Frequency modulation atomic force microscopy, introduced by Albrecht, Grütter, Horne and Rugar in 1991, 
is a mode of nc-AFM where the change in resonant frequency of the sensor is tracked directly, by always exciting the sensor on resonance. To maintain excitation on resonance the electronics must keep a 90° phase difference between the excitation and response of the sensor. This is either done by driving the sensor with the deflection signal phase shifted by 90°, or by using an advanced phase-locked loop which can lock to a specific phase. The microscope can then use the change in resonant frequency (f) as the SPM reference channel, either in feedback mode, or it can be recorded directly in constant height mode.
While recording frequency-modulated images, an additional feedback loop is normally used to keep the amplitude of resonance constant, by adjusting the drive amplitude. By recording the drive amplitude during the scan (usually referred to as the damping channel as the need for a higher drive amplitude corresponds to more damping in the system) a complementary image is recorded showing only non-conservative forces. This allows conservative and non-conservative forces in the experiment to be separated.