[SI-LIST] Re: AW: TDR impedance measurement and rise time
- From: Peter.Pupalaikis@xxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 7 Apr 2009 09:08:09 -0400
Hi all,
We manufacture our TDR such that the user can program the risetime so that
it is slower than the actual risetime of the step which is about 15 ps.
The TDR can be normalized using SOLT calibration and the actual step that
is shown becomes a convolution process as has been previously
mentioned.
I think that the question of impedance measurement is a philosophical one.
Is there really "an impedance?". The impedance varies with frequency
(convert S11 to Z parameters) and with time (the impedance profile).
Looking at an impedance profile, it is easy to get worked up over large,
but
short impedance discontinuities when the signal being transmitted has a
slower risetime than the incident step used used to measure the impedance
profile (or the high frequency content of the S-parameters, if S-parameters
are used to extract the profile). The reality is that if you limit the
frequency
content of the step used to measure an impedance profile, the bumps get
smeared out by the slower risetime and they don't look so bad - in this
manner, the profile provides a measurement that shows more realistically
how the discontinuities will affect your data transmission.
Similarly, if you are eye-balling things with a TDR measurement, the slower
risetime TDR helps you make better sense of things.
By the way, Howard Johnson had an excellent video where he addressed this
topic. I forget where I saw this video, but it might have been related
to some of the work he did with Xilinx a few years back. I remember that
in the video he asked why the engineer was using such a fast TDR when
his signals were so much slower. Maybe Howard can point out where to find
it.
Pete Pupalaikis
LeCroy Corporation
"Havermann, Gert"
<Gert.Havermann@H
ARTING.com> To
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Subject
04/07/2009 08:29 [SI-LIST] AW: TDR impedance
AM measurement and rise time
Hi Istvan,
I also don't know a way of changing the risetime at the TDR, but you can do
a postprocessing to reduce the bandwidth (i.e. the risetime).
We do the measurements with maximum resolution, and measure the risetime at
the desired reference plane to be sure about the risetime at the DUT. If a
lower bandwidth is needed, we use a filter or window function in ADS or
H-Spice.
In General, one should look at the risetime of the silicon he is using,
because the clockrate doesn't tell you much about the bandwidth in yous
system. Since the Die size is constantly shrinking, the signal risetime
keeps on decreasing.
There are two ways to handle risetime related SI-problems on low speed
signals: Filter your signals right at the transmitter, or design your
transmission path according to the risetime of the transmitting silicon.
BR
Gert Havermann
HARTING
Please visit us at the Hannover Messe from April 20 to 24, 2009.
You can find us in hall 11, stand C13.
http://www.hannovermesse.de/
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Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Nagy István
Gesendet: Dienstag, 7. April 2009 13:59
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] TDR impedance measurement and rise time
hi
If we measure PCB test coupons with a TDR to determine characteristic
impedance, should we set the rise time to be the same as the signal's rise
time? is it possible to set it at all?
what i found on the internet, the TDR manufacturers try to make rise time
to be as low as possible, like 15ps..., and thats it.
If the rise time is always 15ps, then i think it will always measure the
impedance on a very high frequency, 2/t_rise or something, so several
gigahertz. Usually on a board we have different signals, some are running
100MHz analog, some other are 800MT/s digital, or 2.5Gb/s digital.
shouldn't we do different setups for these, to get impedances on the
signal's operating frequency?
Someone from a Fab told me, that the "TDR is not frequency dependent". so
they dont take the signal's frequency into account.
what is the correct handling of signaling frequency for impedance
measurements, and simulations?
regards,
Istvan Nagy
CCT
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