scipy.signal._filter_design:iirfilter

Parameters

N : int

The order of the filter.

Wn : array_like

A scalar or length-2 sequence giving the critical frequencies.

For digital filters, Wn 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. (Wn is thus in half-cycles / sample.)

For analog filters, Wn is an angular frequency (e.g., rad/s).

When Wn is a length-2 sequence, Wn[0] must be less than Wn[1].

rp : float, optional

For Chebyshev and elliptic filters, provides the maximum ripple in the passband. (dB)

rs : float, optional

For Chebyshev and elliptic filters, provides the minimum attenuation in the stop band. (dB)

btype : {'bandpass', 'lowpass', 'highpass', 'bandstop'}, optional

The type of filter. Default is 'bandpass'.

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'

• Bessel/Thomson: 'bessel'

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.