Polarization Conversion from Linear to Circular for VLBI and to Stokes Parameters in the DBBC
Koyel Das, Alan Roy; MPIfR, Bonn and Gino Tuccari; IRA-INAF, Noto
Abstract:
VLBI requires that each station respond with high purity to circular polarization and this is most commonly achieved using an orthomode transducer ahead of the first low-noise amplifier. However, such polarizers have narrow fractional bandwidth, which conflicts with the increasing demand from spectroscopic users for front ends with octave bandwidths to reach interesting lines, since such wide bandwidth is most easily achieved with linear polarizers.
To enable the design of a single broad-band receiver to meet the needs of both VLBI and spectroscopy, we are demonstrating the conversion from linear to circular polarization at IF in the DBBC. At the heart of the conversion is an equivalent to the Hilbert transform, which introduces an accurate 90 degree phase shift between the two linears over an arbitrarily broad fractional bandwidth. Preceding the 90 degree phase shift is an equalizer to measure and match the amplitude and phase response of the two linear polarization channels. Following the 90 degree phase shift is planned a D-term correction, in which a small amount of one polarization will be subtracted from the other and the output power minimized by adjusting the amplitude and phase response of an adaptive filter. The result should be circular polarization with high purity and zero leakage, which will be passed on in the time domain to the later existing stages of processing in the DBBC. A straightforward extension would be conversion to Stokes parameters in parallel, providing in addition a broad-band multi-channel polarimeter for single-dish observations.
I will present the design concept and status of the project.