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 Ortomode Transducers (OMTs) for dual-polarization radio astronomy of observation 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 180° power combiners, a stripline crossover section and two oval waveguide outputs (external dimensions of $2.85\times 1.27 \text{mm}^{2})$. 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 $44\times 44\text{mm}^{2}$.

1. Introduction

We are designing an active Orthomode Transducer (OMT) in the framework of work package 1 of the AETHRA (Advanced European Technologies for Heterodyne Receiver for Astronomy) programme, aiming at investigating the new 35 nm gate length mHEMT (Metamorphic High Electron Mobility Transistor) technology for improving MMIC LNA performance and developing a prototype of dual polarization MMIC cryogenic receiver array to be installed on the focal plane of the IRAM 30-m telescope on Pico Veleta, Spain [1]. The integrated receiver array, including the active OMT, will be cooled at the physical temperature of ≈15 K provided by a commercial cryogenic refrigerator.