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Module « scipy.signal »

Fonction cheb2ord - module scipy.signal

Signature de la fonction cheb2ord

def cheb2ord(wp, ws, gpass, gstop, analog=False, fs=None) 

Description

help(scipy.signal.cheb2ord)

Chebyshev type II filter order selection.

Return the order of the lowest order digital or analog Chebyshev Type II
filter that loses no more than `gpass` dB in the passband and has at least
`gstop` dB attenuation in the stopband.

Parameters
----------
wp, ws : float
    Passband and stopband edge frequencies.

    For digital filters, these are in the same units as `fs`. By default,
    `fs` is 2 half-cycles/sample, so these are normalized from 0 to 1,
    where 1 is the Nyquist frequency. (`wp` and `ws` are thus in
    half-cycles / sample.)  For example:

        - Lowpass:   wp = 0.2,          ws = 0.3
        - Highpass:  wp = 0.3,          ws = 0.2
        - Bandpass:  wp = [0.2, 0.5],   ws = [0.1, 0.6]
        - Bandstop:  wp = [0.1, 0.6],   ws = [0.2, 0.5]

    For analog filters, `wp` and `ws` are angular frequencies (e.g., rad/s).
gpass : float
    The maximum loss in the passband (dB).
gstop : float
    The minimum attenuation in the stopband (dB).
analog : bool, optional
    When True, return an analog filter, otherwise a digital filter is
    returned.
fs : float, optional
    The sampling frequency of the digital system.

    .. versionadded:: 1.2.0

Returns
-------
ord : int
    The lowest order for a Chebyshev type II filter that meets specs.
wn : ndarray or float
    The Chebyshev natural frequency (the "3dB frequency") for use with
    `cheby2` to give filter results. If `fs` is specified,
    this is in the same units, and `fs` must also be passed to `cheby2`.

See Also
--------
cheby2 : Filter design using order and critical points
buttord : Find order and critical points from passband and stopband spec
cheb1ord, ellipord
iirfilter : General filter design using order and critical frequencies
iirdesign : General filter design using passband and stopband spec

Examples
--------
Design a digital bandstop filter which rejects -60 dB from 0.2*(fs/2) to
0.5*(fs/2), while staying within 3 dB below 0.1*(fs/2) or above
0.6*(fs/2). Plot its frequency response, showing the passband and
stopband constraints in gray.

>>> from scipy import signal
>>> import matplotlib.pyplot as plt
>>> import numpy as np

>>> N, Wn = signal.cheb2ord([0.1, 0.6], [0.2, 0.5], 3, 60)
>>> b, a = signal.cheby2(N, 60, Wn, 'stop')
>>> w, h = signal.freqz(b, a)
>>> plt.semilogx(w / np.pi, 20 * np.log10(abs(h)))
>>> plt.title('Chebyshev II bandstop filter fit to constraints')
>>> plt.xlabel('Normalized frequency')
>>> plt.ylabel('Amplitude [dB]')
>>> plt.grid(which='both', axis='both')
>>> plt.fill([.01, .1, .1, .01], [-3,  -3, -99, -99], '0.9', lw=0) # stop
>>> plt.fill([.2,  .2, .5,  .5], [ 9, -60, -60,   9], '0.9', lw=0) # pass
>>> plt.fill([.6,  .6,  2,   2], [-99, -3,  -3, -99], '0.9', lw=0) # stop
>>> plt.axis([0.06, 1, -80, 3])
>>> plt.show()

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