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Spectrum Analysis of Signals

This example shows downstream spectrum analysis of signals using MATLAB®, Communications Toolbox™ and DSP System Toolbox™. You can either use captured signals, or receive signals in real time using the RTL-SDR Radio, ADALM-PLUTO Radio or USRP™ Radio. You can change the radio's center frequency to tune the radio to a band where a signal is present. You can then use the spectrum analyzer to view and make measurements on the received spectrum.

Required Hardware and Software

To run this example using captured signals, you need the following software:

Communications Toolbox

DSP System Toolbox

To receive signals in real time, you also need one of the following hardware:

For a full list of Communications Toolbox supported SDR platforms, refer to the "MATLAB and Simulink Hardware Support for SDR" section of Software-Defined Radio (SDR).

Example Code

The receiver asks for user input and initializes variables. Then, it calls the signal source and FM broadcast receiver in a loop. The loop also keeps track of the radio time using the frame duration and lost samples reported by the signal source.

For the option to change default settings, set |cmdlineInput| to 1.

cmdlineInput = false;
if cmdlineInput
%     Request user input from the command-line for application parameters
    userInput = helperSpectralAnalysisUserInput;
%     Set initial parameters
    [SAParams, sigSrc] = helperSpectralAnalysisConfig(userInput);
else
%     Set initial parameters
    load defaultInputSpecAnalysis.mat
    [SAParams, sigSrc] = helperSpectralAnalysisConfig;
end

Setup

Create spectrumAnalyzer object and configure based on user input

hSpectrum = spectrumAnalyzer(...
    'Name',             'Passband Spectrum',...
    'Title',            'Passband Spectrum', ...
    'Method',           'Welch', ...
    'SpectrumType',     'Power density', ...
    'FrequencySpan',    'Full', ...
    'SampleRate',       SAParams.FrontEndSampleRate, ...
    'SpectralAverages', 50, ...
    'FrequencyOffset',  SAParams.CenterFrequency, ...
    'YLimits',          [-120 10], ...
    'YLabel',           'Magnitude-squared, dB', ...
    'Position',         figposition([50 30 30 40]));

Stream processing

View the spectrum. While the spectrum analyzer is running, you can measure peaks, occupied bandwidth, and other properties of the signal.

% Initialize radio time
radioTime = 0;

% Main loop
while radioTime < userInput.Duration
  % Receive baseband samples (Signal Source)
  if SAParams.isSourceRadio
      if SAParams.isSourcePlutoSDR
          rcv = sigSrc();
          lost = 0;
          late = 1;
      elseif SAParams.isSourceUsrpRadio
          rcv= sigSrc();
          lost = 0;
      else
          [rcv,~,lost,late] = sigSrc();
      end
  else
    rcv = sigSrc();
    lost = 0;
    late = 1;
  end
  
    rcv = rcv - mean(rcv);  % Remove DC component.
    step(hSpectrum, rcv);

  % Update radio time. If there were lost samples, add those too.
  radioTime = radioTime + SAParams.FrontEndFrameTime + ...
    double(lost)/SAParams.FrontEndSampleRate;
end

% Release all System objects
release(sigSrc);
release(hSpectrum);

Conclusion

In this example, you used Communications Toolbox™ System objects to analyze the spectrum of a received signal.