scipy.signal.cheb1ord#

scipy.signal.cheb1ord(wp, ws, gpass, gstop, analog=False, fs=None)[source]#

Chebyshev type I filter order selection.

Return the order of the lowest order digital or analog Chebyshev Type I filter that loses no more than gpass dB in the passband and has at least gstop dB attenuation in the stopband.

Parameters:

wp, wsfloat

Passband and stopband edge frequencies.

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. (wp and ws are thus in half-cycles / sample.) 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).

gpassfloat

The maximum loss in the passband (dB).

gstopfloat

The minimum attenuation in the stopband (dB).

analogbool, optional

When True, return an analog filter, otherwise a digital filter is returned.

fsfloat, optional

The sampling frequency of the digital system.

Added in version 1.2.0.

Returns:

ordint: The lowest order for a Chebyshev type I filter that meets specs.
wnndarray or float: The Chebyshev natural frequency (the “3dB frequency”) for use with cheby1 to give filter results. If fs is specified, this is in the same units, and fs must also be passed to cheby1.

See also

cheby1: Filter design using order and critical points
buttord: Find order and critical points from passband and stopband spec
cheb2ord, ellipord
iirfilter: General filter design using order and critical frequencies
iirdesign: General filter design using passband and stopband spec

Examples

Design a digital lowpass filter such that the passband is within 3 dB up to 0.2*(fs/2), while rejecting at least -40 dB above 0.3*(fs/2). Plot its frequency response, showing the passband and stopband constraints in gray.

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

>>> N, Wn = signal.cheb1ord(0.2, 0.3, 3, 40)
>>> b, a = signal.cheby1(N, 3, Wn, 'low')
>>> w, h = signal.freqz(b, a)
>>> plt.semilogx(w / np.pi, 20 * np.log10(abs(h)))
>>> plt.title('Chebyshev I lowpass filter fit to constraints')
>>> plt.xlabel('Normalized frequency')
>>> plt.ylabel('Amplitude [dB]')
>>> plt.grid(which='both', axis='both')
>>> plt.fill([.01, 0.2, 0.2, .01], [-3, -3, -99, -99], '0.9', lw=0) # stop
>>> plt.fill([0.3, 0.3,   2,   2], [ 9, -40, -40,  9], '0.9', lw=0) # pass
>>> plt.axis([0.08, 1, -60, 3])
>>> plt.show()

../../_images/scipy-signal-cheb1ord-1.png