How to center the FFT of sine wave in matplotlib?-CodePudding

I want to plot the FFT of a sine wave using matplotlib and I want to plot a single line at a frequency where the sine wave belongs.

here's my code

import numpy as np
import matplotlib.pyplot as plt


f = 3
t = np.arange(-np.pi, np.pi, 0.01)
y = np.sin(2*np.pi*f*t)


plt.subplot(2, 1, 1)
plt.plot(t, y)


ft = np.fft.fft(y)

plt.subplot(2, 1, 2)
plt.plot(ft.real)

plt.show()

And, here's the output of the above code.

I want to plot the FFT as a single line, and this single line will be straight up at the given sine wave's frequency. Suppose, if I have two sine waves of 20Hz and 40Hz added together like sine(2*pi*20*t) sine(2*pi*40*t) then their FFT should have two straight lines at 20Hz and 30Hz.

How do I fix my code to get what I wanted? I've seen many questions here on StackOverflow but I did not understand them. Any help would be appreciated.

CodePudding user response：

There are a few things to unpack here.

Use `np.fft.rfft` for real signals

Since your signal is real-valued (i.e. no complex numbers) there is no information contained in the second half of your fft. You should use the rfft in stead for real signals.

See

Use np.fft.rfft for real signals

Use `np.fft.rfft` for real signals