ensure that the xtal and its associated decoupling caps are as close as
possible to the processor clock pins.
Put ground planes around it on top /bottom layers and locally stitch top /
bottom planes with as many vias as you can fit in (I tend to use 1mm via /
0.5mm hole for this)

You may also need to decouple all your I/O lines with a few pF - I use SM
capacitor or C-R networks for this purpose and it works well - look at the
Bourns or Phillips websites for these parts.
Hope this helps...

-----Original Message-----
From: Greg Olson [mailto:[EMAIL PROTECTED]]
Sent: 26 July 2001 15:41
To: Protel EDA Forum
Subject: [PEDA] EMC question


Hi all..

I've got a 2.5"x3.5" plug on daughterboard which contains my processor,
memory, flash, etc... The processor is a 20MHz AM186 driven by a 20MHz
crystal. I just had this system in for FCC EMC testing and found that the
20MHz crystal is bleeding its harmonics (especially 80MHz) all over
everything! It's getting onto the IO lines from the daughterboard to the
main board and from there onto just about every wire leaving the enclosure!

Can anyone give suggestions as to the best way to isolate this clock signal
in a relatively tight space? This is a 4 layer board, top layer signal, next
layer ground, then Vcc then signal again.

Thanks in advance,

Greg Olson
DSX Access Systems


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