Writing A Touchstone® File

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This example shows how to write out the data in rfckt objects you create in the MATLAB® workspace into an industry-standard data file: Touchstone®. You can use these files in third-party tools.

This simple example shows how to create and analyze an RLCG transmission line object. It then shows how to write the analyzed result into a Touchstone file, read it back in, and compare the file data to the original result.

Create an RF Circuit Object to Represent an RLCG Transmission Line

Create an rfckt.rlcgline object to represent an RLCG transmission line. Create variables to represent the transmission line parameters and use the set method to update the rfckt.rlcgline object to use these values.

ckt1 = rfckt.rlcgline;
R = 0.002;                              % ohm/m 
G = 0.002;                              % S/m 
mu_0 = pi*4e-7;                         % H/m 
L = mu_0;                               % H/m
c = 299792458;                          % m/s 
epsilon_0 = 1/(mu_0*c^2);               % F/m 
C = epsilon_0;                          % F/m         
linelen = 10;                           % m 
Z_vacuum = sqrt(mu_0/epsilon_0);        % ohms
ckt1.R = R;
ckt1.G = G;
ckt1.L = L;
ckt1.C = C;
ckt1.LineLength = linelen;

Copy the Circuit Object

Make a copy of the first rfckt object. Then change the capacitance of the new object to introduce mismatch that will appear as a finite reflection coefficient on the Smith chart.

ckt2 = copy(ckt1)

ckt2 = 
   rfckt.rlcgline with properties:

              Freq: 1.0000e+09
                 R: 0.0020
                 L: 1.2566e-06
                 C: 8.8542e-12
                 G: 0.0020
          IntpType: 'Linear'
        LineLength: 10
          StubMode: 'NotAStub'
       Termination: 'NotApplicable'
             nPort: 2
    AnalyzedResult: []
              Name: 'RLCG Transmission Line'

ckt2.C = 0.5*ckt1.C;

Cascade Two Circuit Objects

Create an rfckt.cascade object that cascades the two transmission lines together.

ckt3 = rfckt.cascade('Ckts',{ckt1,ckt2});

Analyze the Cascade and Plot S-Parameter Data

Use the analyze method of the rfckt.cascade object to analyze the cascade in the frequency domain. Then, use the smithplot method to plot the object's S11 on a Smith chart.

freq = logspace(0,8,20);
analyze(ckt3,freq); 
figure
smithplot(ckt3,[1,1],'LegendLabels','S11 Original');

Write out the Data to an S2P File

Use the rfwrite function to write the data to a file.

workingdir = tempname;
mkdir(workingdir)
filename = fullfile(workingdir,'myrlcg.s2p');
if exist(filename,'file')
    delete(filename)
end
filedata = ckt3.AnalyzedResult.S_Parameters;
rfwrite(filedata,freq,filename)

Inspect the S2P File

Use the type function to display the contents of the .s2p file in the MATLAB command window to see the Touchstone file format.

type(filename)

