Goertzel Algorithm Implementation

Oct 13, 2025 by pinkandtrash.com 108 views

Introduction

In the realm of signal processing, the Fast Fourier Transform (FFT) is a widely used algorithm for converting time-domain signals into their frequency-domain representations. When working with IQ data from Software Defined Radios (SDRs), it's essential to understand the differences in FFT results obtained from various tools and platforms, such as MATLAB, GNU Radio, and others. In this article, we will delve into the world of FFT, exploring the intricacies of this algorithm and its implementation in MATLAB and GNU Radio.

Understanding FFT

The FFT is an efficient algorithm for calculating the discrete Fourier transform (DFT) of a sequence. It's a crucial tool in signal processing, used in various applications, including filtering, modulation analysis, and spectral estimation. The FFT algorithm works by breaking down the DFT into smaller, more manageable components, which are then combined to produce the final frequency-domain representation.

MATLAB and FFT

MATLAB is a high-level programming language and environment specifically designed for numerical computation and data analysis. It provides an extensive range of built-in functions for signal processing, including the FFT. In MATLAB, the FFT is implemented using the fft() function, which takes a time-domain signal as input and returns its frequency-domain representation.

% Generate a sample time-domain signal
t = 0:0.01:1;
x = sin(2*pi*10*t) + 0.5*sin(2*pi*20*t);

% Perform FFT on the signal
X = fft(x);

% Plot the frequency-domain representation
figure;
plot(abs(X));
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title('Frequency-Domain Representation of the Signal');

GNU Radio and FFT

GNU Radio is an open-source software development toolkit that provides a comprehensive framework for building software-defined radios. It includes a wide range of signal processing blocks, including the FFT. In GNU Radio, the FFT is implemented using the fft block, which can be used to perform FFT on a signal.

# Import the necessary modules
import gr

# Generate a sample time-domain signal
t = np.arange(0, 1, 0.01)
x = np.sin(2*np.pi*10*t) + 0.5*np.sin(2*np.pi*20*t)

# Perform FFT on the signal
X = gr.fft(x)

# Plot the frequency-domain representation
import matplotlib.pyplot as plt
plt.plot(np.abs(X))
plt.xlabel('Frequency (Hz)')
plt.ylabel('Magnitude')
plt.title('Frequency-Domain Representation of the Signal')
plt.show()

Comparing FFT Results

When comparing the FFT results obtained from MATLAB and GNU Radio, we notice some differences in the frequency-domain representations. These differences can be attributed to the implementation details of the FFT algorithm in each platform.

% Perform FFT on the signal using the FFTW library
X_fftw = fft(x, 'fftw');

% Plot the frequency-domain representation
figure;
plot(abs(X_fftw));
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title('Frequency-Domain Representation of the Signal using FFTW');

# Perform FFT on the signal using the FFTW library
X_fftw = gr.fft(x, 'fftw')

# Plot the frequency-domain representation
import matplotlib.pyplot as plt
plt.plot(np.abs(X_fftw))
plt.xlabel('Frequency (Hz)')
plt.ylabel('Magnitude')
plt.title('Frequency-Domain Representation of the Signal using FFTW')
plt.show()

Conclusion

In conclusion, the FFT is a powerful algorithm for converting time-domain signals into their frequency-domain representations. When working with IQ data from SDRs, it's essential to understand the differences in FFT results obtained from various tools and platforms, such as MATLAB and GNU Radio. By exploring the intricacies of the FFT algorithm and its implementation in these platforms, we can gain a deeper understanding of the signal processing techniques used in SDRs.

Future Work

In future work, we plan to explore the use of the Goertzel algorithm to extract the magnitude and phase information at a specific frequency under test. This will involve implementing the Goertzel algorithm in MATLAB and GNU Radio, and comparing the results obtained from each platform.

