低通濾波器實驗程式碼，這是參考別人網上的程式碼，所以自己也分享一下，共同進步
# -*- coding: utf-8 -*-

import numpy as np
from scipy.signal import butter, lfilter, freqz
import matplotlib.pyplot as plt


def butter_lowpass(cutoff, fs, order=5):
    nyq = 0.5 * fs
    normal_cutoff = cutoff / nyq
    b, a = butter(order, normal_cutoff,
 
 btype='low', analog=False)
    return b, a


def butter_lowpass_filter(data, cutoff, fs, order=5):
    b, a = butter_lowpass(cutoff, fs, order=order)
    y = lfilter(b, a, data)
    return y  # Filter requirements.


order = 6
fs = 30.0 # sample rate, Hz
cutoff = 3.667 # desired cutoff frequency of the filter, Hz # Get the filter coefficients so we can check its frequency response.
 

b, a = butter_lowpass(cutoff, fs, order) # Plot the frequency response.
w, h = freqz(b, a, worN=800)
plt.subplot(2, 1, 1)
plt.plot(0.5*fs*w/np.pi, np.abs(h), 'b')
plt.plot(cutoff, 0.5*np.sqrt(2), 'ko')
plt.axvline(cutoff, color='k')
plt.xlim(0, 0.5*fs)
plt.title("Lowpass Filter Frequency Response")
plt.xlabel(
 
'Frequency [Hz]')
plt.grid() # Demonstrate the use of the filter. # First make some data to be filtered.
T = 5.0 # seconds
n = int(T * fs) # total number of samples
t = np.linspace(0, T, n, endpoint=False) # "Noisy" data. We want to recover the 1.2 Hz signal from this.
data = np.sin(1.2*2*np.pi*t) + 1.5*np.cos(9*2*np.pi*t) + 0.5*np.sin(12.0*2*np.pi*t) # Filter the data, and plot both the original and filtered signals.
y = butter_lowpass_filter(data, cutoff, fs, order)
plt.subplot(2, 1, 2)
plt.plot(t, data, 'b-', label='data')
plt.plot(t, y, 'g-', linewidth=2, label='filtered data')
plt.xlabel('Time [sec]')
plt.grid()
plt.legend()
plt.subplots_adjust(hspace=0.35)
plt.show()

實際程式碼，沒有整理，可以讀取txt文字檔案，然後進行低通濾波，並將濾波前後的波形和FFT變換都顯示出來

# -*- coding: utf-8 -*-

import numpy as np
from scipy.signal import butter, lfilter, freqz
import matplotlib.pyplot as plt
import os


def butter_lowpass(cutoff, fs, order=5):
    nyq = 0.5 * fs
    normal_cutoff = cutoff / nyq
    b, a = butter(order, normal_cutoff, btype='low', analog=False)
    return b, a


def butter_lowpass_filter(data, cutoff, fs, order=5):
    b, a = butter_lowpass(cutoff, fs, order=order)
    y = lfilter(b, a, data)
    return y  # Filter requirements.


order = 5
fs = 100000.0 # sample rate, Hz
cutoff = 1000 # desired cutoff frequency of the filter, Hz # Get the filter coefficients so we can check its frequency response.
# b, a = butter_lowpass(cutoff, fs, order) # Plot the frequency response.
# w, h = freqz(b, a, worN=1000)
# plt.subplot(3, 1, 1)
# plt.plot(0.5*fs*w/np.pi, np.abs(h), 'b')
# plt.plot(cutoff, 0.5*np.sqrt(2), 'ko')
# plt.axvline(cutoff, color='k')
# plt.xlim(0, 1000)
# plt.title("Lowpass Filter Frequency Response")
# plt.xlabel('Frequency [Hz]')
# plt.grid() # Demonstrate the use of the filter. # First make some data to be filtered.
# T = 5.0 # seconds
# n = int(T * fs) # total number of samples
# t = np.linspace(0, T, n, endpoint=False) # "Noisy" data. We want to recover the 1.2 Hz signal from this.
# # data = np.sin(1.2*2*np.pi*t) + 1.5*np.cos(9*2*np.pi*t) + 0.5*np.sin(12.0*2*np.pi*t) # Filter the data, and plot both the original and filtered signals.


path = "*****"

for file in os.listdir(path):
    if file.endswith("txt"):
        data=[]
        filePath = os.path.join(path, file)
        with open(filePath, 'r') as f:
            lines = f.readlines()[8:]
            for line in lines:
                # print(line)
                data.append(float(line)*100)
        # print(len(data))
        t1=[i*10 for i in range(len(data))]
        plt.subplot(231)
        # plt.plot(range(len(data)), data)
        plt.plot(t1, data, linewidth=2,label='original data')
        # plt.title('ori wave', fontsize=10, color='#F08080')
        plt.xlabel('Time [us]')
        plt.legend()

