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Characterizing a low-noise amplifier by measuring its S-parameters

This example shows you how to automate the measurement of 2-port S-parameter measurements from a Keysight Technologies® (formerly Agilent Technologies®) network analyzer using MATLAB® and Instrument Control Toolbox™. The example also visualizes the measurements in MATLAB and saves the measurements to a Touchstone data file using functions from the RF Toolbox™.


Scattering parameters, or S-parameters, describe the behavior of individual components, or networks of components in linear electrical networks. Measurements of S-parameters for components in an electrical network may be used to model performance, simulate behavior, design matching networks, etc. This MATLAB example connects to and configures a Keysight (Agilent) PNA (N5222A) to measure the S-parameters of a low noise amplifier (LNA). The instrument used in this example is running firmware version A.09.90.02. The Standard Commands for Programmable Instruments (SCPI) for the PNA in this example may need to be modified if you are using a different instrument or firmware. The SCPI command set for most instruments is available in the instrument programmer's manual from your instrument vendor.


To execute this example you need a Keysight (Agilent) PNA connected to your device under test. The parameters in this example are specific to the measurements for the low-noise amplifier we are using and will likely need to be modified to make measurements on your device under test.

This example uses functions from the Instrument Control Toolbox and the RF Toolbox.

Instrument Connection and Configuration

Define the parameters used to configure the instrument before you make the measurement. In this example we connect to a Keysight (Agilent) PNA using a VISA interface to an instrument on a TCPIP network. Based on your measurement of interest, you may need to modify some of the following parameters.

% Define instrument VISA address. The VISA address of the instrument
% may be obtained from the instrument's user interface or your VISA
% configuration utility
instrumentVISAAddress = 'TCPIP0::';
% Define frequency range of 2.3GHz to 2.6GHz
frequencyRange = [2.3e9 2.6e9];
% Number of points in measurement
numPoints = 401;

% Create a VISA connection to interface with instrument
instrObj = visa('agilent',instrumentVISAAddress);
% Set up connection parameters for transfer of measurement data from the
% instrument
instrObj.InputBufferSize = 10e6;
instrObj.ByteOrder = 'littleEndian';
% Open connection to the instrument and clear hardware buffer of instrument

% Display information about instrument
IDNString = query(instrObj,'*IDN?');
fprintf('Connected to: %s\n',IDNString);
Connected to: Agilent Technologies,N5222A,US51220131,A.09.90.02

Measurement Setup

Set up the instrument to make the 2-port S-parameter measurements.

% Prompt user to ask if they want to calibrate the instrument
doCalibration = questdlg('Do you want to calibrate your PNA?', 'ExampleMeasureSparameter','yes','no','no');
if strcmpi(doCalibration,'yes')
    % Launch wizard-guided calibration on the instrument and prompt user to confirm completion
    fprintf(instrObj,'SYSTem:CORR:WIZ MAIN');
    hMsgBox = msgbox('Press OK when calibration is complete', 'ExampleMeasureSparameter','help','modal');
    % Wait till user has confirmed that the calibration is complete

% Perform a System Preset

% Wait till system is ready as Preset could take time
opcStatus = 0;
    opcStatus = str2double(query(instrObj, '*OPC?'));

% Define a measurement name and parameter
fprintf(instrObj,'CALCulate:PARameter:DEFine:EXT ''SParamMeasurementS11'',S11');
fprintf(instrObj,'CALCulate:PARameter:DEFine:EXT ''SParamMeasurementS12'',S12');
fprintf(instrObj,'CALCulate:PARameter:DEFine:EXT ''SParamMeasurementS21'',S21');
fprintf(instrObj,'CALCulate:PARameter:DEFine:EXT ''SParamMeasurementS22'',S22');

% Create a new display window and turn it on
fprintf(instrObj,'DISPlay:WINDow1:STATE ON');

% Associate the measurements to WINDow1
fprintf(instrObj,'DISPlay:WINDow1:TRACe1:FEED ''SParamMeasurementS11''');
fprintf(instrObj,'DISPlay:WINDow1:TRACe2:FEED ''SParamMeasurementS12''');
fprintf(instrObj,'DISPlay:WINDow1:TRACe3:FEED ''SParamMeasurementS21''');
fprintf(instrObj,'DISPlay:WINDow1:TRACe4:FEED ''SParamMeasurementS22''');

% Turn ON the Title, Frequency, and Trace Annotation to allow for
% visualization of the measurements on the instrument display
fprintf(instrObj,'DISPlay:WINDow1:TITLe:STATe ON');
fprintf(instrObj,'DISPlay:ANNotation:FREQuency ON');
fprintf(instrObj,'DISPlay:WINDow1:TRACe1:STATe ON');
fprintf(instrObj,'DISPlay:WINDow1:TRACe2:STATe ON');
fprintf(instrObj,'DISPlay:WINDow1:TRACe3:STATe ON');
fprintf(instrObj,'DISPlay:WINDow1:TRACe4:STATe ON');

% Turn OFF averaging
fprintf(instrObj,'SENSe1:AVERage:STATe OFF');

