scipy.signal.cheby2¶

scipy.signal.
cheby2
(N, rs, Wn, btype='low', analog=False, output='ba')[source]¶ Chebyshev type II digital and analog filter design.
Design an Nthorder digital or analog Chebyshev type II filter and return the filter coefficients.
Parameters: N : int
The order of the filter.
rs : float
The minimum attenuation required in the stop band. Specified in decibels, as a positive number.
Wn : array_like
A scalar or length2 sequence giving the critical frequencies. For Type II filters, this is the point in the transition band at which the gain first reaches rs. For digital filters, Wn is normalized from 0 to 1, where 1 is the Nyquist frequency, pi radians/sample. (Wn is thus in halfcycles / sample.) For analog filters, Wn is an angular frequency (e.g. rad/s).
btype : {‘lowpass’, ‘highpass’, ‘bandpass’, ‘bandstop’}, optional
The type of filter. Default is ‘lowpass’.
analog : bool, optional
When True, return an analog filter, otherwise a digital filter is returned.
output : {‘ba’, ‘zpk’, ‘sos’}, optional
Type of output: numerator/denominator (‘ba’), polezero (‘zpk’), or secondorder sections (‘sos’). Default is ‘ba’.
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
Secondorder sections representation of the IIR filter. Only returned if
output=='sos'
.Notes
The Chebyshev type II filter maximizes the rate of cutoff between the frequency response’s passband and stopband, at the expense of ripple in the stopband and increased ringing in the step response.
Type II filters do not roll off as fast as Type I (
cheby1
).The
'sos'
output parameter was added in 0.16.0.Examples
Plot the filter’s frequency response, showing the critical points:
>>> from scipy import signal >>> import matplotlib.pyplot as plt
>>> b, a = signal.cheby2(4, 40, 100, 'low', analog=True) >>> w, h = signal.freqs(b, a) >>> plt.semilogx(w, 20 * np.log10(abs(h))) >>> plt.title('Chebyshev Type II frequency response (rs=40)') >>> plt.xlabel('Frequency [radians / second]') >>> plt.ylabel('Amplitude [dB]') >>> plt.margins(0, 0.1) >>> plt.grid(which='both', axis='both') >>> plt.axvline(100, color='green') # cutoff frequency >>> plt.axhline(40, color='green') # rs >>> plt.show()