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besselap
Bessel analog lowpass filter prototype.
[z,p,k] = besselap(n)
[z,p,k] = besselap(n)
returns the zeros, poles, and gain of an order n Bessel analog lowpass filter prototype. It returns the poles in the length n column vector p and the gain in scalar k. z is an empty matrix, because there are no zeros. n must be less than or equal to 25. The transfer function is
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 [1]. The magnitude of the filter is less thansqrt(1/2) at the unity cutoff frequency 
c = 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
besselap finds the filter roots from a look-up table constructed using the Symbolic Math Toolbox.