Chemical differentiator model. (A) Input–output relation of the chemical differentiator. Phase shift between input and output signals is shown. Within the differentiator band, the phase shift ϕ approximately equals 90°, with a tolerance of 30°. (B) Amplitude gain as function of the input frequency. For input frequencies 0 < f < 6 Hz (the differentiation band is in gray), the amplitude gain ρ increases linearly. (C) Model Ca²⁺ signal used as input for the chemical differentiator (green) and the output parameter C (purple) resulting from the model. (D) Time derivative of the input Ca²⁺ signal and the output C perfectly superimposes. a.u., arbitrary unit.