Please go to "" or the website "" to see "Scanning frequency comb microscopy--A new method in scanning probe microscopy", AIP Advances, volume 8, issue 12, Paper 125203 (December 2018). This is the first publication following our announcement in a post-deadline poster at Microscopy&Microanalysis in Baltimore last August. Previously we focused a mode-locked laser on the tunneling junction of a scanning tunneling microscope (STM) to generate a microwave frequency comb with hundreds of harmonics at integer multiples of the laser pulse repetition frequency. Each harmonic sets the present state-of-the-art for narrow linewidth (e.g. 0.1 Hz for 200th harmonic at 14.85 GHz) enabling this harmonic with a power of several attowatts to be measured with 20 dB of signal/noise. We also noted that the spreading resistance in a semiconductor sample attenuates the harmonics so the carrier density at the tunneling junction may be determined by measuring this attenuation. Now we describe a new instrument in which these measurements may be made with no applied DC bias and no DC tunneling current by controlling the tip-sample distance with feedback based on the harmonics.