ENBr, EQUIVALENT NOISE BANDWIDTH RATIO ENBr * f3dB Brick wall filter 1.00 1 pole filter (- 20dB/Decade) 1.57 2 poles filter (- 40dB/Decade) 1.22 3 poles filter (- 60dB/Decade) 1.15 4 poles filter (- 80dB/Decade) 1.13 5 poles filter (-100dB/Decade) 1.11 The values in the following table are for normalized lowpass filter functions with infinite Q and exact conformance to design equations. ***************************** It is easy to compute analytically the ENBR of a n poles Buttterworth filter pi ENBr = ------------ 2 n sin(pi/2) 1 pole Butterworth filter 1.5708 2 poles Butterworth filter 1.1107 3 poles Butterworth filter 1.0472 4 poles Butterworth filter 1.0262 5 poles Butterworth filter 1.0166 6 poles Butterworth filter 1.0115 7 poles Butterworth filter 1.0084 8 poles Butterworth filter 1.0065 9 poles Butterworth filter 1.0051 10 poles Butterworth filter 1.0041 ****************************** For Chebyshev, the ENBr depends of course on the ripple tou have chosen: Ripple -> 0.01dB 0.1dB 0.25dB 0.5dB 1.0dB 2 poles Chebyshev filter 3.6672 2.1444 1.7449 1.4889 1.2532 3 poles Chebyshev filter 1.9642 1.4418 1.2825 1.1666 1.0411 4 poles Chebyshev filter 1.5039 1.2326 1.1405 1.0656 0.9735 5 poles Chebyshev filter 1.3114 1.1417 1.0780 1.0208 0.9433 6 poles Chebyshev filter 1.2120 1.0937 1.0448 0.9970 0.9272 7 poles Chebyshev filter 1.1537 1.0653 1.0251 0.9828 0.9175 8 poles Chebyshev filter 1.1166 1.0471 1.0125 0.9736 0.9113 9 poles Chebyshev filter 1.0914 1.0347 1.0038 0.9674 0.9071 10 poles Chebyshev filter 1.0736 1.0258 0.9977 0.9629 0.9041 ****************************** For Bessel 1 pole Bessel filter 1.57 2 poles Bessel filter 1.56 3 poles Bessel filter 1.08 4 poles Bessel filter 1.04 5 poles Bessel filter 1.04 6 poles Bessel filter 1.04