Conversion from linear to circular polarization in FPGA in real time

ABSTRACT Radio astronomical receivers are now expanding their frequency range to cover large (octave) fractional bandwidths for sensitivity and spectral flexibility, which makes the design of good analogue circular polarizers challenging. Better polarization purity requires a flatter phase response over increasingly wide bandwidth, which is most easily achieved with digital techniques. They offer the ability to form circular polarization with perfect polarization purity over arbitrarily wide fractional bandwidths, due to the ease of introducing a perfect quadrature phase shift. In analogue systems the quadrature phase shift is not accurate in the regions away from the design point or frequency. In digital systems on the contrary, it is possible to introduce the exact quadrature phase shift vectorially to each frequency point in the band thus producing a perfect quadrature phase shift throughout the band. Further, the rapid improvements in field programmable gate arrays provide the high processing power, low cost, portability and reconfigurability needed to make practical the implementation of the formation of circular polarization digitally. It will be possible to carry out broadband polarization observations. Circular polarization is used in very long baseline interferometry (VLBI) due to geometrical and stability considerations. VLBI is often used to explore polarization of radio emission, which often occurs due to synchrotron mechanism, Zeeman effect in atoms and molecules, cyclotron radiation and plasma oscillations in the solar atmosphere. Also VLBI finds application in methods like rotation measure synthesis that can be used to find the magnetic field strength and whose multiwavelength observations determine the direction of magnetic field. So a digital circular polarizer would find a considerable application in VLBI systems. Here I explore the performance of a circular polarizer implemented with digital techniques. I designed a digital circular polarizer in which the intermediate frequency signals from a receiver with native linear polarizations were sampled and converted to circular polarization. The frequency-dependent instrumental phase difference and gain scaling factors were determined using an injected noise signal and applied to the two linear polarizations to equalize the transfer characteristics of the two polarization channels. This equalization was performed in 512 frequency channels over a 500 MHz bandwidth. Circular polarization was formed by quadrature phase shifting and summing the equalized linear polarization signals. I obtained polarization purity of -58 dB corresponding to a D-term of 0.0012 over the whole bandwidth. This value of D-term is an upper limit. This technique enables construction of broad-band radio astronomy receivers with native linear polarization to form circular polarization for VLBI.