scipy.signal.besselap#
- scipy.signal.besselap(N, norm='phase')[source]#
Return (z,p,k) for analog prototype of an Nth-order Bessel filter.
- Parameters:
- Nint
The order of the filter.
- norm{‘phase’, ‘delay’, ‘mag’}, optional
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. [6]
The magnitude response asymptotes are the same as a Butterworth filter of the same order with a cutoff of 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. [1]
New in version 0.18.0.
- Returns:
- zndarray
Zeros of the transfer function. Is always an empty array.
- pndarray
Poles of the transfer function.
- kscalar
Gain of the transfer function. For phase-normalized, this is always 1.
See also
bessel
Filter design function using this prototype
Notes
To find the pole locations, approximate starting points are generated [2] for the zeros of the ordinary Bessel polynomial [3], then the Aberth-Ehrlich method [4] [5] is used on the Kv(x) Bessel function to calculate more accurate zeros, and these locations are then inverted about the unit circle.
References
[1]C.R. Bond, “Bessel Filter Constants”, http://www.crbond.com/papers/bsf.pdf
[2]Campos and Calderon, “Approximate closed-form formulas for the zeros of the Bessel Polynomials”, arXiv:1105.0957.
[3]Thomson, W.E., “Delay Networks having Maximally Flat Frequency Characteristics”, Proceedings of the Institution of Electrical Engineers, Part III, November 1949, Vol. 96, No. 44, pp. 487-490.
[4]Aberth, “Iteration Methods for Finding all Zeros of a Polynomial Simultaneously”, Mathematics of Computation, Vol. 27, No. 122, April 1973
[5]Ehrlich, “A modified Newton method for polynomials”, Communications of the ACM, Vol. 10, Issue 2, pp. 107-108, Feb. 1967, DOI:10.1145/363067.363115
[6]Miller and Bohn, “A Bessel Filter Crossover, and Its Relation to Others”, RaneNote 147, 1998, https://www.ranecommercial.com/legacy/note147.html