s defined as half-power, so -3dB, not half-amplitude (-6dB)
    like for `scipy.signal.fiwin`.
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.
ftype : str, optional
    The type of IIR filter to design:

        - Butterworth   : 'butter'
        - Chebyshev I   : 'cheby1'
        - Chebyshev II  : 'cheby2'
        - Cauer/elliptic: 'ellip'

output : {'ba', 'zpk', 'sos'}, optional
    Filter form of the output:

        - second-order sections (recommended): 'sos'
        - numerator/denominator (default)    : 'ba'
        - pole-zero                          : 'zpk'

    In general the second-order sections ('sos') form  is
    recommended because inferring the coefficients for the
    numerator/denominator form ('ba') suffers from numerical
    instabilities. For reasons of backward compatibility the default
    form is the numerator/denominator form ('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
iirfilter : General filter design using order and critical frequencies

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

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

>>> wp = 0.2
>>> ws = 0.3
>>> gpass = 1
>>> gstop = 40

>>> system = signal.iirdesign(wp, ws, gpass, gstop)
>>> w, h = signal.freqz(*system)

>>> fig, ax1 = plt.subplots()
>>> ax1.set_title('Digital filter frequency response')
>>> ax1.plot(w, 20 * np.log10(abs(h)), 'b')
>>> ax1.set_ylabel('Amplitude [dB]', color='b')
>>> ax1.set_xlabel('Frequency [rad/sample]')
>>> ax1.grid(True)
>>> ax1.set_ylim([-120, 20])
>>> ax2 = ax1.twinx()
>>> phase = np.unwrap(np.angle(h))
>>> ax2.plot(w, phase, 'g')
>>> ax2.set_ylabel('Phase [rad]', color='g')
>>> ax2.grid(True)
>>> ax2.axis('tight')
>>> ax2.set_ylim([-6, 1])
>>> nticks = 8
>>> ax1.yaxis.set_major_locator(matplotlib.ticker.LinearLocator(nticks))
>>> ax2.yaxis.set_major_locator(matplotlib.ticker.LinearLocator(nticks))

r