[SI-LIST] TDR impedance measurement and rise time
- From: Nagy István <buenos@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 7 Apr 2009 13:59:22 +0200 (CEST)
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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