Build Simulink Model from Rational Function

Read Single-Ended 4-Port S-Parameters and Convert Them to Differential 2-Port S-Parameters

Read a Touchstone data file, default.s4p, into an sparameters object. The parameters in this data file are the 50-ohm S-parameters of a single-ended 4-port passive circuit, measured at 1496 frequencies ranging from 50 MHz to 15 GHz. Then, get the single-ended 4-port S-parameters from the data object, and use the matrix conversion function s2sdd to convert them to differential 2-port S-parameters.

filename = 'default.s4p';
backplane = sparameters(filename);
data = backplane.Parameters;
freq = backplane.Frequencies;
z0 = backplane.Impedance;

Convert to 2-port differential S-parameters. This operation pairs together odd-numbered ports first, followed by the even-numbered ports. If a different configuration has been used to measure the single ended S-parameters, you can specify a different second argument in the s2sdd command. For example, option "2" will allow you to pair the input and output ports in ascending order. Alternatively, you can use the command snp2smp to change the port order.

diffdata = s2sdd(data,1);
diffz0 = 2*z0;

Compute Transfer Function and Its Rational Function Representation

First, use the s2tf function to compute the differential transfer function. Then, use the rationalfit function to compute the closed form of the transfer function and store it in an rfmodel.rational object. The rationalfit function fits a rational function object to the specified data over the specified frequencies.

difftf = s2tf(diffdata,diffz0,diffz0,diffz0);
fittol = -30;          % Rational fitting tolerance in dB
delayfactor = 0.9;     % Delay factor
rationalfunc = rationalfit(freq,difftf,fittol,'DelayFactor', delayfactor)
npoles = length(rationalfunc.A);
fprintf('The derived rational function contains %d poles.\n', npoles);

rationalfunc = 

   rfmodel.rational with properties:

        A: [20x1 double]
        C: [20x1 double]
        D: 0
    Delay: 6.0172e-09
     Name: 'Rational Function'

The derived rational function contains 20 poles.

Get Numerator and Denominator of Laplace Transform S-Domain Transfer Functions

This example uses Laplace Transform S-Domain transfer functions to represent the backplane in the Simulink model. Convert the poles and corresponding residues of the rational function object into numerator and denominator form for use in the Laplace Transform transfer function blocks. Each transfer function block represents either one real pole and the corresponding real residue, or a pair of complex conjugate poles and residues, so the transfer function block always has real coefficients. For this example, the rational function object contains 2 real poles/residues and 6 pairs of complex poles/residues, so the Simulink model contains 8 transfer function blocks.

A = rationalfunc.A;
C = rationalfunc.C;
den = cell(size(A));
num = cell(size(A));
k = 1;                          % Index of poles and residues
n = 0;                          % Index of numerators and denominators
while k <= npoles
    if isreal(A(k))             % Real poles
        n = n + 1;
        num{n} = C(k);
        den{n} = [1, -A(k)];
        k = k + 1;
    else                        % Complex poles
        n = n + 1;
        real_a = real(A(k));
        imag_a = imag(A(k));
        real_c = real(C(k));
        imag_c = imag(C(k));
        num{n} = [2*real_c, -2*(real_a*real_c+imag_a*imag_c)];
        den{n} = [1, -2*real_a, real_a^2+imag_a^2];
        k = k + 2;
    end
end
den = den(1:n);
num = num(1:n);

Build Simulink Model of Backplane

Build a Simulink model of the backplane using the Laplace Transform transfer functions. Then, connect a random source to the input of the backplane and a scope to its input and output.

modelname = fliplr(strtok(fliplr(tempname), filesep));
simulink_rfmodel_build_rational_system_helper(modelname , numel(num))
simulink_rfmodel_add_source_sink_helper(modelname)

Figure 1. Simulink model for a rational function

Simulate Simulink Model of Rational Function

When you simulate the model, the Scope shows the impact of the differential backplane on the random input signal.

set_param([modelname,'/Rational Model Output'], 'Open', 'on')
sc = get_param([modelname,'/Rational Model Output'],'ScopeConfiguration');
sc.Position = [200, 216, 901, 442];
sim(modelname);

Close Model

close_system(modelname, 0)