Spectrum Software has released Micro-Cap 11, the eleventh generation of our
SPICE circuit simulator.
For users of previous Micro-Cap versions, check out the
new features available in the latest version. For those of you who are
new to Micro-Cap, take our
features tour to see what Micro-Cap has to offer.
Converting S-Parameters to Y-Parameters
Manufacturers of RF devices often supply S-parameters for their parts in the form of tables of magnitude and phase versus frequency. Micro-Cap 6 cannot directly import S-parameter tables, so how can you use them? The answer is to convert the S-parameter equations to Y-parameter equations and then implement them with Laplace table sources. Y-parameters may be calculated directly from S-parameters using the following standard formulas (See "Microwave Circuit Design", by Vendelin, Pavio, and Rhoda page 16).
D = ((1+S11)*(1+S22)-S12*S21)
y11 = ((1-S11)*(1+S22)+S12*S21) / D
y12 = -2*S12 / D
y21 = -2*S21 / D
y22 = ((1+S11)*(1-S22)+S12*S21) / D
All we need to do is to read in the S-parameters from a file, calculate the equivalent Y-parameter table values at each frequency using the formulas above, and write a file with four tables of values, one each for y11, y12, y21, and y22. The table file is then imported into a circuit like the figure below that implements the Y-parameter equations with Laplace table voltage-controlled current sources.
This equivalent circuit implements the standard Y-parameter equations:
I1 = y11*V1 + y12*V2
I2 = y21*V1 + y22*V2
where V1 and I1 are the input port voltage and current and V2 and I2 are the output port voltage and current.
While the conversion formulas look simple, they require complex arithmetic. To ease the conversion, we have made a DOS program called STOY(S to Y converter) available for download from our web site at www.spectrum-soft.com.
The program is run with command line arguments that list the input and output file names:
stoy infile > outfile
stoy GG10V10M.STP > 194.OUT
STOY input file format
The program reads in an ASCII text file containing the S-parameters in standard tabular value
format as follows:
Using the STOY output
When the 194.OUT file generated by STOY is imported into a circuit with a statement like
the symbolic variables, y11, y12, y21, and y22 defined by these statements are available for use in the circuit. In the sample circuits to follow we have set the VALUE attribute of each source to one of these four variable names. Definitions of s11, s12, s21, and s22 are also included so that a quick comparison can be made between the measurements of the S-parameters on the Y-equivalent circuit and the original S-Parameters can be compared.
Figure 18 shows how to use the equivalent Y-parameter circuit to model a two-port whose parameters are imported from a file (194.out) generated by the STOY program.
This is a very general circuit that can model any two-port. You need only change the name of the input file.
Here is a plot of the measured S11 and S21 parameters, together with a comparison plot of the original S11 and S21 parameters.
Figure 20 shows the equivalent Y-parameter set up to measure the S22 and S12 parameters.
Figure 21 shows a plot of the measured S22 and S12 parameters, together with a comparison plot of the original S22 and S12 parameters.
In general, the fit between measured and modeled values is exact at the original data points, whereas logarithmic interpolation used between data points produces some slight discrepancies.