  Context: 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. 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. Aims: Here
we explore the performance of a circular polarizer implemented with digital
techniques. Methods: We 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 512 MHz bandwidth.
Circular polarization was formed by quadrature phase shifting and summing the
equalized linear polarization signals. Results: We obtained polarization purity
of -25 dB corresponding to a D-term of 0.06 over the whole bandwidth.
Conclusions: This technique enables construction of broad-band radio astronomy
receivers with native linear polarization to form circular polarization for
VLBI.
