  We investigate sideband spectroscopy of a trapped ion using a probe laser
phase modulated at the trap drive frequency. The enhanced sensitivity of our
technique over traditional sideband spectroscopy allows us to detect stray
fields of $0.01\,\mathrm{V/m}$ on a timescale of a few minutes and detect
differential phases of $5\,\mu\mathrm{rad}$ between applied ac potentials. We
also demonstrate the ability suppress Doppler shifts from excess motion to well
below the limit imposed by the intrinsic motion of the ion in the vibrational
ground-state. The technique we introduce can be readily implemented in any ion
trap system that utilizes sideband spectroscopy for micromotion compensation
and can be seamlessly integrated into experiments in a fully automated way
