Division Note:

In the frequency range in which the filter amplitude characteristics remain flat, the phase shift Φ of a Butterworth filter can be approximated by

• Φ = 81 x (f/f₀) degrees for second order

• Φ = 150 x (f/f₀) degrees for fourth order

• Φ = 294 x (f/f₀) degrees for eighth order

The time delay for all filter orders is: t = (Φ/360) x (1/f₀)