scipy.signal._filter_design:iirdesign

Parameters

wp, ws : float or array like, shape (2,)

Passband and stopband edge frequencies. Possible values are scalars (for lowpass and highpass filters) or ranges (for bandpass and bandstop filters). 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. 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). Note, that for bandpass and bandstop filters passband must lie strictly inside stopband or vice versa. Also note that the cutoff at the band edges for IIR filters is 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.