Bessel analog lowpass filter prototype
[z,p,k] = besselap(n)
[z,p,k] = besselap(n) returns
the poles and gain of an order-
n Bessel analog
lowpass filter prototype.
be less than or equal to 25. The function returns the poles in the
n column vector
the gain in scalar
z is an empty matrix because there are no zeros. The transfer
besselap 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.
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
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
 Rabiner, L. R., and B. Gold. Theory and Application of Digital Signal Processing. Englewood Cliffs, NJ: Prentice-Hall, 1975, pp. 228–230.