FFT analysis in Simulink - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an FFT block to the Simulink model.

Simulink

add_block('dspsigops/FFT', '[1]');

Drag options to blanks, or click blank then click option'

AmyModel/FFT

BmyModel/Sine Wave

CmyModel/Scope

DmyModel/Gain

Attempts:

3 left

💡 Hint

Common Mistakes

Using a block path that does not exist in the library.

Using a destination path that conflicts with existing blocks.

The FFT block should be added with a unique path like 'myModel/FFT' to the Simulink model.

2fill in blank

medium

Complete the code to set the FFT block's input port to the signal named 'inputSignal'.

Simulink

add_line('myModel/[1]', 'myModel/FFT/1');

Drag options to blanks, or click blank then click option'

AoutputSignal

BcontrolSignal

CinputSignal

DnoiseSignal

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the input to a signal that does not exist.

Using the output signal name instead of the input.

The FFT block input should be set to the signal named 'inputSignal' to analyze that signal.

3fill in blank

hard

Fix the error in the code to correctly run the FFT analysis simulation.

Simulink

simOut = sim('[1]');

Drag options to blanks, or click blank then click option'

AmyModel.txt

BmyModel

CmyModel.mdl

DmyModel.slx

Attempts:

3 left

💡 Hint

Common Mistakes

Including file extensions in the model name string.

Using a file type that Simulink does not recognize.

The 'sim' function requires the model name without file extension to run the simulation.

4fill in blank

hard

Fill both blanks to extract the FFT magnitude and frequency from the simulation output.

Simulink

fftData = simOut.[1];
frequencies = simOut.[2];

Drag options to blanks, or click blank then click option'

AfftMagnitude

BfftFrequency

CfftMag

Dfreq

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect or non-existent variable names.

Mixing magnitude and frequency variable names.

The simulation output contains 'fftMag' for magnitude and 'freq' for frequency data.

5fill in blank

hard

Fill all three blanks to plot the FFT magnitude versus frequency with labels.

Simulink

plot([1], [2]);
xlabel('[3]');

Drag options to blanks, or click blank then click option'

Afrequencies

BfftData

CFrequency (Hz)

DfftMagnitude

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping x and y data in the plot function.

Forgetting to label the x-axis.

Plot frequency on x-axis and FFT magnitude on y-axis, labeling x-axis as 'Frequency (Hz)'.

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