References

[1] Oppenheim, A. V., & Schafer, R. W. (2010). Discrete-Time Signal Processing (3rd ed.). Prentice Hall.
[2] Proakis, J. G., & Manolakis, D. G. (2014). Digital Signal Processing: Principles, Algorithms, and Applications (4th ed.). Pearson Education.
[3] GNU Radio Documentation. (n.d.). Retrieved from https://www.gnuradio.org/doc/doxygen/

Appendix

MATLAB Code

% Generate a sample time-domain signal
t = 0:0.01:1;
x = sin(2*pi*10*t) + 0.5*sin(2*pi*20*t);

% Perform FFT on the signal
X = fft(x);

% Plot the frequency-domain representation
figure;
plot(abs(X));
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title('Frequency-Domain Representation of the Signal');

GNU Radio Code

# Import the necessary modules
import gr

# Generate a sample time-domain signal
t = np.arange(0, 1, 0.01)
x = np.sin(2*np.pi*10*t) + 0.5*np.sin(2*np.pi*20*t)

# Perform FFT on the signal
X = gr.fft(x)

# Plot the frequency-domain representation
import matplotlib.pyplot as plt
plt.plot(np.abs(X))
plt.xlabel('Frequency (Hz)')
plt.ylabel('Magnitude')
plt.title('Frequency-Domain Representation of the Signal')
plt.show()

Goertzel Algorithm Implementation

% Implement the Goertzel algorithm
function [m, p] = goertzel(x, f)
    n = length(x);
    m = zeros(1, n);
    p = zeros(1, n);
    w = 2 * pi * f;
    for i = 1:n
        m(i) = 2 * cos(w) * m(i-1) - m(i-2) + 2 * sin(w) * x(i);
        p(i) = 2 * sin(w) * m(i-1) + x(i);
    end
end

# Implement the Goertzel algorithm
def goertzel(x, f):
    n = len(x)
    m = [0] * n
    p = [0] * n
    w = 2 * math.pi * f
    for i in range(1, n):
        m[i] = 2 * math.cos(w) * m[i-1] - m[i-2] + 2 * math.sin(w) * x[i]
        p[i] = 2 * math.sin(w) * m[i-1] + x[i]
    return m, p

Introduction

In our previous article, we explored the world of FFT, discussing its implementation in MATLAB and GNU Radio. We also touched upon the differences in FFT results obtained from these platforms. In this article, we will address some of the frequently asked questions related to FFT, MATLAB, and GNU Radio.

Q: What is the difference between FFT and DFT?

A: The Fast Fourier Transform (FFT) is an efficient algorithm for calculating the discrete Fourier transform (DFT) of a sequence. While both FFT and DFT are used to convert time-domain signals into their frequency-domain representations, the main difference lies in the computational complexity. The DFT has a time complexity of O(n^2), whereas the FFT has a time complexity of O(n log n), making it much faster for large datasets.

Q: How do I implement the FFT in MATLAB?

A: In MATLAB, you can implement the FFT using the fft() function. This function takes a time-domain signal as input and returns its frequency-domain representation. For example:

% Generate a sample time-domain signal
t = 0:0.01:1;
x = sin(2*pi*10*t) + 0.5*sin(2*pi*20*t);

% Perform FFT on the signal
X = fft(x);

% Plot the frequency-domain representation
figure;
plot(abs(X));
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title('Frequency-Domain Representation of the Signal');

Q: How do I implement the FFT in GNU Radio?

A: In GNU Radio, you can implement the FFT using the fft block. This block takes a time-domain signal as input and returns its frequency-domain representation. For example:

# Import the necessary modules
import gr

# Generate a sample time-domain signal
t = np.arange(0, 1, 0.01)
x = np.sin(2*np.pi*10*t) + 0.5*np.sin(2*np.pi*20*t)

# Perform FFT on the signal
X = gr.fft(x)

# Plot the frequency-domain representation
import matplotlib.pyplot as plt
plt.plot(np.abs(X))
plt.xlabel('Frequency (Hz)')
plt.ylabel('Magnitude')
plt.title('Frequency-Domain Representation of the Signal')
plt.show()

Q: What is the difference between the FFT and Goertzel algorithms?

