Design Bessel filter of Order with CutoffFreq frequencies and of FilterType type. Set Analog to True, to request an analog filter design in s-plane or set it to false to obtain a digital filter design in z-plane. CutoffFreq must be in range between 0 and 1 (Sampling frequency = 2). 

IIrFrequencyTransform specifies when and how will the frequency band transformation be applied. Bessel filters typically do not preserve a flat group delay once transformed to z-domain. The routine uses bilinear transformation to map from s to z-domain. Use matched-Z transform to preserve more phase properties of the Bessel filters in the z-domain. The resulting transfer function is returned in the zero-pole form, with z,p,k variables.