Make sure that the ground side of the decoupling caps for the crystal go straight
to the GND of the processor with as thick a
trace as possible. Preferably a polygon fill throughout the processor & crystal area.
If you are using just a normal trace here,
believe it or not, you may have just made a tuned antenna which can multiply the
signal strength of the 3rd harmonic coming from the
crystal's decoupling current through those caps, 80Mhz.
____________
Brian Guralnick
----- Original Message -----
From: "Greg Olson" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Thursday, July 26, 2001 10:41 AM
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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