PLL-based high-speed demodulation of FM signals for real-time AFM applications

In this paper we present a new architecture for PLL-based high-speed demodulation of frequency-modulated AFM signals. In our approach, we use single-sideband frequency up-conversion to translate the AFM signal from the position sensitive detector to a fixed intermediate frequency of 10 MHz. In this way, we fully benefit from the excellent noise performance of PLL-based FM demodulators still avoiding the intrinsic bandwidth limitation of such systems. Furthermore, the system becomes independent of the cantilever´s resonance frequency. To investigate if the additional noise introduced by the single-sideband upconverter degrades the system noise figure we present a model of the AM-to-FM noise conversion in the PLL phase detector. Using this model, we can predict an upper corner frequency for the demodulation bandwidth above which the converted noise from the single-sideband upconverter becomes the dominant noise source and therefore begins to deteriorate the overall system performance. The approach is validated by measured data obtained with a PCB-based prototype implementing the proposed demodulator architecture.