Power spectral density of the phase variation Δφ obtained considering the externally (upper panel) and the internally (lower panel) driven damped harmonic oscillator with noise. The black line represents the results obtained for a single realization (the same that was considered in Figs 2 and 3), while the green line represents the average over 100 realizations. The hypotenuse of the red triangle has a f⁻² slope characteristic of red noise. Notice that the transition from red noise to white noise (flat spectrum) at f ∼ η/(2|$\pi$|⁠) (the blue vertical line in the upper panel is defined by f = η/(2|$\pi$|⁠)) does not happens in the modified case.