Tom,
Yes, but that's the ideal case or a DUT with a nice launch. I guess I
wasn't specific enough as I work in a setting where typical channel
consists of all forms of discontinuities.
If you throw in connectors and other bad discontinuities that are in the
measurement and close to the launch, your accuracy of the apparent
impedance down the line (after all the speed bumps and potholes) can turn
pretty crappy even if there isnt much loss!
On Jul 6, 2016 6:46 PM, "Tom Dagostino" <tom@xxxxxxxxxxxxxxxxx> wrote:
Ryan
With any measurement system the more loss you have between the point of
entry and the location of the measurement the less information you will
have. With TDR looking at a constant impedance line should not show any
"crap" with increased distance. What you have lost is the ability to
differentiate features from each other. If you have a launched risetime of
20 psec into a lossy line the ability to resolve features decreases as the
risetime (bandwidth) degrades. At the launch point and a 20 psec risetime
you should be able to see two features that are 40 psec apart very nicely.
But after a bandwidth degradation of 2 (double the risetime) caused by
losses you will not be able to differentiate the two features spaced 40
psec
apart.
Tom Dagostino
971-279-5325
tom@xxxxxxxxxxxxxxxxx
Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Ryan Lott
Sent: Wednesday, July 06, 2016 2:54 PM
To: haroldw@xxxxxxxxxx
Cc: SI-LIST
Subject: [SI-LIST] Re: S-parameter TDR: Freq to Time Domain Mapping
Harold,
For a typical uniform transmission line, depending on the length and size
of
the interconnect, there is a region in Impedance vs Frequency plot where
impedance is pretty much determined by sqrt(r/jwc). This is called the RC
region. After that region, the impedance pretty much constantly remains
100Ohms let's say as example. WAG here, but maybe if you can find the
transition region and force a constant value for the RC region, this can
help you achieve what you're looking for.
Just to throw this in here, please beware that TDR itself has a lot of
quirks. As you go further and further down the line, your accuracy pretty
much goes to crap.
Interesting thought though.
-Ryan
On Wed, Jul 6, 2016 at 4:22 PM, Harold Wang <haroldw@xxxxxxxxxx> wrote:
Hi Experts,S-parameter data.
When performing TDR simulation using S-parameteres, we know that the
final steady-state impedance (Zfinal) is equivalent to the DC
impedance of the DUT and the initial impedance (Zinit, t=0) is
represented by the impedance at the highest recorded frequency in the
When doing a TDR on an uniform lossy transmission line, we expect the
impedance plot to raise from Zinit to Zfinal due to resistive loss in
the transmission line. Can we predict the slope of this rise? For
instance, if I moved 50ps into this uniform lossy transmission line,
what is the impedance rise that I should expect? What frequency point
does that map to in the S-parameter data?
The end goal is to separate the natural rise of an uniform lossy
transmission line from true impedance increase due to structural change.
Thanks,
Harold
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