Python，FFT 和 DC 偏移分析

Question

I test an FFT on a square signal (100Hz, 0V-5V) of 50% duty cycle and i don't understand why my DC offset is huge?

In theory it should be 2.5V?

Thanks, Best regards.

The fundamental is ok and others harmonics are correct.

square signal 100Hz, TTL compatible 0V-5V, 50% duty cycle

FFT, DC offset problem, fundamental ok, harmonics ok

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

#
# configuration
# time analyse = L * (1/Fsample)
#
L = 512 # lenght buffer
Fsample = 2000 # frequency sample
Fsignal = 100 # frequency

#********************************

# perdiode sample
Tsample = 1.0/Fsample

# time vector, start = 0s, stop = 0.1024s, step = (0.1024s / (1/Fe))
t = np.linspace(0.0, L*Tsample, L)

# signal definition, DC offet = 2.5V, Amplitude = 2.5V
signal = 2.5 + 2.5*signal.square(2 * np.pi * Fsignal * t, 0.5)

# plot time signal
plt.subplot(2,1,1)
plt.plot(t, signal)

# fft of time signal
yf = fft(signal)

# time vector of fft 
xf = np.linspace(0.0, 1.0/(2.0*Tsample), L//2)

# plot spectral
plt.subplot(2,1,2)
plt.plot(xf, 2.0/L * np.abs(yf[0:L//2]))

Answer 1

On the last line, the normalization factor was wrong.

It was not 2/L but 1/L .

The correct normalization factor plt.plot(xf, 1.0/L * np.abs(yf[0:L//2]))

The code work fine now !

FFT correct amplitude