m ('ba'), where the 'b'
    and the 'a' in 'ba' refer to the commonly used names of the
    coefficients used.

    Note: Using the second-order sections form ('sos') is sometimes
    associated with additional computational costs: for
    data-intense use cases it is therefore recommended to also
    investigate the numerator/denominator form ('ba').

fs : float, optional
    The sampling frequency of the digital system.

    .. versionadded:: 1.2.0

Returns
-------
b, a : ndarray, ndarray
    Numerator (`b`) and denominator (`a`) polynomials of the IIR filter.
    Only returned if ``output='ba'``.
z, p, k : ndarray, ndarray, float
    Zeros, poles, and system gain of the IIR filter transfer
    function.  Only returned if ``output='zpk'``.
sos : ndarray
    Second-order sections representation of the IIR filter.
    Only returned if ``output='sos'``.

See Also
--------
butter : Filter design using order and critical points
cheby1, cheby2, ellip, bessel
buttord : Find order and critical points from passband and stopband spec
cheb1ord, cheb2ord, ellipord
iirdesign : General filter design using passband and stopband spec

Notes
-----
The ``'sos'`` output parameter was added in 0.16.0.

Examples
--------
Generate a 17th-order Chebyshev II analog bandpass filter from 50 Hz to
200 Hz and plot the frequency response:

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

>>> b, a = signal.iirfilter(17, [2*np.pi*50, 2*np.pi*200], rs=60,
...                         btype='band', analog=True, ftype='cheby2')
>>> w, h = signal.freqs(b, a, 1000)
>>> fig = plt.figure()
>>> ax = fig.add_subplot(1, 1, 1)
>>> ax.semilogx(w / (2*np.pi), 20 * np.log10(np.maximum(abs(h), 1e-5)))
>>> ax.set_title('Chebyshev Type II bandpass frequency response')
>>> ax.set_xlabel('Frequency [Hz]')
>>> ax.set_ylabel('Amplitude [dB]')
>>> ax.axis((10, 1000, -100, 10))
>>> ax.grid(which='both', axis='both')
>>> plt.show()

Create a digital filter with the same properties, in a system with
sampling rate of 2000 Hz, and plot the frequency response. (Second-order
sections implementation is required to ensure stability of a filter of
this order):

>>> sos = signal.iirfilter(17, [50, 200], rs=60, btype='band',
...                        analog=False, ftype='cheby2', fs=2000,
...                        output='sos')
>>> w, h = signal.freqz_sos(sos, 2000, fs=2000)
>>> fig = plt.figure()
>>> ax = fig.add_subplot(1, 1, 1)
>>> ax.semilogx(w, 20 * np.log10(np.maximum(abs(h), 1e-5)))
>>> ax.set_title('Chebyshev Type II bandpass frequency response')
>>> ax.set_xlabel('Frequency [Hz]')
>>> ax.set_ylabel('Amplitude [dB]')
>>> ax.axis((10, 1000, -100, 10))
>>> ax.grid(which='both', axis='both')
>>> plt.show()

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