Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
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Below are all of the forum threads, including all
the responses to the original posts.
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
|Kirt Blattenberger |
Posted: Mon May 02, 2005 10:59 pm
Joined: Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
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!