Square Wave: Program 1

Fourier Series

The Fourier series is a way to represent a periodic function as a sum of simple sine and cosine waves. It breaks down any periodic function or signal into the sum of an infinite series of sines and cosines. This concept is widely used in fields such as signal processing, acoustics, and heat transfer, as it provides a tool for analyzing the frequency components of signals.

Key Concepts

Periodic Function: A function \( f(x) \) is called periodic with period \( T \) if \( f(x + T) = f(x) \) for all \( x \). In a Fourier series, we are interested in expressing this periodic function as a combination of simpler periodic functions (sines and cosines).
Orthogonal Basis: The sines and cosines form an orthogonal basis for the space of periodic functions. This means that any periodic function can be expressed as a linear combination of sine and cosine functions.
Harmonics: The sine and cosine functions are often called the harmonics of the original periodic function. The fundamental harmonic corresponds to the lowest frequency \( \frac{1}{T} \), while higher harmonics correspond to integer multiples of this frequency.

Fourier Series Mathematical Form

For a periodic function \( f(x) \) with period \( T \), the Fourier series is expressed as:

\[ \small f(x) = a_0 + \sum_{n=1}^{\infty} \left( a_n \cos \left( \frac{2\pi n}{T} x \right) + b_n \sin \left( \frac{2\pi n}{T} x \right) \right) \]

Where:

\( a_0 \) is the average (DC component) of the function over one period.
\( a_n \) and \( b_n \) are the Fourier coefficients for the cosine and sine terms, respectively.
\( n \) is the harmonic number, representing the frequency multiple of the base frequency.

Fourier Coefficients

The Fourier coefficients \( a_0 \), \( a_n \), and \( b_n \) are calculated as follows:

\( a_0 \) (the DC term):

\[ a_0 = \frac{1}{T} \int_0^T f(x) \, dx \]

\( a_n \) (cosine coefficients):

\[ a_n = \frac{2}{T} \int_0^T f(x) \cos \left( \frac{2\pi n}{T} x \right) \, dx \]

\( b_n \) (sine coefficients):

\[ b_n = \frac{2}{T} \int_0^T f(x) \sin \left( \frac{2\pi n}{T} x \right) \, dx \]

import numpy as np
import matplotlib.pyplot as plt
from scipy import signal
from scipy.integrate import quad

x_ = np.linspace(-20, 20, 10000)
T = 10  # Time period
n2 = 10  # Number of Harmonics
n1 = n2 + 1

# Define the square wave function
def f(x):
    return signal.square(2 * np.pi * (1/T) * x)

# Define a0
a0, _ = quad(lambda x: f(x), -T/2, T/2)
a0 = (2/T) * a0
print('a0', a0)

# Define an, bn
def integrand_an(x, n):
    return f(x) * np.cos(n * 2 * np.pi * x / T)

def integrand_bn(x, n):
    return f(x) * np.sin(n * 2 * np.pi * x / T)

anm = [(2/T) * quad(lambda x: integrand_an(x, i), -T/2, T/2)[0] for i in range(0, n1)]
bnm = [(2/T) * quad(lambda x: integrand_bn(x, i), -T/2, T/2)[0] for i in range(0, n1)]

print('an', anm)
print('bn', bnm)

# Fourier Series function
def fs(x):
    sum = a0 / 2
    for i in range(1, n1):
        sum += bnm[i] * np.sin(i * 2 * np.pi * x / T) + anm[i] * np.cos(i * 2 * np.pi * x / T)
    return sum

y = np.array([f(i) for i in x_])
fss = np.array([fs(i) for i in x_])

plt.plot(x_, y, color="blue", label="Signal")
plt.plot(x_, fss, color="red", label="Fourier series approximation")
plt.title("Fourier Series approximation number of harmonics: " + str(n2))
plt.legend()
plt.savefig('plot.png')

Output 1

Square Wave: Program 2

Fourier series approximation of a square wave signal (without using scipy.signal)

import numpy as np
import matplotlib.pyplot as plt
from scipy.integrate import quad

x_ = np.linspace(-20, 20, 10000)
T = 10  # Time period
n2 = 10  # Number of Harmonics
n1 = n2 + 1

# Define the square wave function without scipy.signal
def square_wave(x):
    return np.sign(np.sin(2 * np.pi * x / T))

# Define a0
a0, _ = quad(lambda x: square_wave(x), -T/2, T/2)
a0 = (2 / T) * a0
print('a0', a0)

# Define an and bn
def integrand_an(x, n):
    return square_wave(x) * np.cos(n * 2 * np.pi * x / T)

def integrand_bn(x, n):
    return square_wave(x) * np.sin(n * 2 * np.pi * x / T)

anm = [(2 / T) * quad(lambda x: integrand_an(x, i), -T/2, T/2)[0] for i in range(0, n1)]
bnm = [(2 / T) * quad(lambda x: integrand_bn(x, i), -T/2, T/2)[0] for i in range(0, n1)]

print('an', anm)
print('bn', bnm)

# Fourier Series function
def fourier_series(x):
    result = a0 / 2
    for i in range(1, n1):
        result += anm[i] * np.cos(i * 2 * np.pi * x / T) + bnm[i] * np.sin(i * 2 * np.pi * x / T)
    return result

# Compute the square wave and Fourier series approximation
y = square_wave(x_)
fss = fourier_series(x_)

# Plot the results
plt.plot(x_, y, color="blue", label="Square Wave Signal")
plt.plot(x_, fss, color="red", label="Fourier Series Approximation")
plt.title(f"Fourier Series Approximation with {n2} Harmonics")
plt.legend()
plt.savefig('plot.png')
plt.show()

Fourier Series

Square Wave: Program 1

Fourier Series

Key Concepts

Fourier Series Mathematical Form

Fourier Coefficients

\( a_0 \) (the DC term):

\( a_n \) (cosine coefficients):

\( b_n \) (sine coefficients):

Output 1

Square Wave: Program 2

Output 2