caiyu/LibreVNA
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Fix calculation of through standard

Browse Source
This commit is contained in:
Jan Käberich 2024-02-18 11:33:15 +01:00
parent 4aa1564de0
commit 993957d4d2
1 changed files with 19 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

31
Software/PC_Application/LibreVNA-GUI/Calibration/calstandard.cpp
View File

@ -574,18 +574,25 @@ Sparam Through::toSparam(double freq)
return Sparam(interp[0], interp[1], interp[2], interp[3]);
} else {
// calculate effect of through
double through_phaseshift = -2 * M_PI * freq * delay * 1e-12;
double through_att_db = loss * 1e9 * 4.3429 * delay * 1e-12 / Z0 * sqrt(freq / 1e9);;
double through_att = pow(10.0, -through_att_db / 10.0);
auto through = polar<double>(through_att, through_phaseshift);
// Assume symmetric and perfectly matched through for other parameters
auto S = Sparam(0.0, through, through, 0.0);
// update S parameters if Z0 does not match system impedance exactly
if(Z0 != 50.0) {
auto abcd = ABCDparam(S, Z0);
S = Sparam(abcd, 50.0);
}
return S;
// nomenclature and formulas from https://loco.lab.asu.edu/loco-memos/edges_reports/report_20130807.pdf
auto w = 2.0 * M_PI * freq;
auto f_sqrt = sqrt(freq / 1e9);
auto offset_impedance = Z0; // lossless characteristic impedance
auto offset_loss = loss * 1e9;
auto offset_delay = delay * 1e-12;
auto Z_c = std::complex<double>(offset_impedance + (offset_loss / (2*w)) * f_sqrt, -(offset_loss / (2*w)) * f_sqrt);
auto gamma_l = std::complex<double>(offset_loss*offset_delay/(2*offset_impedance)*f_sqrt, w*offset_delay+offset_loss*offset_delay/(2*offset_impedance)*f_sqrt);
auto p = exp(-gamma_l); // propagation factor
auto Z_r = std::complex<double>(50.0);
auto Gamma = (Z_c - Z_r) / (Z_c + Z_r);
// S-parameter of a transmission line, see e.g. "Specifying Calibration Standards and Kits for Keysight Vector Network Analyzers"
// Keysight application note equations (6) and (7)
auto Sxx = Gamma * (1.0 - p*p) / (1.0 - p*p*Gamma*Gamma);
auto Sxy = p * (1.0 - Gamma*Gamma) / (1.0 - p*p*Gamma*Gamma);
return Sparam(Sxx, Sxy, Sxy, Sxx);
}
}