        # filter_data = data[30000:35000]
        # filter_data=data[60000:80000]
        # filter_data2=data[60000:80000]
        # filter_data = data[80000:100000]
        # filter_data = data[100000:120000]
        filter_data = data[120000:140000]

        filter_data2=filter_data
        t2=[i*10 for i in range(len(filter_data))]
        plt.subplot(232)
        plt.plot(t2, filter_data, linewidth=2,label='cut off wave before filter')
        plt.xlabel('Time [us]')
        plt.legend()
        # plt.title('cut off wave', fontsize=10, color='#F08080')

        # filter_data=zip(range(1,len(data),int(fs/len(data))),data)
        # print(filter_data)
        n1 = len(filter_data)
        Yamp1 = abs(np.fft.fft(filter_data) / (n1 / 2))
        Yamp1 = Yamp1[range(len(Yamp1) // 2)]
        # x_axis=range(0,n//2,int(fs/len
        # 計算最大賦值點頻率
        max1 = np.max(Yamp1)
        max1_index = np.where(Yamp1 == max1)
        if (len(max1_index[0]) == 2):
            print((max1_index[0][0] )* fs / n1, (max1_index[0][1]) * fs / n1)
        else:
            Y_second = Yamp1
            Y_second = np.sort(Y_second)
            print(np.where(Yamp1 == max1)[0] * fs / n1,
                  (np.where(Yamp1 == Y_second[-2])[0]) * fs / n1)
        N1 = len(Yamp1)
        # print(N1)
        x_axis1 = [i * fs / n1 for i in range(N1)]

        plt.subplot(233)
        plt.plot(x_axis1[:300], Yamp1[:300], linewidth=2,label='FFT data')
        plt.xlabel('Frequence [Hz]')
        # plt.title('FFT', fontsize=10, color='#F08080')
        plt.legend()
        # plt.savefig(filePath.replace("txt", "png"))
        # plt.close()
        # plt.show()



        Y = butter_lowpass_filter(filter_data2, cutoff, fs, order)
        n3 = len(Y)
        t3 = [i * 10 for i in range(n3)]
        plt.subplot(235)
        plt.plot(t3, Y, linewidth=2, label='cut off wave after filter')
        plt.xlabel('Time [us]')
        plt.legend()
        Yamp2 = abs(np.fft.fft(Y) / (n3 / 2))
        Yamp2 = Yamp2[range(len(Yamp2) // 2)]
        # x_axis = range(0, n // 2, int(fs / len(Yamp)))
        max2 = np.max(Yamp2)
        max2_index = np.where(Yamp2 == max2)
        if (len(max2_index[0]) == 2):
            print(max2, max2_index[0][0] * fs / n3, max2_index[0][1] * fs / n3)
        else:
            Y_second2 = Yamp2
            Y_second2 = np.sort(Y_second2)
            print((np.where(Yamp2 == max2)[0]) * fs / n3,
                  (np.where(Yamp2 == Y_second2[-2])[0]) * fs / n3)
        N2=len(Yamp2)
        # print(N2)
        x_axis2 = [i * fs / n3 for i in range(N2)]

        plt.subplot(236)
        plt.plot(x_axis2[:300], Yamp2[:300],linewidth=2, label='FFT data after filter')
        plt.xlabel('Frequence [Hz]')
        # plt.title('FFT after low_filter', fontsize=10, color='#F08080')
        plt.legend()
        # plt.show()
        plt.savefig(filePath.replace("txt", "png"))
        plt.close()
        print('*'*50)

        # plt.subplot(3, 1, 2)
        # plt.plot(range(len(data)), data, 'b-', linewidth=2,label='original data')
        # plt.grid()
        # plt.legend()
        #
        # plt.subplot(3, 1, 3)
        # plt.plot(range(len(y)), y, 'g-', linewidth=2, label='filtered data')
        # plt.xlabel('Time')
        # plt.grid()
        # plt.legend()
        # plt.subplots_adjust(hspace=0.35)
        # plt.show()
        '''
        # Y_fft = Y[60000:80000]
        Y_fft = Y
        # Y_fft = Y[80000:100000]
        # Y_fft = Y[100000:120000]
        # Y_fft = Y[120000:140000]
        n = len(Y_fft)
        Yamp = np.fft.fft(Y_fft)/(n/2)
        Yamp = Yamp[range(len(Yamp)//2)]

        max = np.max(Yamp)
        # print(max, np.where(Yamp == max))

        Y_second = Yamp
        Y_second=np.sort(Y_second)
        print(float(np.where(Yamp == max)[0])* fs / len(Yamp),float(np.where(Yamp==Y_second[-2])[0])* fs / len(Yamp))
        # print(float(np.where(Yamp == max)[0]) * fs / len(Yamp))
        '''