembly consisting of three blocks. The mechanical structure incorporates a circular waveguide input (diameter 3 mm) that transitions the signals to four symmetrical double-balanced microstrip probes, two rectangular waveguide 1800 power combiners, a stripline crossover section and two oval waveguide outputs (external dimensions of 2.85×1.27 mm 2 ). The MMIC LNA chips are electrically close to the OMT circular waveguide input, thus resulting in low insertion loss before the RF signal amplification, which is key to achieve low-noise receiver performance. The two planar OMT designs differ on the MMIC number (one with 8 MMICs, the other with 6 MMICs) and on the output waveguide transitions. In both OMT designs, the two output waveguides are parallel to the input waveguide axis; this feature is desirable for use of the OMTs in focal plane arrays in order to minimize the single-pixel footprint size. The receiver modules are designed to have a maximum footprint in the direction orthogonal to the optical axis of 44×44 mm².