How to limit frequency range using scipy FFT

Question

I am using FFT do find the frequencies of a signal. I am only interested in a certain range of frequencies, between 1 and 4 Hz.

I have this code to compute frequencies:

from scipy.fft import rfft, rfftfreq, irfft

plt.plot(d)
plt.show()
N = len(d)
yf = rfft(d)
xf = rfftfreq(N, 1 / sample_rate) # 29
plt.plot(xf, np.abs(yf))
plt.show()

Which results in:

How do I modify my code so that xf and yf only correspond to frequencies in my desired range of 1-4 Hz, instead of the 0-15 seen in the plot?

Answer 1

You can use xlim feature of matplotlib to modify x axis .

Here is the example code that you can refer.

from scipy.fft import fft, fftfreq
import numpy as np
# Number of sample points
N = 600
# sample spacing
T = 1.0 / 800.0
x = np.linspace(0.0, N*T, N, endpoint=False)
y = np.sin(50.0 * 2.0*np.pi*x) + 0.5*np.sin(80.0 * 2.0*np.pi*x)
yf = fft(y)
xf = fftfreq(N, T)[:N//2]
import matplotlib.pyplot as plt
plt.plot(xf, 2.0/N * np.abs(yf[0:N//2]),'b')
plt.plot()
plt.grid()
plt.show()

plt.plot(xf, 2.0/N * np.abs(yf[0:N//2]),'b')
plt.xlim(0,100) # you need this
plt.grid()
plt.show()