The Scanning Microdeformation Microscope, as many other scanning probe microscopes developed in the last years, is a kind of ac force microscope using the near-field acoustic interaction. The heart of the system is an electromechanical oscillator made of a silicon cantilever, a diamond or sapphire tip, associated with a bimorph piezoelectric transducer and a specific amplifier. The specificity of the system is the way of detection of the oscillation frequency performed electrically through the admittance of the piezoelectric transducer. In this paper, we describe the technique of detection involved in the microscope. A modelling of the complete behavior of the electromechanical oscillator performed with the finite element method (FEM) (of simulation) is presented. A comparison between experimental and theoretical behavior shows a very good agreement.