[SI-LIST] Re: PCB impedance coupon measurement
- From: David Instone <dave.instone@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 24 Aug 2006 09:34:34 +0100
Kenny,
Yes, TDR can be done on PCB traces, making the connection can be
tricky and requires experience to get it right. On a T network it is
almost impossible to evaluate where any mismatches are. The two arms of
the T appear in parallel, hence you see the impedance drop to the
parallel value after the junction, reflections from mismatches during
this part of the TDR display could be from mismatches on either arm. Re
reflections are also a problem, layer peeling software can eliminate
this problem on single traces but it can't cope with a T topology. If
one arm is longer than the other then things get really confusing,
experiment with putting a short and a termination in turn on the end of
the shorter arm. The short will tell you where on the trace the short
arm ends and then when it's terminated any reflections after that point
stand a good chance of being on the longer arm rather than re
reflections on the shorter arm. If you have boards to spare then
cutting traces can help evaluation.
If your traces are strip line between power and gnd planes and the
board is unpopulated then either fit the decoupling caps or fit shorts
in their place.
Regards
Dave
Kenny Frohlich wrote:
> Hi all,
> Since we're on the subject of PCB impedance coupon measurement, I have a
> question on TDR test. I understand that TDR test is performed at the
> impedance coupon on the PCB. Can a TDR measurement be taken on an actual
> trace on the PCB? If a net is routed as a T-topology, can a TDR measurement
> take on this net?
>
> Thanks,
> Kenny
>
> Geetha Balasubramanian <geethukarrthik@xxxxxxxxx> wrote:
> Dear Members,
> Generally what is the frequency range for PCB impedance coupon measurement?
> Is it possible to measue the impedance of the coupon at a specific discrete
> frequency?
> For better accuracy is it necessary to have 2 seperate bands one for USB full
> speed (12 Mbps) design and another for USB high speed (480 Mbps) design?
>
> Thanking you all in anticipation
>
> Geetha
>
>
>
>
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