besselap(N, norm='phase')
To find the pole locations, approximate starting points are generated for the zeros of the ordinary Bessel polynomial , then the Aberth-Ehrlich method is used on the Kv(x) Bessel function to calculate more accurate zeros, and these locations are then inverted about the unit circle.
The order of the filter.
Frequency normalization:
phase
The filter is normalized such that the phase response reaches its midpoint at an angular (e.g., rad/s) cutoff frequency of 1. This happens for both low-pass and high-pass filters, so this is the "phase-matched" case.
The magnitude response asymptotes are the same as a Butterworth filter of the same order with a cutoff of :None:None:`Wn`.
This is the default, and matches MATLAB's implementation.
delay
The filter is normalized such that the group delay in the passband is 1 (e.g., 1 second). This is the "natural" type obtained by solving Bessel polynomials
mag
The filter is normalized such that the gain magnitude is -3 dB at angular frequency 1. This is called "frequency normalization" by Bond.
Zeros of the transfer function. Is always an empty array.
Poles of the transfer function.
Gain of the transfer function. For phase-normalized, this is always 1.
Return (z,p,k) for analog prototype of an Nth-order Bessel filter.
bessel
Filter design function using this prototype
The following pages refer to to this document either explicitly or contain code examples using this.
scipy.signal._filter_design.bessel
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