  We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a
balanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding
to AM-PM induced phase noise 60 dB below the single-sideband relative intensity
noise of the laser. This enables us to generate 8 GHz microwave signals from a
commercial Er-fibre comb with a single-sideband residual phase noise of -131
dBc/Hz at 1 Hz offset frequency and -148 dBc/Hz at 1 kHz offset frequency.
