DSP Blockset

Displaying Signals in the Frequency-Domain

The Spectrum Scope block can display the frequency spectra of time-domain input data. It differs from the Vector Scope by computing the FFT of inputs to transform them to the frequency domain.

Example: Displaying Frequency-Domain Data

In the model below, the frequency content of two frame-based signals is simultaneously displayed on the scope.

To create the model, first load the mtlb signal:

• load mtlb            % Contains variables 'mtlb' and 'Fs'
  

Specify the following parameter values in the Signal From Workspace block:

• Signal = mtlb
• Sample time = 1
• Samples per frame = 16
• Form output after final data value = Cyclic Repetition

Specify the following parameter values in the Digital Filter Design block:

• Filter Type = Lowpass
• Design Method = FIR (Window)
• Filter Order (Specify order) = 22
• Window Specifications (Window) = Hamming
• Frequency Specifications (wc) = 0.25
• Frequency Specifications (Units) = Normalized (0 to 1)
• Magnitude Specifications (Units) = dB

Specify the following parameter values in the Scope properties pane of the Spectrum Scope block:

• Buffer input =
• Buffer size = 128
• Buffer overlap = 64
• Specify FFT length =
• Number of spectral averages = 2

With these settings, the Spectrum Scope block buffers each input channel to a new frame size of 128 (from the original frame size of 16) with an overlap of 64 samples between consecutive frames. Because Specify FFT length is not selected, the frame size of 128 is used as the number of frequency points in the FFT. This is the number of points plotted for each channel every time the scope display is updated.

You may want to set the Stop time in the Simulation Parameters dialog box to inf to allow the simulation to run longer. See Improving the Appearance of the Display for some tips on improving the scope display.

Displaying Signals in the Time-Domain

Displaying Matrices