We wanted to check and make sure that the relative phase of the two inputs ( local oscillator and photodiode signal ) to the demod board is such that the Q output is maximised. We displayed the I and Q signals on the oscilloscope in XYmode with I along the X direction. If Q is maximised (and therefore I is minimised) the oscillocope trace would be perfectly vertical since all the signal would be in Q and none in I. Initially we noted that the trace was slightly rotated to the CCW of the vertical and that a short cable was present in the PD input line. Removing this rotated the trace CW and made it pretty much vertical. The screen shot of the oscilloscope is below.