Frequency Offset Calibration Transmitter with USRP Hardware
This example shows how to use the USRP™ System objects to measure and calibrate for transmitter/receiver frequency offset at the receiver.
The USRP Transmitter sends a sine wave at 100Hz with the MATLAB script, FrequencyOffsetCalibrationTransmitterUSRPHardwareExample.m
, to the USRP receiver. The USRP Receiver monitors received signals, calculates the transmitter/receiver frequency offset and displays it in the MATLAB command window for calibration with the MATLAB script, FrequencyOffsetCalibrationReceiverUSRPHardwareExample.m
.
Introduction
The example provides the following information about the USRP transmitter/receiver link:
The quantitative value of the frequency offset
A graphical view of the spur-free dynamic range of the receiver
A graphical view of the qualitative SNR level of the received signal
To calibrate the frequency offset between two USRP devices, run FrequencyOffsetCalibrationTransmitterUSRPHardwareExample.m
on one USRP radio, while simultaneously running FrequencyOffsetCalibrationReceiverUSRPHardwareExample.m
on another USRP radio. The CenterFrequency property of the SDRu transmitter and receiver System objects should have the same value.
To compensate for a transmitter/receiver frequency offset, add the displayed frequency offset value to the Center Frequency of the SDRu Receiver System object. Be sure to use the sign of the offset in your addition. Once you've done that, the spectrum displayed by the receiver's spectrum analyzer System object should have its maximum amplitude at roughly 0 Hz.
Please refer to the Simulink(R) model Frequency Offset Calibration with USRP Hardware in Simulink for a block diagram view of the system.
Required Hardware and Software
To run this example, ensure that the center frequency of the SDRu Transmitter and Receiver System objects is within the acceptable range of your USRP daughter board and the antennas you are using. You need one of the following:
USRP N2xx or B2xx series radio and Communications Toolbox Support Package for USRP Radio. For information on supported radios, see Supported Hardware and Required Software.
USRP E3xx, N3xx, X3xx, or X4xx series radio and Wireless Testbench Support Package for NI USRP Radios. For information on supported radios, see Supported Radio Devices (Wireless Testbench).
Discover Radio
Discover radio(s) connected to your computer. This example uses the first USRP radio found using the findsdru
function. Check if the radio is available and record the radio type. If no available radios are found, the example uses a default configuration for the system but does not run the main loop.
connectedRadios = findsdru;
Checking radio connections...
if strncmp(connectedRadios(1).Status, 'Success', 7) radioFound = true; platform = connectedRadios(1).Platform; switch connectedRadios(1).Platform case {'B200','B210'} address = connectedRadios(1).SerialNum; case {'N200/N210/USRP2','E320','X300','X310','X410','N300','N310','N320/N321'} address = connectedRadios(1).IPAddress; end else radioFound = false; address = '192.168.10.2'; platform = 'N200/N210/USRP2'; end
Initialization
Set the properties of the sine wave source, the SDRu transmitter, and the spectrum analyzer System object.
bbTxFreq = 100; % Transmitted baseband frequency rfTxFreq = 1.85e9; % Nominal RF transmit center frequency prmFreqCalibTx = configureFreqCalibTx(platform, rfTxFreq, bbTxFreq); hSineSource = dsp.SineWave (... 'Frequency', prmFreqCalibTx.SineFrequency, ... 'Amplitude', prmFreqCalibTx.SineAmplitude,... 'ComplexOutput', prmFreqCalibTx.SineComplexOutput, ... 'SampleRate', prmFreqCalibTx.Fs, ... 'SamplesPerFrame', prmFreqCalibTx.SineFrameLength, ... 'OutputDataType', prmFreqCalibTx.SineOutputDataType); % The host computer communicates with the USRP radio using the SDRu % transmitter System object. B200 and B210 series USRP radios are % addressed using a serial number while USRP2, N200, N210, X300 and X310 % radios are addressed using an IP address. The parameter structure, % prmFreqCalibTx, sets the CenterFrequency, Gain, and InterpolationFactor % arguments. % Set up radio object to use the found radio switch platform case {'B200','B210'} radio = comm.SDRuTransmitter( ... 'Platform', platform, ... 'SerialNum', address, ... 'MasterClockRate', prmFreqCalibTx.MasterClockRate, ... 'CenterFrequency', prmFreqCalibTx.USRPTxCenterFrequency, ... 'Gain', prmFreqCalibTx.USRPGain,... 'InterpolationFactor', prmFreqCalibTx.USRPInterpolationFactor) case {'E320','X300','X310','X410','N300','N310','N320/N321'} radio = comm.SDRuTransmitter( ... 'Platform', platform, ... 'IPAddress', address, ... 'MasterClockRate', prmFreqCalibTx.MasterClockRate, ... 'CenterFrequency', prmFreqCalibTx.USRPTxCenterFrequency, ... 'Gain', prmFreqCalibTx.USRPGain,... 'InterpolationFactor', prmFreqCalibTx.USRPInterpolationFactor) case {'N200/N210/USRP2'} radio = comm.SDRuTransmitter( ... 'Platform', platform, ... 'IPAddress', address, ... 'CenterFrequency', prmFreqCalibTx.USRPTxCenterFrequency, ... 'Gain', prmFreqCalibTx.USRPGain,... 'InterpolationFactor', prmFreqCalibTx.USRPInterpolationFactor) end
radio = comm.SDRuTransmitter with properties: Platform: 'N200/N210/USRP2' IPAddress: '192.168.10.2' ChannelMapping: 1 CenterFrequency: 1.8500e+09 LocalOscillatorOffset: 0 Gain: 23 PPSSource: 'Internal' EnableTimeTrigger: false ClockSource: 'Internal' EnableMIMOCableSync: false MasterClockRate: 100000000 InterpolationFactor: 500 TransportDataType: 'int16' EnableBurstMode: false
% Use spectrumAnalyzer to display the spectrum of the transmitted % signal. hSpectrumAnalyzer = spectrumAnalyzer(... 'Name', 'Frequency of the Sine waveform sent out',... 'Title', 'Frequency of the Sine waveform sent out',... 'FrequencySpan', 'Full', ... 'SampleRate', prmFreqCalibTx.Fs, ... 'YLimits', [-70,30],... 'FrequencySpan', 'Start and stop frequencies', ... 'StartFrequency', -100e3, ... 'StopFrequency', 100e3,... 'Position', figposition([50 30 30 40]));
Stream Processing
Loop until the example reaches the target number of frames.
% Check for the status of the USRP radio if radioFound for iFrame = 1: prmFreqCalibTx.TotalFrames sinewave = step(hSineSource); % generate sine wave step(radio, sinewave); % transmit to USRP radio end % Display the spectrum after the simulation. step(hSpectrumAnalyzer, sinewave); else warning(message('sdru:sysobjdemos:MainLoop')) end
Warning: No available USRP(R) devices found. Skipping main loop.
Release System Objects
release (hSineSource);
release (radio);
clear radio
Conclusion
In this example, you used Communications Toolbox™ System objects to build a signal source to send a reference tone at 100 Hz. This signal is to be used as a calibration signal for a USRP receiver.
Appendix
The following scripts are used in this example.
configureFreqCalibTx.m