[SI-LIST] Re: Question about S-Parameter theory
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>, "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>, "Rolf" <rofini1975@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 7 Dec 2009 09:57:13 -0700
Rolf,
Forgot to add that the length of line that you are testing is important
also. With 50 ohm terminations on a 60 ohm line you get a repetitive S11
like Patrick said. Ex: at 1GHz S11 has minimums at 2GHz, 4GHz, 6GHz..etc:
given that the electrical length of the line is lambda/4 (90deg). At 180
deg you have mimimums at 2,3,4,5 GHz. The longer the line the more mimimums
you get. The impedance difference between the terminations and the line
under test determines the maximum value of S11. The greater the difference
between the two, the greater the magnitude of the S11 maxima.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Ken Cantrell
Sent: Monday, December 07, 2009 8:44 AM
To: Zabinski, Patrick; Rolf; si-list@xxxxxxxxxxxxx
Subject: [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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