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FM Demodulation and Spectrum Analysis with USRP Radio

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This example shows how to demodulate and perform wideband and narrowband analysis of the FM radio spectrum using a USRP™ radio. The wideband analysis involves capturing the entire FM band and identifying the strongest station, while the narrowband analysis focuses on real-time processing of a single FM station signal.

To run this example, you require, a USRP radio and Communications Toolbox Support Package for USRP Radio add on.

Select the Acquisition Type

Select the type of acquisition from the drop down menu.

acqType = "Wideband | 20MSPS";

Set the sample rate, center frequency, and the number of samples per frame depending on the acquisition type.

if acqType == "Wideband | 20MSPS"
    fs = 20e6;
    fc = 98.0e6;
    samplesPerFrame = 65536;

else
    fs = 500e3;
    fc = 101.1e6;
    samplesPerFrame = 5000;
end

Configure SDRu Receiver System Object

Set the receive gain in dB.

rxGain = 45.0;

Create a receiver System object.

rx = comm.SDRuReceiver(Platform="B210",SerialNum='3136D5F',...
    CenterFrequency=fc,ChannelMapping=1,Gain=rxGain,...
    TransportDataType='int16',OutputDataType='Same as transport data type',...
    SamplesPerFrame=samplesPerFrame);

Set the decimation factor.

rx.DecimationFactor = round(rx.MasterClockRate./fs);

Calculate the sample rate by dividing the master clock rate by the decimation factor.

actualSampleRate = rx.MasterClockRate./rx.DecimationFactor;
Ts = 1/actualSampleRate;

Set the desired frame duration.

frameDuration = samplesPerFrame.*(1/actualSampleRate);

Receive and Demodulate FM Signal

Create an FM demodulator System object.

fmDemodulate = comm.FMBroadcastDemodulator( ...
    SampleRate=actualSampleRate, ...
    FrequencyDeviation=75e3, ...
    PlaySound=true, ...
    Stereo=true, ...
    AudioSampleRate=48e3);

Create a spectrum analyzer System object to visualize the captured FM signal.

spectrumScope = spectrumAnalyzer(SampleRate=actualSampleRate,...
    ViewType="spectrum",SpectrumType="power",...
    PlotAsTwoSidedSpectrum=true,Method="welch",...
    AveragingMethod="exponential",SpectrumUnits="dBFS",...
    FullScaleSource="property",FullScale=2^11,...
    AxesScaling="manual",YLimits=[-80 -60],...
    ForgettingFactor=0.95,FrequencyOffset=fc);

Enable peak finder to identify the strongest FM stations.

spectrumScope.PeakFinder.Enabled      = true;
spectrumScope.PeakFinder.NumPeaks     = 6; % for narrowband acquisition, set the number of peaks to 1
spectrumScope.PeakFinder.LabelPeaks   = true;
spectrumScope.PeakFinder.LabelFormat = "x";

Receive the FM signal.

overrun = 0; 
for i=1:1000
     [iqdata,dataLen,overrun] = rx();     
     spectrumScope(iqdata)     
     drawnow    
     if (acqType == "Narrowband | 750kSPS")         
         fmDemodulate(double(iqdata)./2^11);                      
     end
end

Release the receiver System object rx.

release(rx);