Spectrum Analysis of Signals

This example shows you how to undertake spectral analysis of signals. You can either use recorded data from a file, 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.

For Simulink® implementation of this example, see Spectrum Analysis of Signals in Simulink.

Required Hardware and Software

By default, this example runs using recorded data from a file. Optionally, you can receive signals over-the-air. For this, you also need one of the following:

• RTL-SDR radio and Communications Toolbox Support Package for RTL-SDR Radio

• ADALM-PLUTO radio and Communications Toolbox Support Package for ADALM-PLUTO Radio

• 200-Series USRP Radio and Communications Toolbox Support Package for USRP Radio. For information on how to map an NI™ USRP device to an Ettus Research™ 200-series USRP device, see Supported Hardware and Required Software.

• 300-Series USRP Radio and Wireless Testbench Support Package for NI USRP Radios. For information on how to map an NI USRP device to an Ettus Research 300-series USRP device, see Supported Radio Devices (Wireless Testbench).

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 System objects to analyze the spectrum of a received signal.