巴特沃思低通滤波器

Question

I am not getting the desired output, it is cutting off the lower freq. no matter if I set it to 'low' or 'high'.

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

def butter_filter(data, butterCutoffFreq, order=1):
    b, a = signal.butter(order, butterCutoffFreq, 'low', analog=True)
    y = signal.lfilter(b, a, data)
    return y

"""
    White noise
"""
N = 1024
dt = np.float64(1)
y = np.random.normal(loc=0.0, scale=1.0, size=N)
t = np.arange(start=0, stop=N, step=dt)
butterCutoffFreq = 0.5 * 1/dt

amps = np.fft.rfft(y)
freqs = np.fft.rfftfreq(y.size, dt)
#plt.plot(freqs, np.abs(amps), color='b')

ampsFiltered = butter_filter(y, butterCutoffFreq)
ampsFiltered = np.fft.rfft(ampsFiltered)

plt.plot(freqs, np.abs(ampsFiltered), color='g')
plt.show()

Answer 1

Your problem is whith the param analog=True remove it and it will work.

lfilter() can only use a digital filter. It doesn't make sense to filter a digital signal (your white noise assumed regularly sampled) with an analog filter.