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Rev Sci Instrum. 2011 Jul;82(7):073707. doi: 10.1063/1.3608447.

Wideband phase-locked loop circuit with real-time phase correction for frequency modulation atomic force microscopy.

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  • 1Frontier Science Organization, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan. fukuma@staff.kanazawa-u.ac.jp


We have developed a wideband phase-locked loop (PLL) circuit with real-time phase correction for high-speed and accurate force measurements by frequency modulation atomic force microscopy (FM-AFM) in liquid. A high-speed operation of FM-AFM requires the use of a high frequency cantilever which, however, increases frequency-dependent phase delay caused by the signal delay within the cantilever excitation loop. Such phase delay leads to an error in the force measurements by FM-AFM especially with a low Q factor. Here, we present a method to compensate this phase delay in real time. Combined with a wideband PLL using a subtraction-based phase comparator, the method allows to perform an accurate and high-speed force measurement by FM-AFM. We demonstrate the improved performance by applying the developed PLL to three-dimensional force measurements at a mica/water interface.

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