FFT analysis in Simulink - Mini Project: Build & Apply

FFT analysis in Simulink

📖 Scenario: You are working on analyzing signals in Simulink. You want to learn how to perform a Fast Fourier Transform (FFT) analysis to see the frequency components of a signal.

🎯 Goal: Build a Simulink model that generates a sine wave signal, configures FFT analysis parameters, applies the FFT block to the signal, and displays the frequency spectrum.

📋 What You'll Learn

Create a sine wave source block with specific frequency and amplitude

Add a variable to set the FFT length

Use the FFT block to analyze the sine wave signal

Display the FFT output using a Spectrum Scope block

💡 Why This Matters

🌍 Real World

FFT analysis is used in engineering and science to understand the frequency content of signals, such as audio, vibration, or communication signals.

💼 Career

Knowing how to perform FFT analysis in Simulink is useful for engineers working in signal processing, control systems, and embedded system design.

1

Create a sine wave signal source

In your Simulink model, add a Sine Wave block. Set its Amplitude parameter to 1 and Frequency parameter to 50 Hz. This will create a sine wave signal to analyze.

Simulink

% Add a Sine Wave block with Amplitude 1 and Frequency 50 Hz
% Your code here

Hint

Use the Simulink library browser to find the Sine Wave block. Double-click it to set the amplitude and frequency.

2

Set FFT length parameter

Create a variable called fftLength in the MATLAB workspace and set it to 256. This variable will be used to configure the FFT block length.

Simulink

% Add a variable called fftLength and set it to 256
% Your code here

Hint

In the MATLAB command window or script, type fftLength = 256;

3

Add FFT block and configure it

In your Simulink model, add an FFT block from the DSP System Toolbox. Set its FFT length parameter to the variable fftLength. Connect the output of the Sine Wave block to the input of the FFT block.

Simulink

% Add an FFT block and set its FFT length to fftLength
% Connect Sine Wave output to FFT input
% Your code here

Hint

Use the DSP System Toolbox to find the FFT block. Set the FFT length parameter to fftLength by typing fftLength in the parameter field.

4

Display FFT output with Spectrum Scope

Add a Spectrum Scope block from the DSP System Toolbox. Connect the output of the FFT block to the input of the Spectrum Scope block. Run the simulation and observe the frequency spectrum of the sine wave.

Simulink

% Add a Spectrum Scope block
% Connect FFT output to Spectrum Scope input
% Run the simulation
% Your code here

Hint

Find the Spectrum Scope block in DSP System Toolbox and connect it to the FFT output. Run the simulation to view the spectrum.

Practice

1. What is the main purpose of using FFT analysis in Simulink?

easy

A. To filter out noise from a signal

B. To generate random signals for simulation

C. To display signals only in the time domain

D. To convert a signal from time domain to frequency domain

Solution

1. Step 1: Understand FFT function

   FFT (Fast Fourier Transform) changes signals from time view to frequency view.

2. Step 2: Identify Simulink FFT usage

   Simulink uses FFT blocks to analyze frequency components inside signals.

3. Final Answer:

   To convert a signal from time domain to frequency domain -> Option D

4. Quick Check:

   FFT = Time to Frequency [OK]

Hint: FFT always changes time signals to frequency signals [OK]

Common Mistakes:

• Confusing FFT with signal generation
• Thinking FFT filters noise directly
• Assuming FFT shows only time domain

2. Which Simulink block is used to generate a test signal for FFT analysis?

easy

A. Signal Generator

B. Spectrum Scope

C. FFT

D. Gain

Solution

1. Step 1: Identify signal source blocks

   Signal Generator block creates test signals like sine waves for simulation.

2. Step 2: Match block to purpose

   FFT block analyzes signals, Spectrum Scope displays frequency, Gain changes amplitude.

3. Final Answer:

   Signal Generator -> Option A

4. Quick Check:

   Signal Generator = Test signal source [OK]

Hint: Signal Generator creates signals; FFT analyzes them [OK]

Common Mistakes:

• Choosing FFT as signal source
• Confusing Spectrum Scope with signal creation
• Selecting Gain block incorrectly

3. Given this Simulink setup: a sine wave at 50 Hz connected to an FFT block, what frequency peak will the Spectrum Scope show?

medium

A. 0 Hz

B. 100 Hz

C. 50 Hz

D. No peak

Solution

1. Step 1: Understand input signal frequency

   The sine wave frequency is 50 Hz, so the main frequency component is at 50 Hz.

2. Step 2: Analyze FFT output

   FFT shows peaks at frequencies present in the input; here, a peak at 50 Hz appears.

3. Final Answer:

   50 Hz -> Option C

4. Quick Check:

   Sine wave frequency = FFT peak frequency [OK]

Hint: FFT peak matches input sine wave frequency [OK]

Common Mistakes:

• Choosing 0 Hz as peak frequency
• Doubling frequency to 100 Hz incorrectly
• Expecting no peak from sine wave

4. You connected a Signal Generator to an FFT block but see no frequency peaks in Spectrum Scope. What is the likely error?

medium

A. Signal Generator frequency set to 0 Hz

B. FFT block not connected to Signal Generator output

C. Spectrum Scope block missing

D. Signal Generator amplitude set too high

Solution

1. Step 1: Check signal flow connections

   FFT block must be connected to the Signal Generator output to analyze the signal.

2. Step 2: Identify cause of no peaks

   No connection means FFT receives no input, so no frequency peaks appear.

3. Final Answer:

   FFT block not connected to Signal Generator output -> Option B

4. Quick Check:

   Connection missing = no FFT input [OK]

Hint: Always connect blocks properly to see FFT results [OK]

Common Mistakes:

• Assuming zero frequency causes no peaks
• Thinking Spectrum Scope absence causes no peaks
• Blaming amplitude for missing peaks

5. You want to analyze a signal composed of two sine waves at 30 Hz and 70 Hz in Simulink. Which setup will correctly show both frequencies in the FFT output?

hard

A. Use two Signal Generators summed together, connected to FFT block

B. Use one Signal Generator set to 100 Hz connected to FFT block

C. Use one Signal Generator at 30 Hz connected to Spectrum Scope only

D. Use two FFT blocks connected separately to Signal Generators at 30 Hz and 70 Hz

Solution

1. Step 1: Combine signals for FFT input

   Summing two Signal Generators (30 Hz and 70 Hz) creates a combined signal with both frequencies.

2. Step 2: Connect combined signal to FFT block

   FFT block analyzes combined input and shows peaks at both 30 Hz and 70 Hz.

3. Step 3: Evaluate other options

   Single frequency or separate FFT blocks won't show both frequencies together in one spectrum.

4. Final Answer:

   Use two Signal Generators summed together, connected to FFT block -> Option A

5. Quick Check:

   Sum signals before FFT to see all frequencies [OK]

Hint: Sum signals before FFT to capture all frequencies [OK]

Common Mistakes:

• Using one generator at wrong frequency
• Connecting FFT blocks separately without summing
• Not summing signals before FFT