[SI-LIST] Re: TDR impedance measurement and rise time
- From: Long Yang <long.0.yang@xxxxxxxxx>
- To: Nagy István <buenos@xxxxxxxxxxx>
- Date: Tue, 7 Apr 2009 21:08:23 +0800
Istvan:
Yes, you can set the rising time of TDR as the same as that of
signal's rising time.
But you should make sure the rising time less than 1/2 of time delay of the
test coupons, or the waveform in TDR will be masked.
The rising time of TDR is related to the dynamic range of TDR. By FFT, less
rising time (time-domian) means higher dynamic range(frequency domain).
But less rising time is not responsible to the higher frequency impedance
measured. The impedance is measured by terminal voltage and current at the
sampling point. Think about an extreme case, the PCB coupons are all perfect
conductors and loss-less insulators. This case is not related to
frequency...
Long Yang
http://blog.ednchina.com/spices/
Emo Philips <http://www.brainyquote.com/quotes/authors/e/emo_philips.html>
- "A computer once beat me at chess, but it was no match for me at
kick
boxing."
On Tue, Apr 7, 2009 at 7:59 PM, Nagy István <buenos@xxxxxxxxxxx> wrote:
> 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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