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ADS 2011 Coax Model and using it as a Balun for impedance matching

Question asked by paulh on Feb 2, 2013
Latest reply on Feb 3, 2013 by paulh
Hello ADS users : )

My question pertains to the 4 terminal coax cable model in ADS. I am using this model as part of a schematic to transform a 50 ohm source to 
a blanced 5 ohm load. The circuit is the input impedance match for a pair of fets.

Anyway I created the schematic and made a simple proto type of the design. Testing the design in the real world at 80 Mhz with a HP8753 VNA my S11 input returnloss is not even close to what the simulator produces. This has kind of got me curious as to why the huge difference between what the sim produces and what the VNA says it is. I verified the coax cable I am using with a caliper so that its dimensions are correct. Verified the sim produced no errors etc.. All Look's good.

Running the simulator the S11 is better than -20dB over a 20Mhz bandwidth. In the real world the results are very different typically only -4dB with the VNA.
I think possibly that the coax model is the culprit.

I was wondering can the coax model in ADS be really used in a balun configuration to produce viable results. I am kind of scratching my head here so any
help would be greatly appreciated. I attach a schematic of the circuit and a pic of the actual proto. Perhaps there might be a more effective way to simulate this type of circuit with ADS ?

The balun configuration is a 1:1 in cascade with a 9:1

Thanks Paul.  

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