[SI-LIST] AW: AW: Help

  • From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
  • To: "herman_westra@xxxxxxxxxxx" <herman_westra@xxxxxxxxxxx>, "shine2828@xxxxxxxxx" <shine2828@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 19 Dec 2012 16:36:41 +0000

Herman,

thanks for the additional explanation. my answer might have been too brief. 
What I meant is, that every S-parameter set (especially in touchstone format) 
comes with a common reference, and this can result in strange or wrong results 
depending on what you put together, and how.

E.g. if you have a channel consisting of two "isolated" Transceivers connected 
with only a twisted pair for data. In simulation, you could use 2 S-Parameter 
sets (one transceiver, one cable) If you connect all reference points of the 
s-parameters to GND, you open a very short return path for common modes which 
is definitely not there in reality.

Or imagine a huge daugtercard. the "GND" will not have a clean 0 voltage all 
across the plane, but S-parameters of this daughtercard will have this fixed 0 
Voltage, as they only have one reference node.

When you directly measure S-Parameters with a VNA, the coax cables bring the 
GND for each Port directly to the DUT. If your DUT has different reference 
voltages at each port, you can not measure directly.

This is what I meant with "all referenced to GND"

BR
Gert



----------------------------------------
Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 Espelkamp; 
Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB 8808; 
Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning, 
Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost

-----Ursprüngliche Nachricht-----
Von: herman_westra@xxxxxxxxxxx [mailto:herman_westra@xxxxxxxxxxx]
Gesendet: Mittwoch, 19. Dezember 2012 16:54
An: Havermann, Gert; shine2828@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Betreff: RE: [SI-LIST] AW: Help

Gert,

[...] that S-Parameters are based on the idea of "GND", and they all reference 
to GND. [...]

Not quite so.

S-parameters describe the relationship (magnitude and phase) of 'power waves' 
(voltage, current of a sinusoidal signal referenced to a specified Z0) incident 
onto and scattered from a multi-port electrical network (Sji).
Each port has two terminals (+ and - to allow the phase relationships to be 
described unambiguously).
S-parameters contain no information about the relationship (and do not require 
a connection) between any of the two terminals of a port to any terminal of any 
other port. Ports of an S-parameter network are floating (with respect to each 
other and wrt to "GND"). All current going into a port's + terminal comes out 
of the - terminal of the same port (or vice versa).
The ONLY thing that S-parameters describe is (again) the magnitude/phase 
relationship between a wave entering the network at port i and the wave 
scattered (reflected/transmitted) from port j (Sji).

In many circuit simulators the minus terminals of the ports of 
S-parameter-based models are tied together to a common pin. This is not related 
to how the S-parameters were measured/EM-simulated and does not change the 
behavior of the model. The common pin can be connected to the circuit 
simulator's node 0 or GND, but this is not a requirement and only dependent on 
the intended overall circuit to be simulated.

Regards,

Herman Westra
Application Engineer
Agilent EEsof EDA


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Havermann, Gert
Sent: Tuesday, 18 December, 2012 14:05
To: shine2828@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AW: Help

Joy,

S-Parameter have advantages and disadvantages you should know about before you 
start using them.
One advantage is, that S-Parameters can include quite complex channel 
geometries with coupling, components, connectors and discontinuities which are 
hard to model just with transmission lines.
One disadvantage is, that S-Parameters are based on the idea of "GND", and they 
all reference to GND.

There are many other aspects. S-Parameters are just as good as the way they 
were achieved, and transmission lines are just as good as their model.

There has been a nice tutorial at DC2012:
http://blogs.synopsys.com/analoginsights/files/2012/02/BradBrim_TutorialSlides_final1.pdf
This is also helpful information:
http://www.eetimes.com/electronics-news/4154786/Using-S-parameters-for-signal-integrity-analysis

There are many other links to be found with google, bing or other search 
engines.

BR
Gert



----------------------------------------
Absender ist HARTING Electronics GmbH, Marienwerderstraße 3, D-32339 Espelkamp; 
Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.: HRB 8808; 
Vertretungsberechtige Geschäftsführer: Dipl.-Kfm. Edgar-Peter Düning, 
Dipl.-Ing. Torsten Ratzmann, Dr.-Ing. Alexander Rost

-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im 
Auftrag von Joy Christian
Gesendet: Dienstag, 18. Dezember 2012 12:06
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Help

Hi,
I am using Scattering parameter based channel in my design. Can anyone please 
tell me the advantages of Scattering parameter over transmission line?

Thanks,
Joy
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