Digital Orthomode Transducer

Status: Mature

Introduction

An orthomode transducer (OMT) is a device that separates the received astronomical signal into two orthogonal linearly-polarized components. By measuring the apparent power of the signals at probes in different angles, one can compute the real polarization angle of the input signal. One of the main problems with this kind of devices in RF is that slight errors in manufacture, or phase/amplitude mismatch in the RF components can lead to improper measurements in the probes, most notably measuring a slight amount of power when the signal polarization is orthogonal with the probe angle (no power measurement is expected in this case). This type of effect is called 'cross-polarization'.

In a similar way to the digital calibration of the Sideband Separating Receiver, the components mismatch along with the manufacturing errors can be corrected in the digital domain, using a set of measured constants. With the application of this technique one can calibrate OMTs up to the numerical precision of the digital system used, for which we have achieved over 40dB of cross-polarization isolation.

Documentation

• First proposal of digital calibrated OMT with thorough mathematical derivation: https://arxiv.org/abs/1008.3501

• Andres Alvear's M.Sc. Thesis on Digital OMT in ROACH2: http://www.das.uchile.cl/lab_mwl/publicaciones/Tesis/cf_alvear_ac.pdf

• Roberto Fuentes' Engineering Thesis on radiation pattern measurement with Digital OMT (spanish): http://www.das.uchile.cl/lab_mwl/publicaciones/Tesis/memoria_roberto_fuentes.pdf

• Following papers of MWL od Digital OMT:

  • https://ui.adsabs.harvard.edu/abs/2016SPIE.9914E..1EA/abstract

Code and Models

• ROACH2 models and bofs (no scripts): https://github.com/FrancoCalan/simulink_models/tree/master/Digital_OMT/ROACH2