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Non-contact scanning probe microscopy (SPM) has developed into a powerfultechnique to image many different properties of samples. The conventionalmethod involves monitoring the amplitude, phase or frequency of a cantileveroscillating at or near its resonant frequency as it is scanned across thesurface of a sample. For high Q factor cantilevers, monitoring the resonantfrequency is the preferred method in order to obtain reasonable scan times.This can be done by using a phase-locked-loop (PLL). PLLs can be obtained ascommercial integrated circuits, but these do not have the frequency resolutionrequired for SPM. To increase the resolution, all-digital PLLs requiringsophisticated digital signal processors or field programmable gate arrays havealso been implemented. We describe here a hybrid analog/digital PLL where mostof the components are implemented using discrete analog integrated circuits,but the frequency resolution is provided by a direct digital synthesis chipcontrolled by a simple PIC microcontroller. The PLL has excellent frequencyresolution and noise, and can be controlled and read by a computer via a USBconnection.

A Hybrid Analog/Digital Phase-Locked Loop for Frequency Mode Non-contact Scanning Probe Microscopy