[SI-LIST] Re: Question about S-Parameter theory

Rolf,
Or to look at it another way, terminate the DUT in 60 ohms and you get your
textbook S-parameters.

Ken

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Zabinski, Patrick
Sent: Monday, December 07, 2009 6:31 AM
To: Rolf; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Question about S-Parameter theory



> 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.


Ah, close, but not quite.

S-parameters do assume that the external equipment has no reflections.
That is, the internal termination of the VNA (or simulation) is ideally
matched to the 50 Ohm cable.

However, S-parameters do not assume that your output port is matched to
the equipment.

> I will always have reflections on port 2 when my DUT is
> unequal to 50 ohms.

Exactly - and these reflections will be captured in the S-parameters.

> How does that fit together with my
> example above??

Because of the Port 2 reflections, the effective relections at Port 1
(S11) will by cyclical in nature across the frequency spectrum, which is
due to the constructive/destructive contributions of the two ports.  If
you look at S11, you'll see resonances across the spectrum, where the
spread of the resonances is related to the half-wavelength of the
physical structure.

Pat


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