S-parameter and Touchstone

[tyassin]

Hi

Maybe a trivial question....
I have an S-parameter file containing data for a three port balun. One input port is 50 ohm and the two output ports are 25 ohm.

I can see in the touchstone file from the vendor that it is measured in 50 ohm inviroment. BUT when I import the touchstone file in Designer I get the correct amplitude and phase balance and match at port 1. However the match at port two is very poor. COmpared to the datasheet the matching should be better that 15 dB at all ports but port 2 & 3 only give around 7-8 dB RL.

What is going wrong here...is it a setup problem in port impedance or what???

Thank you

 

[madengr]

The balun requires ports 2 and 3 be driven simultaneously but out of phase. Measuring S22 as you are just leaves port 3 terminated but un-driven. You can:

1) Use an ideal balun to convert it back to single ended. See attached picture.
2) Lift the ground on a measurement port (ADS I think allows this) and connect it to port 3.
3) Use a mode converter like in MWO.

 

[tyassin]

Hello Madengr

I was not aware of this property of a balun.
You say that you must also drive the port. But in your picture how do you drive the port. I see is as you just connect a port as well although through a transformer.

Thank you very much.

Regards

 

[toonafishy]

@DS does allow the S-parameter termination to not be grounded. So you should connect the TERM across the differential with neither pin grounded. This is the easiest way to do it.

You can simulate the device as a two port (s1-s2, s2-s3, s1-s3) but you then must convert the 2 port S parameters to 3 port to get the correct results. There is an @DS example in the Examples folder that show how to do it this way. That is how I measure 3 and 4 port devices on my 2 port VNA.

 

[madengr]

It's a linear simulator so if you are measuring S22 it drives a small signal into port 2 and measures the reflection. The problem is that ports 2 and 3 are a balanced pair, so in order to get a minimum reflection out of port 2 you must be driving port 3 with the same signal but out of phase.

Also, a linear simulator really doesn't drive the circuit with a signal but computes everything with transfer functions. The units of volts cancel out in the end but it's handy to think of it that way.