Tutorial: Computing a Fast Fourier Transform
Learn how to generate a 1000 Hz plus 3000 Hz sinusoidal wave with additive noise, apply a Chebyshev Type II filter, compute a Fast Fourier Transform (FFT) and power spectral density, display simulation results in a vector scope, and downsample the output of a buffer.
Files for This Tutorial
filterPSD.scm
Generating a Sinusoidal Wave with Additive Noise
Create a model and define a diagram context for a sinusoidal wave with additive noise.
 On the ribbon, select New Model, then Save Model.
 In the dialog that appears, enter the path to your working directory with the file name: filterPSD_practice.scm.

From the Palette Browser, add the following blocks into the
modeling window:
 From SampleClock block, 1 Random block, and2 SineWaveGenerator blocks. , drag 1
 From Sum block. , drag 1

For each block: doubleclick the block, enter the parameter values as
indicated, then click OK.
 On the Sum block, doubleclick, and for Number of
inputs, enter: 3; for all Input Ports, enter:
"+" .
Block Define Parameter SampleClock Sample period: 1/Fs Random Distribution: Normal SineWaveGenerator (1) Frequency: 2*pi*1000 SineWaveGenerator (2) Frequency: 2*pi*3000 Sum  Number of inputs: 3
 For all Input Ports: "+"
 On the Sum block, doubleclick, and for Number of
inputs, enter: 3; for all Input Ports, enter:
"+" .

Connect the blocks and adjust the links like this:
 On the ribbon, select the Diagram tool .
 On the Context tab, enter: Fs = 10000, and click Apply.
Applying a Chebyshev Type II Low Pass Filter
Apply the OML function, Cheby2, in a diagram context. The Cheby2 function generates the coefficient of the transfer function of the low pass filter.
 On the ribbon, select the Diagram tool .

On the Context tab, enter the command:
[num,den]=cheby2(3,20,2000/5000);
This command generates the coefficient of the transfer function of the low pass filter, which has an order of 3, a cutoff frequency of 2000 Hz, and a stop band attenuation of 20 dB. The returned variables num and den are used as coefficients of the numerator and denominator of a discrete transfer function representing the filter.
 From the DiscrTransFunc block into the diagram. , drag 1

On the DiscrTransFunc block, doubleclick, and enter
these parameter values:

Connect the blocks so that the sine wave with additive noise passes through the
digital low pass filter implemented with a discrete transfer function:
Computing the FFT and Power Spectral Density
Define blocks in your model to compute the Fast Fourier Transform and Power Spectral Density.
 From the Buffer block into the diagram. , drag 1
 On the Buffer block, doubleclick. In the block dialog, for length, enter: N.
 From the FFT block into the diagram. , drag 1

Connect the blocks like this:
 On the ribbon, select the Diagram tool .

On the Context tab, enter the variable: N=1024.
This variable specifies for 1,024 points of data to be sampled and sent to the FFT block.Note: The onesided power spectral density is computed with the equation P= 2 ∗ ff  ^ 2/(F∗N)
 From the Abs and 1 Power block into the diagram. , drag 1
 On the Power block, doubleclick. In the block dialog, for power, enter: 2.
 From the Gain block into the diagram. , drag 1
 On the Gain block, doubleclick. In the block dialog, for Gain, enter: 2/ (Fs*N).

Connect the blocks like this:
 From the MatrixExtractor block into the diagram. , drag 1

On the MatrixExtractor block, doubleclick, enter the
following parameter values, and click OK:
 For Index vector I, select : (all), which is the default.
 For Index vector J, select Defined as parameter, and enter [1:N/2+1].
 Select Onebased indexing.

Connect the blocks like this:

Click and drag your mouse left to right to boxselect the blocks
FFT, Abs,
Power, and Gain.
 With the blocks selected, from the ribbon, click, Create Super Block .

In the tree of the Property Editor, under General, click the field for
Name, and enter PSD.
The super block reflects the new name:
Displaying Results with a VectorScope Block
View simulation results in a power spectrum density graph.

From the Palette Browser, add the following blocks into
your diagram:
 From Transpose block. , drag 1
 From VectorScope block. , drag 1
 From Constant block. , drag 1
 On the Constant block, doubleclick. In the block dialog, for the parameter Constant, enter [0:Fs/N:Fs/2]', and click OK.
 On the VectorScope block, doubleclick. From the block dialog, select the box for External X Value and Plot after simulation ends, and click OK.

Assemble and connect the blocks like this:

On the ribbon, select Run.

Doubleclick the VectorScope block.
The VectorScope window appears displaying a plot of the simulation results. To fit the view of the plot into the window, middlemouse click.

On the Yaxis block, rightclick. In the floating panel
that appears, from the Scale drop down list, select
db10.
The power spectrum density graph appears as you see in the following figure. Two peaks at frequencies are present: 1000Hz and 3000Hz. The 3000Hz peak is lower because the original signal is attenuated by the Chebychev lowpass filter.
Downsample the Output of the Buffer
Reduce the sampling frequency of the buffer.

From the ReSampleClock block into the
diagram.
, drag 1 Both activation input ports of the Buffer block are triggered by the SampleClock block at 10,000Hz. This trigger forces the blocks downstream, including the FFT and VectorScope blocks, to run at 10,000Hz which can result in unnecessary computation and scope generation. The addition of the ReSampleClock block ensures that the Buffer block updates the output only when required.

Assemble and connect the blocks like this:

On the ReSampleClock block, doubleclick. From the block
dialog that appears, for Down Sampling Factor, enter
N.
Changing this parameter ensures the outgoing activation is triggered whenever N points are sampled by the Buffer block when full.

On the ribbon, select Run.
Running the simulation takes less time to finish than in the previous iteration, though the results are similar: