[SI-LIST] Re: Question about S-Parameter theory
- From: "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>
- To: "Rolf" <rofini1975@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 7 Dec 2009 07:31:08 -0600
> 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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