[SI-LIST] Re: Trace layout
- From: Hassan Ali <hali@xxxxxxxxxxxxxx>
- To: "'khalida@xxxxxxxxxxx'" <khalida@xxxxxxxxxxx>
- Date: Tue, 7 Aug 2001 11:36:31 -0400
Khalid,
Signals at 2.5Gbps (400ps bit width) will most likely have rise/fall times
of 100ps or less. That means, the bandwidth of the signal is around 3.5GHz
(based on BW=0.35/Tr rule) corresponding to an effective wavelength of just
about 4 cm on typical board materials.
effective wavelength = speed of light in vacuum/[BW*sqrt(effective
dielectric constant)]
A copper trace length of about quarter wavelength, 1 cm in this case, (and
multiples of quarter wavelength) is a perfect antenna. So traces on such a
board will radiate like hell and they will certainly pose a big EMI problem
if in (unshielded) microstrip format. If in stripline format they will still
radiate but they are somewhat shielded by the stripline upper and bottom
copper planes. Even with striplines you have to use other EMI shielding
solutions because the radiation doesn't just come from traces - high speed
devices and connectors, also radiate - some of them very badly.
Hassan.
> -----Original Message-----
> From: Khalid Ansari [mailto:khalida@xxxxxxxxxxx]
> Sent: Friday, August 03, 2001 6:42 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Trace layout
>
>
>
> I attended a presentation where the speaker mentioned that for signals
> running around 2.5 Gbps you should not use microstrip it should always
> be stripline. Is this true, if so why?
>
> If a board doesn't have an EMI issue then it should be OK to
> use microstrip.
> Also you get around 150 ps/in delay in microstrip compared to almost
> 200 ps/in in stripline. Unless you use some sort of edge
> mounted connector
> that has connections on layers where you are laying out your
> striplines
> you should try to use microstrip otherwise you will be going
> through quite
> a bit of vias????? Is the effect of the via that drastic? I
> have come across
> people with different opinions on this question.
>
> Any comments will be appreciated.
>
> Khalid
>
>
>
>
>
>
>
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