[SI-LIST] Re: Causality test

  • From: "Aubrey Sparkman" <asparky@xxxxxxxxxxxxxxxxxxxxxxxx>
  • To: "'Scott McMorrow'" <scott@xxxxxxxxxxxxx>, "'Luciano Boglione'" <l.boglione@xxxxxxxx>
  • Date: Sun, 22 Aug 2010 16:38:16 -0500

Scott,
In your two cases below, I have the impression you are assuming that the
person testing the s parameters has no knowledge of the construction of the
object being represented by the s parameters.  In that generality, you are
quite correct.

However, as I read the original question, the case where the tester has
knowledge of the device under test and perhaps enven designed a test
structure is not eliminated.  Would you like commment on the specific case
where one has a a set of data from a known test structure?


Aubrey Sparkman
512-461-6165
asparky@xxxxxxxxxxxxxxxxxxxxxxxx

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Saturday, August 21, 2010 6:20 PM
To: Luciano Boglione
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Causality test

In general, no.

Take the following 2 cases.

Hypothetical material with zero loss and Er = 1 Case 1 - A single-ended
trace that is 10 inch long in this material Case 2 - A single-ended trace
that is folded back and forth in a 1 inch distance with a total length of 10
inchs

In both cases the S2p file describes a trace that is X mm long.
In case 1 a signal traveling from Port 1 to Port 2 will travel at the speed
of light and arrive at port 2 approximately 847.25 ps after being launched
from Port 1.

In case 2 a signal traveling from Port 1 to Port 2 will travel at the speed
of light, but will arrive at port 2 sooner than 847.25 ps after being
launched, because some of the signal is coupled across the folded section
and will begin arriving 84.725 ps after the launch at Port 1.

Both systems are causal.  Measurements of s-parameters of these cases would
be causal, yet one will show the far end signal beginning to transition much
earlier than the other.  How could this be discerned by a test?


regards

Scott



Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

TeraspeedR is the registered service mark of Teraspeed Consulting Group LLC


On 8/21/2010 7:05 PM, Luciano Boglione wrote:
> Hi folks,
>
> Is there a "test" that can be executed directly on a given S parameter set
that can confirm causality of the network? Put yourself in the best possible
conditions (e.g. off the top of my head, frequency ranges from DC to
infinity continuously). I'm asking for something similar to a passivity test
in the frequency domain.
>
> Thanks,
> Luciano
>
>
>
>
>
>
> .
>
>
> Sent via BlackBerry from T-Mobile
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