A calibrated digital sideband separating spectrometer for radio astronomy applications

High image rejection receivers are of particular interest at places where the atmospheric noise contribution is high or when radio frequency interference is present. It is also important when the radio source is complex having many/broad spectral lines. Unavoidable gain and phase imbalances strongly limits the achievable sideband rejection ratio of analog instruments to about 10-20dB for high sensitivity, broadband receivers. The increasing power of digital processing hardware has opened the door for a new approach which is based on performing the IF recombination using digital technology. This method allows correcting the amplitude and phase imbalances of the analog front end in the digital back end to achieve high sideband rejection ratios.

We are working in the implementation of a digital sideband separating FFT spectrometer using a FPGA-based platform called the ROACH (Reconfigurable Open Architecture Computing Hardware). The ROACH is developed by the CASPER group (Berkeley). Two ROACH boards were acquired and fitted with Virtex-5 chips thanks to the donation of Xilinx Inc. The results of the research were very positive showing an important improvement with respect to current analog technology. The next step is to test the new spectrometer on the 1.2m mm-wave telescope and to search for new instruments to integrate the technology.