问题
I'm trying to make sense of the output produced by the python FFT library.
I have a sqlite database where I have logged several series of ADC values. Each series consist of 1024 samples taken with a frequency of 1 ms.
After importing a dataseries, I normalize it and run int through the fft
method. I've included a few plots of the original signal compared to the FFT output.
import sqlite3
import struct
import numpy as np
from matplotlib import pyplot as plt
import time
import math
conn = sqlite3.connect(r"C:\my_test_data.sqlite")
c = conn.cursor()
c.execute('SELECT ID, time, data_blob FROM log_tbl')
for row in c:
data_raw = bytes(row[2])
data_raw_floats = struct.unpack('f'*1024, data_raw)
data_np = np.asarray(data_raw_floats)
data_normalized = (data_np - data_np.mean()) / (data_np.max() - data_np.min())
fft = np.fft.fft(data_normalized)
N = data_normalized .size
plt.figure(1)
plt.subplot(211)
plt.plot(data_normalized )
plt.subplot(212)
plt.plot(np.abs(fft)[:N // 2] * 1 / N)
plt.show()
plt.clf()
The signal clearly contains some frequencies, and I was expecting them to be visible from the FFT output.
What am I doing wrong?
回答1:
You need to make sure that your data is evenly spaced when using np.fft.fft
, otherwise the output will not be accurate. If they are not evenly spaced, you can use LS periodograms for example: http://docs.astropy.org/en/stable/stats/lombscargle.html.
Or look up non-uniform fft.
About the plots:
I don't think that you are doing something obviously wrong. Your signal consists a signal with period in the order of magnitude 100
, so you can expect a strong frequency signal around 1/period=0.01
. This is what is visible on your graphs. The time-domain signals are not that sinusoidal, so your peak in the frequency domain will be blurry, as seen on your graphs.
来源:https://stackoverflow.com/questions/54865473/understanding-the-output-from-the-fast-fourier-transform-method