The scanning probe microscope has a primary control loop (7, 11, 12) for keeping the phase and/or amplitude of deflection at constant values as well as a secondary control loop (9) that e.g. keeps the frequency of the cantilever oscillation constant by applying a suitable DC voltage to the probe while, at the same time, a conservative AC excitation is applied thereto. By actively controlling the frequency with the first control loop (7, 11, 12) and subsequently controlling the DC voltage in order to keep the frequency constant, a fast system is created that allows to determine the contact potential difference or a related property of the sample (3) quickly.