Convert a table of S param to Z param

Hi,

I have text file which has freq and s21 value(In cartesian form and polar form ).

Need a help in convertering those S-paramteres to Z- parameters

I tried to use s2z command, but i doesn't work in this.

Could any one please help me resolve the problem?

Answer 1

[William Rose]

@Venkatkumar M,

How do you know s2z does not work? Do you get an error message, and if so, what?

The input to s2z must be complex numbers, in the form Re + i*Im. The file you provided has a header line and three columns: frequency, real part, imaginary part. It uses two different separators: tab and comma. That is annoying because it complicates the reading in of the data.

I assume that the data is in real, imaginary format. Change it to complex:

z=Re+1i*Im

Then use s2z.

[Venkatkumar M]

@William Rose

Thank you for the help, could you please help me to convert the z parameter to table form.

data = readcell('s21.txt');
[rows, columns] = size(data);
counter = 0;
for row = 1 : rows
    thisCellContents = data{row, 1};
    if contains(thisCellContents, 'Freq');
        % Skip a text line in the middle of all the data.
        continue;
    end
    counter = counter + 1;
    column1(counter) = str2double(thisCellContents(1:21));
    column2(counter) = str2double(thisCellContents(23:end-1));
    column3(counter) = data{row, 2};
end
fprintf('Done parsing %d lines of data to get %d numbers per vector.\n', rows, counter);

a=column1;
b=column2.';
c=column3.';
T = complex(b,c);
Z=s2z(T,50);
%%% COnvert above Z to table form%%%%
X=struct2table(Z);

[William Rose]

@Venkatkumar M,

It was easier for me to import the text file you provided to Excel. Therefore I did that. Then I saved the data, without the header line, to a text file, with tab separators (not tab plus comma). That file is attached to this message. It is easy to load the data from that file and convert columns 2 and 3 from s plane to z plane:

A=load('s21.txt','-ascii');
s=A(:,2)+1i*A(:,3);
z=s2z(s);

Try it.

[William Rose]

@Venkatkumar M,

A plot of the real and imaginary parts versus frequency shows that the values of s(f) repeat, and the repeat period is about 1.5E8 Hz:

A=load('s21.txt','-ascii');
figure; plot(A(:,1),A(:,2),'-r',A(:,1),A(:,3),'-b');
grid on; legend('Real','Imag'); 
xlabel('Frequency'); ylabel('s(f)');

Therefor, let us plot s(f) for one cycle of frequencies. At higher frequencies, s(f) repeats itself.

N1=find(A(:,1)>=1.5e8,1);  %index corresponding to one cycle of s(f)
figure; plot(A(1:N1,2),A(1:N1,3),'-kx'); grid on; axis equal; 
xlabel('Re(s)'); ylabel('Im(s)'); title('s plane');

s2z adds an extra dimension to the array, which complicates plotting. Squeeze array z after creating it with s2z, to eliminate the extra dimension:

s=A(:,2)+1i*A(:,3);
z=s2z(s);
z=squeeze(z);

Next: plot z(f) for one cycle:

figure; 
plot(real(z(1:N1)),imag(z(1:N1)),'-bx'); title('z plane'); 
grid on; axis equal

The default reference impedance for s2z() is 50 Ω. That is why real(s)=0 maps to real(z)=50. Good luck.

[Venkatkumar M]

@William Rose Simple question is it posisble to do the same process with freq and polar form data(s21_db.txt)?

[William Rose]

@Venkatkumar M, yes you can do it.

First, you must get the data from file s21_db.txt into a numeric array or vectors. I used the Import Tool in Matlab to import the data from file s21_db.txt. In the ImportData tool, I defined delimiters tab, comma, left parenthesis, right parenthesis, and "dB". I told import Tool to treatmultiple delimiters as one. I imported the columns as three column vectors: freq, magdB, and angle.

Then you convert the data from [dB, angle in degrees] to a vector of complex numbers as follows:

s=10.^(magdB/20).*(cos(angle*pi/180)+1i*sin(angle*pi/180));

Now you convert the complex vector s to z values:

z=s2z(s);

or you can do

z=squeeze(s2z(s));

to eliminate the extra dimension that is created by s2z().

[Venkatkumar M]

@William Rose

Is there way to delimeter (tab, comma, left parenthesis, right parenthesis, and "dB") this thing using command delimeter in readtable function in editor instead importing it and naming it

[William Rose]

@Venkatkumar M, Yes, you can use readtable to read in the file s21_dB.txt. See the following:

delims={'dB,','(',')','tab',char(176)};
T=readtable('s21_db.txt','Delimiter',delims);
whos('T');

  Name          Size             Bytes  Class    Attributes

  T         15957x3             384383  table              

The variable names are the default values: T.Var1, T.Var2, T.Vvar3. If you want to rename the variable name, do the following:

T = renamevars(T,["Var1","Var2","Var3"],["freq","magdB","phase"]);

Use the vectors in the table to compute the complex numbers for s2z.

s=10.^(T.magdB/20).*(cos(pi*T.phase/180)+1i*sin(pi*T.phase/180));
z=squeeze(s2z(s));
whos('z');

  Name          Size             Bytes  Class     Attributes

  z         15957x1             255312  double    complex   

Try.

[Venkatkumar M]

@William Rose

I am getting error in function renamevars

Undefined function 'renamevars' for input arguments of type 'table'.

Error in s_parm_db_to_z_param_1 (line 7)
T = renamevars(T,["Var1","Var2","Var3"],["freq","magdB","phase"]);

could you please clarify this?

[William Rose]

@Venkatkumar M, I don;t know why you get an error. The error message suggests Matlab is not finding the function renamevars(). renamevars is art of basic Matlab - nt some Toolbox. Therefore it should work.

Did renamevars get overwritten by accident? Try "clear renamevars" to address this possibility, then try your code again.