  Dual sideband (2SB) receivers are well suited for the spectral observation of
complex astronomical signals over a wide frequency range. They are extensively
used in radio astronomy, their main advantages being to avoid spectral
confusion and to diminish effective system temperature by a factor two with
respect to double sideband (DSB) receivers. Using available millimeter-wave
analog technology, wideband 2SB receivers generally obtain sideband rejections
ratios (SRR) of 10-15dB, insufficient for a number of astronomical
applications. We report here the design and implementation of an FPGA-based
sideband separating FFT spectrometer. A 4GHz analog front end was built to test
the design and measure sideband rejection. The setup uses a 2SB front end
architecture, except that the mixer outputs are directly digitized before the
IF hybrid, using two 8bits ADCs sampling at 1GSPS. The IF hybrid is implemented
on the FPGA together with a set of calibration vectors that, properly chosen,
compensate for the analog front end amplitude and phase imbalances. The
calibrated receiver exhibits a sideband rejection ratio in excess of 40dB for
the entire 2GHz RF bandwidth.
