Bessel analog lowpass filter prototype
Frequency Response of an Analog Bessel Filter
Design a 6th-order Bessel analog lowpass filter. Display its magnitude and phase responses.
[z,p,k] = besselap(6); % Lowpass filter prototype [num,den] = zp2tf(z,p,k); % Convert to transfer function form freqs(num,den) % Frequency response of analog filter
n — Filter order
Filter order, specified as an integer scalar.
n must be less
than or equal to 25.
z — Zeros
Zeros of the filter prototype, returned as an empty matrix.
is empty because there are no zeros.
p — Poles
Poles of the filter prototype, returned as a length-
k — Gain
Gain of the filter prototype, returned as a scalar.
besselap function finds the filter roots from a lookup table constructed using Symbolic Math Toolbox™ software.
Analog Bessel filters are characterized by a group delay that is maximally flat at zero frequency and almost constant throughout the passband. The group delay at zero frequency is
besselap function normalizes the poles and gain so that at low
frequency and high frequency the Bessel prototype is asymptotically equivalent to the
Butterworth prototype of the same order . The magnitude of the filter is less than at the unity cutoff frequency Ωc = 1. The transfer
function is expressed in terms of
 Rabiner, L. R., and B. Gold. Theory and Application of Digital Signal Processing. Englewood Cliffs, NJ: Prentice-Hall, 1975, pp. 228–230.
C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.
Usage notes and limitations:
Filter order must be a constant. Expressions or variables are allowed if their values do not change.
Introduced before R2006a