[SI-LIST] Question about S-Parameter theory
- From: "Rolf" <rofini1975@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 7 Dec 2009 14:06:28 +0100
Hi *,
I have a question about S-Parameter theory.
I am quite familiar with reflections but there is one point which is not
clear to me.
Using S-Parameters one can characterize any network. So let's say our DUT is
long transmission line with an impedance of 60 ohm.
My reference system is based on 50 ohms, so I am connecting my DUT between
two 50 transmission lines.
Let's assume an incident wave on port 1 (the left hand side). Since my DUT
does not match the line impedance, of course some portion will be reflected.
Some other portion will be transmitted into my DUT. Now this wave inside the
DUT reaches the port 2 on the right hand side. Since the DUT impedance does
not match the impedance of the transmission line on port 2 there are again
reflections. So far, so good.
My point is now:
When you calculate S11 for example, it's assumed that port 2 is terminated
with the reference impedance of 50. The theory says that in this case no
reflected wave a2 exists. How does that fit together with my example above??
I will always have reflections on port 2 when my DUT is unequal to 50 ohms.
I assume it has something to do with the normalization process, but so far I
was not able to find a reasonable explanation.
Any thoughts or comments on this?
Regards
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