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]))
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 !
