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Balun vs Single-ended return loss measurements on a twisted pair

Question asked by steaky1212 on May 4, 2012

Sorry if this is in the wrong place. 

Historically, I measure the NEXT and Return Loss of a differential twisted pair using a balun. 1 port calibration is performed using an open, short and 100R load using 100thou header pins, with the thru measurement with the baluns back-to-back.

I am trying to move over to the single ended method as per [] but I am seeing a difference in return loss measurements (balun averaging around 6db better), when measuring twisted pair.  
I calibrate open, short and 50R load and thru using 100thou header pins. 

I dont have access to an LCR meter, and I dont know how to characterise the calibration artifacts so I havent selected a cal kit. I'm using the E5071B ENA. Frequency range is from 1MHz to 1GHz. Baluns used are the BH Electronics 040-0192. Test leads are RG402 50R Semi-rigid Coax approx 150mm long. Twisted pair is 1km long, nominal 100R characteristic impedance. Far end is terminated with 50R from each wire down to ground. 

I dont know what other information to give, and I have run out of ideas.

Thanks in advanced.