S Parameters Guidance

[Catalyst]

Hello Everyone,

I am struggling with the concept of S Parameters. Let me just explain where my head is at first.

I have been performing S11 measurements on Antennas to find the VSWR, Impedance...etc...

When I output the data to disk I am left with a data file which has 2 two columns of data.

1. What does this data correspond to?
2. How do i transform them to VSWR / Impedance myself ?


Can anybody kindly clear this up?

Thanks for your time

Cat

 

[dhirajkhanna]

I suppose you are using a network analyzer for your S11 measurements, right?
In that case, if you are in log magnitude format, then the two columns will correspond to frequency and log magnitude. S11 implies that you are transmitting through port1 and measuring the reflected power at port 1. So the second column is the ratio of the reflected power to the incident power, expressed in dB.

 

[Catalyst]

Heres an example out put of the datafile

BEGIN,
6.46E-01,7.40E-01
6.47E-01,7.38E-01
6.48E-01,7.37E-01
6.50E-01,7.35E-01
6.52E-01,7.35E-01
6.54E-01,7.34E-01
6.55E-01,7.31E-01
6.56E-01,7.30E-01
6.58E-01,7.30E-01
...............
continues like so

 

[dhirajkhanna]

To find the impedance, just change the format on the VNA to smith chart. Then your text file will contain three columns, frequency, R and X. Similarly change the format to VSWR and you will get 2 columns- freq and VSWR

 

[Mazz]

Catalyst

usually Spar are expressed in a format (Touchstone) with simple understanding.
This is not your case. Maybe you are using an ATE, so you should look into the user manual.
You know that, just a couple of number, cannot be understood without knowing what they represent.
Mazz

I have done a quick google search, and the format you show seems to ba a part of a CITIfile, see **broken link removed** (really a piece of agilent ADS help).

 

[Catalyst]

Hi all, thanks for the comments.

I have a suspision the data could be the S11 parameters. Don't S Parameters have a magnitude and phase as well ?

How would i convert these to Impedance?

Kind Regards

Cat

 

[biff44]

We could try to guess what the heck you are doing. But it would be better for you to read the manual for the equipment you are using!

If I were to guess, the first column would be the real part of S11, and the 2nd column would be the imaginary part of S11. A little unusual, but you could accidently program your gear to show that.

 

[Catalyst]

In the case that these are the Real and Imaginery parts of the S11 Parameters....how would one be able to convert them to Impedance (Real and Imaginery) components?

I mean from a theoretical point of view, not with settings on the machine. The main problem I am trying to get across is that I do not understand the link between Impedance and S11 values.

Sorry for not being clearer.

Kind Regards

Cat

 

[ananth]

Hey Catalyst,
You can find the conversion formulae in Pozar's book. If you are studying any library will have these. Or u cud also google it.

Off the top of my head, I'd think you might have to convert to ABCD and then to Z parameters.

Like I said, Pozar lists these formulae in the cover pages of his Microwave Engineering book.

As far as the connection between the S parameters and impedane is concerned, if the Impedance where the signal is being input is not matched, you'll get a larger value of S11 indicating signal loss. It basically signifies the reflection of a signal because of an impedance mismatch.

Hope this helps..
Ananth.

 

[eunderhill]

remember S11 is the same as the reflection coefficient. You should know the formula for reflection coefficient (otherwise see Pozar). Basically, you're variables will be:
- reflection coefficient
(Γ, known, IT'S S11),
- input impedance
(50Ω , that's what it always is for Spars), and
- load impedance
(only unknown).

Just solve algebraically for the load impedance.

Good Luck

 

[tvincent]

What Network Analyzer are you using? Enter these measurements to a Smith Chart to get your Impedance. Or use the Network Analyzer options menu. I have found several Smith Charts on the WEB for free. All you need to do is enter the data from your S11 measurements onto a Smith Chart and that provides you with the input impedance for that frequency. You may want to consider this book: Electronic Applications of The Smith Chart by Phillip H. Smith

Good luck

Added after 50 seconds:

What Network Analyzer are you using? Enter these measurements to a Smith Chart to get your Impedance. Or use the Network Analyzer options menu. I have found several Smith Charts on the WEB for free. All you need to do is enter the data from your S11 measurements onto a Smith Chart and that provides you with the input impedance for that frequency. You may want to consider this book: Electronic Applications of The Smith Chart by Phillip H. Smith

Good luck

 

[dissa]

What you are getting is imaginary and real parts of the S11. Want the magnitude? Just square Imag and Real part and add them and then take the square root. To convert to decibel take 20 log of the result. Then you'll see the S11 curve as you see on the screen of the VNA.

Getting the Smith chart is bit tricky. You can use the magnitude os S11 previously calculated and phase which is arc(tan) Im/Re. There are programs to plot smith charts using these two parameters. I know AXUM can plot smith charts. Unfortunately I hate Excel in doing these kind of plots.

Hope this helps

 

[snkhan]

ITs seems that the data is real and imaginary part of impedance.
return loss=√ (Γreal²+Γimg²)

Regards

 

[peleda]

conversion from S to Z,
[Z] = ( + ) * inverse( - )

conversion from Z to S
= inverse([Z] + ) * ([Z] - )
page 177, 3rd ed Pozar