[SI-LIST] Re: Signal Routing in outer layer
- From: ariazi <ariazi@xxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 19 Oct 2004 11:40:11 -0700
=20
> 1).Can we route high speed differential signal in outer layer?.
> 2).If we can route the differential signal in outer layer then, how do =
we
calculate the impedence. >3).What is the minimum tolereance which we can
give for the outer layer impedece.
> 4).If the high speed differential signal can be routed in the outer =
layer
then,how do we control the EMI issues?.
>
>
=20
Dear Bijesh,
1. It is quite feasible to route differential pairs on outer layers (as =
well
as inner layers).
2. Determining the differential impedance is simple. Once when the
cross-sectional geometry
and material properties (of the conductors and substrate) are defined a
field solver can
be employed to determine the impedance. There exits also mathematical
formulas for calculating
differential impedance for the case of edge-coupled microstrip (and also
stripline) with FR-4 substrate;
but the field solver approach can yield more accurate result. One good
reference on this topic is
Eric Bogatin's book, "Signal integrity Simplified", Pages 488-491
3. A common tolerance value for the differential impedance is +/- 10%; =
for
example 100 Ohms +/- 10%.
4. To minimize EMI radiation, the traces of high-speed differential pair
should be accurately length matched and one should avoid creating stubs =
or
routing over plane splits.
Best Regards,
Abe Riazi
ServerWorks
=20
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