Phase-Locked Loop design applied to frequency-modulated atomic force microscope

The atomic force microscope (AFM) introduced the surface investigation with true atomic resolution. In the frequency modulation technique (FM-AFM) both the amplitude and the frequency of oscillation of the micro-cantilever must be kept constant even in the presence of tip–surface interaction forces. For that reason, the proper design of the Phase-Locked Loop (PLL) used in FM-AFM is vital to system performance. Here, the mathematical model of the FM-AFM control system is derived considering high order PLL. In addition a method to design stable third-order Phase-Locked Loops is presented.

Research highlights
► FM-AFM nonlinear model shows the complex interaction between the cantilever and PLL.
► Proper PLL design improve the FM-AFM control system performance.
► Third order PLL is more effective to filter the Double Frequency Jitter.