  Adaptive optics (AO) is a technique allowing to drastically improve
ground-based telescopes angular resolution. The wavefront sensor (WFS) is one
of the key components of such systems, driving the fundamental performance
limitations. In this paper, we focus on a specific class of WFS: the
Fourier-filtering wavefront sensors (FFWFS). This class is known for its
extremely high sensitivity. However, a clear and comprehensive noise
propagation model for any kind of FFWFS is lacking. Considering read-out noise
and photon noise, we derive a simple and comprehensive model allowing to
understand how these noises propagates in the phase reconstruction in the
linear framework. This new noise propagation model works for any kind of FFWFS,
and allows to revisit the fundamental sensitivity limit of these sensors.
Furthermore, a new comparison between widely used FFWFS is held. We focus on
the two main used FFWFS classes: the Zernike WFS (ZWFS) and the pyramid WFS
(PWFS), bringing new understanding of their behavior.