# GHz S MA R 50

1.000000000000000e-09  4.9956688299520730e-01  1.7999980458876163e+02  4.9963354343955202e-01 -1.6640558205811934e-04  4.9963354343955202e-01 -1.6640558205811902e-04  4.9956688299521823e-01  1.7999980458844280e+02
2.636650898730358e-09  4.9956688299243801e-01  1.7999948476878276e+02  4.9963354343260874e-01 -4.3875342748465255e-04  4.9963354343260874e-01 -4.3875342748465299e-04  4.9956688299251434e-01  1.7999948476794214e+02
6.951927961775606e-09  4.9956688297319013e-01  1.7999864151514811e+02  4.9963354338433580e-01 -1.1568396188433882e-03  4.9963354338433602e-01 -1.1568396188433902e-03  4.9956688297372076e-01  1.7999864151293158e+02
1.832980710832436e-08  4.9956688283937206e-01  1.7999641814969570e+02  4.9963354304875479e-01 -3.0501822197363916e-03  4.9963354304875479e-01 -3.0501822197363908e-03  4.9956688284306039e-01  1.7999641814385149e+02
4.832930238571753e-08  4.9956688190907761e-01  1.7999055591120035e+02  4.9963354071582089e-01 -8.0422656731208870e-03  4.9963354071582100e-01 -8.0422656731208905e-03  4.9956688193471843e-01  1.7999055589579137e+02
1.274274985703133e-07  4.9956687544174538e-01  1.7997509923522475e+02  4.9963352449742515e-01 -2.1204646687655957e-02  4.9963352449742515e-01 -2.1204646687656002e-02  4.9956687561999880e-01  1.7997509919459898e+02
3.359818286283781e-07  4.9956683048132772e-01  1.7993434538435341e+02  4.9963341174836157e-01 -5.5909244749526403e-02  4.9963341174836151e-01 -5.5909244749526417e-02  4.9956683172053190e-01  1.7993434527727968e+02
8.858667904100824e-07  4.9956651792079754e-01  1.7982689184853271e+02  4.9963262792700741e-01 -1.4741305049577125e-01  4.9963262792700697e-01 -1.4741305049577147e-01  4.9956652653563866e-01  1.7982689156699070e+02
2.335721469090122e-06  4.9956434507311936e-01  1.7954357698415038e+02  4.9962717895522329e-01 -3.8867473728393093e-01  4.9962717895522341e-01 -3.8867473728393043e-01  4.9956440496207738e-01  1.7954357625600892e+02
6.158482110660261e-06  4.9954924203836615e-01  1.7879662219796427e+02  4.9958930282204372e-01 -1.0247626683562452e+00  4.9958930282204383e-01 -1.0247626683562454e+00  4.9954965834354320e-01  1.7879662053812120e+02
1.623776739188721e-05  4.9944436490450694e-01  1.7682803522724353e+02  4.9932621885777728e-01 -2.7012650168069037e+00  4.9932621885777739e-01 -2.7012650168069023e+00  4.9944725716695926e-01  1.7682803561343289e+02
4.281332398719396e-05  4.9872093087973968e-01  1.7165347141326552e+02  4.9750823089356755e-01 -7.1099543855329870e+00  4.9750823089356744e-01 -7.1099543855329834e+00  4.9874094831180454e-01  1.7165355930454223e+02
1.128837891684688e-04  4.9395192774857949e-01  1.5829285403785633e+02  4.8537498738050983e-01 -1.8527617133746109e+01  4.8537498738050966e-01 -1.8527617133746105e+01  4.9408695423932697e-01  1.5829462565930422e+02
2.976351441631320e-04  4.7005276175625949e-01  1.2736060374891665e+02  4.1944100749863295e-01 -4.5568428419474273e+01  4.1944100749863278e-01 -4.5568428419474301e+01  4.7083855558191262e-01  1.2738801395821361e+02
7.847599703514606e-04  4.3246964060503451e-01  8.0964046625780895e+01  2.4583660567079157e-01 -9.3582775189553644e+01  2.4583660567079157e-01 -9.3582775189553686e+01  4.3540960369119625e-01  8.1178776658663935e+01
2.069138081114790e-03  4.8456534514674954e-01  4.7259771019756514e+01  8.6002402769753919e-02 -1.6278818625706072e+02  8.6002402769753614e-02 -1.6278818625705992e+02  4.9196758351450026e-01  4.7787861008000988e+01
5.455594781168516e-03  6.0419813636585018e-01  2.6872620020357260e+01  1.6637174822162825e-02  8.7039349152634500e+01  1.6637174822162825e-02  8.7039349152634500e+01  6.1766382847138612e-01  2.7689212173598698e+01
1.438449888287663e-02  6.9842374918749306e-01  1.4365722237145548e+01  1.8280441246750511e-03 -1.1221107334109674e+02  1.8280441246684008e-03 -1.1221107334108370e+02  7.2277371883520825e-01  1.5713306512214587e+01
3.792690190732246e-02  7.4788451928626032e-01  6.7469821304155451e+00  2.0574647544194854e-04  1.5947439599551970e+02  2.0574647534614038e-04  1.5947439599551970e+02  7.9041650399708774e-01  8.2310537700086570e+00
1.000000000000000e-01  7.6256263726735674e-01  2.7440654158858386e+00  6.0006898721147904e-05  3.1994461726741314e+01  6.0006898749090796e-05  3.1994461833463021e+01  8.1978106471099599e-01  3.6949462491260925e+00

Compare the Data

Read the data from the file myrlcg.s2p into a new rfckt object and plot S11 on a Smith chart. Visually compare this Smith chart to the previous one to see that the data is the same.

ckt4 = read(rfckt.passive,filename); 
figure
smithplot(ckt4,[1,1],'LegendLabels','S11 from S2P');

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