  We present a scanning tunneling microscopy (STM) technique to simultaneously
measure the topography, the local tunnel barrier height (dI/dz) and the
differential conductivity (dI/dV). We modulate the voltage and tip piezo with
small sinusoidal signals that exceed the cut-off frequency of the STM
electronics and feed the tunneling current into two lock-in amplifiers (LIAs).
We derive and follow a set of criteria for the modulation frequencies to avoid
any interference between the LIA measurements. To validate the technique, we
measure Friedel oscillations and the subtle tunnel barrier difference between
the hcp and fcc stacked regions of the Au(111) herringbone reconstruction.
Finally, we show that our method is also applicable to open feedback loop
measurements by performing grid I(V) spectroscopy.
