RF Toolbox: Plotting S-parameters

Hi,

I recently purchased the RF toolbox addon for MATLAB. I have been using the matching network designer and so far have encountered a strange issue.

The issue is that the S-parameter plots in the matching network designer app are not the same as those generated by manual code, i.e: matchingnetwork(), sparameters() and rfplot().

For example given the following code snippet (for an L match):

f       = 1.5e9;       
Zin     = 50;     
Zl      = 25;     

n = matchingnetwork('CenterFrequency',f,'Bandwidth',7.5e8,'LoadImpedance',Zl,'SourceImpedance',Zin, 'Components',2);
freq = linspace(n.CenterFrequency-n.Bandwidth/2,n.CenterFrequency+n.Bandwidth/2);
S = sparameters(n,freq,50,1)
rfplot(S,1,1)

The following plot is obtained:

Which is completely different to the S1,1 obtained by constructing the same circuit (L-match) in the matching network designer:

Why am I not seeing a minimum point in the first image/plot? I expected the two plots to be the same. Am I plotting S1,1 incorrectly?

Thank you in advance.

Answer 1

[Abhishek]

Hi Arsat,

I understand that you're trying to figure out difference between plot of S parameters using the matchingnetwork function and through app.

Notice that when using the following command for calculating the S parameters, the load impedance information is not being transferred to the sparameter function.

s = sparameters(n,freq,RefZ0,ckt_no)

Consequently, the default value is utilized, and any user-defined values specified during the design of the matching network is ignored. Refer to: Calculate S-parameters for RF data, network, circuit, and matching network objects - MATLAB sparameters (mathworks.com)

As a result, it is recommended to utilize the following command directly without defining the s-parameters:

rfplot(n,freq,1)

This will give you the following result which matches with one produced by Matching Network Designer app.

Also, ensure that the component property aligns with the topologies mentioned in the configuration tab of the application, such as 'L', 'Tee', or 3.

For additional guidance on utilizing the rfplot function, please refer to the associated documentation:

Plot S-parameter data - MATLAB rfplot (mathworks.com)