Imported balun .s3p file with Microwave Office help

[rkwong]

Hello,

I downloaded various S-Parameter .s3p files of 50 to 200 ohm baluns for 2.4Ghz operation from various vendors (i.e. TDK, Anaren, Johanson) and just want to verify that I get 200 ohm on the differential balanced ports. In looking inside the S-parameter files, they define port 1 as the unbalanced port and ports 2 and 3 as the balanced ports. So when I import it, Microwave Office creates a 3 port device. So in Microwave Office, I just create a simple schematic circuit where I attach a grounded 50 ohm resistor to port 1, and attached the Microwave Office Port elements with Z=100 Ohm to each of the balanced ports of the balun. (for example I use Ports 6 and 7). I then create a Smith chart measurement and measure Port Parameter Z66 and Z77 respectively and define a single frequency of 2.4Ghz. However, I am running into problems. The problem is that if I compare results from different vendors supposedly advertising the same 50 to 200ohm @2.4Ghz specs, I get results that are all over the Smith Chart and are not even close to each other. This also means that I'm not getting the 100 ohm impedance expected at each port (assuming a virtual ground in between, since assuming microwave office ports measurements are with respect to ground, so combining the two should yield 200 ohm). In fact, I get a complex result for the impedance but the imaginary parts are not 180 degrees out of phase as expected, but rather off by maybe 20 to 30 degrees. Can someone advise whether this is the proper way to test the imported balun element or propose an alternative in Microwave Office?

Thanks,
rkwong

 

[eastern]

check if the reference impedance of the "touchstone" files are all 50. if not change it manually. many set it to 1 ohm.
I doubt nothing else!

 

[madengr]

This is a common problem, I did it once and learned from it. The output ports 2 and 3 are balanced; therefore ports 2 and 3 must be driven simultaneously with equal magnitude and opposite phase. You must use a multi-mode converter (or a transformer) between the balun and you're measurement port.

 

[rkwong]

Hi,

Thanks for the replies. For eastern's suggestion, I did check the .s3p files and it is using 50 ohm since the anaren and tdk files both have the entry:
# MHZ S MA R 50
which I believe translates into readings are for mhz frequency values, s parameters measurements listed in magnitude angle format, using 50 ohm reference.

For madengr's reply: I do remember you posting a reply to another balun question where you included a picture with the transformer in it. I'll try it out and see. But can you tell me if my measurement ports on the balanced side are set correctily? (i.e. Z=100 for each port on the balanced side for the 50 to 200 balun conversion case) Or should I just use standard 50 ohm, independent of the transformation ratio (i.e. if using 50 to 100 later on)

Thanks,
rkwong

 

[madengr]

I think 50 ohms at the input and 100 on the output is OK. If the input return loss is poor that indicates the balun is not terminated properly. I don't remember what value is used to terminate the MMCONV element in MWO; it's either 100 ohms or 200 ohms.