A: The FFT and Goertzel algorithms are both used to extract the magnitude and phase information at a specific frequency under test. However, the Goertzel algorithm is more efficient for narrowband signals, whereas the FFT is more efficient for wideband signals. The Goertzel algorithm also provides a more accurate estimate of the frequency under test.

Q: How do I implement the Goertzel algorithm in MATLAB?

A: In MATLAB, you can implement the Goertzel algorithm using the following code:

% Implement the Goertzel algorithm
function [m, p] = goertzel(x, f)
    n = length(x);
    m = zeros(1, n);
    p = zeros(1, n);
    w = 2 * pi * f;
    for i = 1:n
        m(i) = 2 * cos(w) * m(i-1) - m(i-2) + 2 * sin(w) * x(i);
        p(i) = 2 * sin(w) * m(i-1) + x(i);
    end
end

Q: How do I implement the Goertzel algorithm in GNU Radio?

A: In GNU Radio, you can implement the Goertzel algorithm using the following code:

# Implement the Goertzel algorithm
def goertzel(x, f):
    n = len(x)
    m = [0] * n
    p = [0] * n
    w = 2 * math.pi * f
    for i in range(1, n):
        m[i] = 2 * math.cos(w) * m[i-1] - m[i-2] + 2 * math.sin(w) * x[i]
        p[i] = 2 * math.sin(w) * m[i-1] + x[i]
    return m, p

Conclusion

In conclusion, the FFT and Goertzel algorithms are both used to extract the magnitude and phase information at a specific frequency under test. While the FFT is more efficient for wideband signals, the Goertzel algorithm is more efficient for narrowband signals. By understanding the differences between these algorithms and their implementation in MATLAB and GNU Radio, you can make informed decisions when working with signal processing applications.

Future Work

In future work, we plan to explore the use of the Goertzel algorithm in more complex signal processing applications, such as modulation analysis and spectral estimation.

References

[1] Oppenheim, A. V., & Schafer, R. W. (2010). Discrete-Time Signal Processing (3rd ed.). Prentice Hall.
[2] Proakis, J. G., & Manolakis, D. G. (2014). Digital Signal Processing: Principles, Algorithms, and Applications (4th ed.). Pearson Education.
[3] GNU Radio Documentation. (n.d.). Retrieved from https://www.gnuradio.org/doc/doxygen/

Appendix

MATLAB Code

% Generate a sample time-domain signal
t = 0:0.01:1;
x = sin(2*pi*10*t) + 0.5*sin(2*pi*20*t);

% Perform FFT on the signal
X = fft(x);

% Plot the frequency-domain representation
figure;
plot(abs(X));
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title('Frequency-Domain Representation of the Signal');

GNU Radio Code

# Import the necessary modules
import gr

# Generate a sample time-domain signal
t = np.arange(0, 1, 0.01)
x = np.sin(2*np.pi*10*t) + 0.5*np.sin(2*np.pi*20*t)

# Perform FFT on the signal
X = gr.fft(x)

# Plot the frequency-domain representation
import matplotlib.pyplot as plt
plt.plot(np.abs(X))
plt.xlabel('Frequency (Hz)')
plt.ylabel('Magnitude')
plt.title('Frequency-Domain Representation of the Signal')
plt.show()

Goertzel Algorithm Implementation

% Implement the Goertzel algorithm
function [m, p] = goertzel(x, f)
    n = length(x);
    m = zeros(1, n);
    p = zeros(1, n);
    w = 2 * pi * f;
    for i = 1:n
        m(i) = 2 * cos(w) * m(i-1) - m(i-2) + 2 * sin(w) * x(i);
        p(i) = 2 * sin(w) * m(i-1) + x(i);
    end
end

# Implement the Goertzel algorithm
def goertzel(x, f):
    n = len(x)
    m = [0] * n
    p = [0] * n
    w = 2 * math.pi * f
    for i in range(1, n):
        m[i] = 2 * math.cos(w) * m[i-1] - m[i-2] + 2 * math.sin(w) * x[i]
        p[i] = 2 * math.sin(w) * m[i-1] + x[i]
    return m, p