% Set the number of points
fprintf(instrObj, sprintf('SENSe:SWEep:POINts %s',num2str(numPoints)));

% Set the frequency ranges
fprintf(instrObj, sprintf('SENSe:FREQuency:STARt %sHz',num2str(frequencyRange(1))));
fprintf(instrObj, sprintf('SENSe:FREQuency:STOP %sHz',num2str(frequencyRange(2))));

% Select measurements and set measurement trigger to immediate
fprintf(instrObj,'CALCulate:PARameter:SELect ''SParamMeasurementS11''');
fprintf(instrObj,'CALCulate:PARameter:SELect ''SParamMeasurementS12''');
fprintf(instrObj,'CALCulate:PARameter:SELect ''SParamMeasurementS21''');
fprintf(instrObj,'CALCulate:PARameter:SELect ''SParamMeasurementS22''');
fprintf(instrObj,'TRIG:SOURce IMMediate');

% Autoscale display
fprintf(instrObj, 'DISPlay:WIND:Y:AUTO');

% Select a single sweep across the frequency range to trigger a measurement
fprintf(instrObj,':SENSe:SWEep:MODE SINGLE');

% Since the instrument may take time to make the measurement, wait until it
% is done before requesting measurement data
opcStatus = 0;
    opcStatus = str2double(query(instrObj, '*OPC?'));

Retrieve measurement data

The instrument can return the measured data in a variety of formats. The most efficient way to transfer the S-parameter measurements is using IEEE 488.2 block format.

% Set instrument to return the data back using binblock format
fprintf(instrObj, 'FORMat REAL,64');

% Set byte order to swapped (little-endian) format. SWAPped is required
% when using IBM compatible computers
fprintf(instrObj, 'FORMat:BORDer SWAP');

% Request 2-port measurement data from instrument
fprintf(instrObj, 'CALC:DATA:SNP:PORTs? ''1,2''');

% Read the measured data
rawDataDB = binblockread(instrObj, 'double'); 

% Read back the number of points in the measurement and reshape the
% measurement data
numPoints = str2double(query(instrObj,' SENSe:SWEep:POINts?'));
% Reshape measurement data to [frequency, real, imag] array
rawDataDB = reshape(rawDataDB, numPoints, 9);

Display any errors

Display the instrument errors to the user if any of the preceding SCPI commands caused an instrument error.

% Loop through the error queue and display all errors
systemError = '';
while isempty(strfind(lower(systemError),'no error'))
    systemError = query(instrObj,'SYSTem:ERRor?');
    fprintf('System Error(s): %s',systemError);
System Error(s): +0,"No error"

Clean up Instrument Connections

Clean up any connections to the instrument after measurements are complete

% Close, delete, and clear instrument connections.
clear instrObj;

Format data into sparameters object for use by RF Toolbox

The RF Toolbox has objects that can be used to store measurements such as the S-parameter measurements obtained from the instrument. The objects can be created by reading in measurements from standard file formats, or, by initializing the object with measurement data.

% Store frequency range of the measurements
freqRange = rawDataDB(:,1);

% Convert retrieved magnitude info from dB
sparamMag = 10.^((1/20).*rawDataDB(:,2:2:8));

% Convert retrieved phase info from degrees to radians
sparamPhase = rawDataDB(:,3:2:9)*(pi/180);

% Extract S-Parameter vectors
rawDataRI = sparamMag.*(cos(sparamPhase)+1i*sin(sparamPhase));
S11 = reshape(rawDataRI(:,1),1,1,numPoints);
S12 = reshape(rawDataRI(:,3),1,1,numPoints);
S21 = reshape(rawDataRI(:,2),1,1,numPoints);
S22 = reshape(rawDataRI(:,4),1,1,numPoints);

% Assemble into a 3D matrix to be consumed by the RF Toolbox
SParameter3Ddata = [S11 S12; S21 S22];

Write out the measured data into a Touchstone file

Touchstone files are a standard file format used to store S-parameter measurement data. RF Toolbox provides standard functions to read and write touchstone files from MATLAB. The S-parameter measurements can be used directly in MATLAB for design purposes, such as this example on designing matching networks for a low-noise amplifier.


Visualize the measurement data in MATLAB

MATLAB can be used to visualize the data in a number of ways. For an easy comparison with what is displayed on the screen of the instrument, plot the frequency vs. complex magnitude of the S-parameter data.

% Create the sparameters object
hSParamData = sparameters(SParameter3Ddata,freqRange);
% Plot the magnitude v/s frequency plot for the S-parameters of the LNA
title('2-Port S-Parameter Measurement Values');

Additional Information

This example shows you how to automate the measurement of S-parameter data from a Keysight Technologies® (formerly Agilent Technologies®) network analyzer using MATLAB and Instrument Control Toolbox. Using Instrument Control Toolbox, it is possible to control a wide range of test & measurement hardware such as signal analyzers, RF signal generators, oscilloscopes, etc. from within MATLAB. MATLAB can be used to visualize the measurement data or use the measurements to analyze and design digital and RF systems using the library of functions in the Signal Processing Toolbox™, DSP System Toolbox™, Communications Toolbox™ and RF Toolbox.