[SI-LIST] AW: TDR impedance measurement and rise time
- From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
- To: Nagy István <buenos@xxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Tue, 7 Apr 2009 14:29:06 +0200
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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-----Ursprüngliche Nachricht-----
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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