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rf terminations - RF Cafe Forums

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 Post subject: rf terminations
Posted: Mon May 02, 2005 6:06 pm 
I dont have the math skills to explain this to a colleague so if someone could help I need a lay explanation to explain why the termination is done at the measuring point at the end of a cable not at the source. He recently tried to show how the voltage from a calibrator output didn't change at the end of the cable when the source was terminated into 50 ohms (proper match)at varying lengths of cable used. I told him this was the incorrect way to do it and that it greatly affected the s paramameters


 
  
 
 Post subject:
Posted: Mon May 02, 2005 10:59 pm 
 
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Joined: Sun Aug 03, 2003 2:02 pm
Posts: 451
Location: Erie, PA
Greetings dv8shane:

Agilent has an excellent page that covers exactly what you are looking for. It even has an animated Java applet that illustrates voltage propagation along a transmission line.


http://www.educatorscorner.com/index.cg ... NT_ID=2483


- Kirt Blattenberger :smt024


 
   
 
 Post subject:
Posted: Tue May 03, 2005 9:49 am 
The shotr open and load are used to calibrate the combined 1-port error network of the cables and the imperfections fo the VNA. A matched termination does not have any reflection nor does a cable matched to the termination. But this is not the case for the real world. Neither the VNA nor the test set cables are true 50 Ohm. Therefore the error introduced by this network must be characterized with known loads placed at one port of the error network, since the VNA is on one side the loads must be at the measurement point. The Short, Open and Load solve for the three terms of the Error network for each port, namely the E11, E22 and ER (E21*E12). The through solves for 4 more parameters.

So, in theory, your friend could be correct for only the special case of an ideal VNA with 50 oHm matched cables and termination. In that case there would be no reflection for the termination so the error network would be determined solely from the response of the open and short. But you could never know if that special case existed with out a SOL cal at the mesaurement plane in the first place!




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