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rf terminations - RF Cafe Forums
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dv8shane
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Post subject: rf terminations
Posted: Mon May 02, 2005 6:06 pm
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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
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Kirt Blattenberger
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Post subject:
Posted: Mon May 02, 2005 10:59 pm
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Site Admin |
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Joined: Sun Aug 03, 2003
2:02 pm Posts: 451 Location: Erie, PA
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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
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Guest |
Post subject:
Posted: Tue May 03, 2005 9:49 am
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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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Posted 11/12/2012
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