What I do is lay a ground (polygon plane) plane directly underneath the crystal and
surrounding components. This plane is additional to the ground plane on the internal
layer. So the polygon is on the same layer as the component. Also, very close
placement to the processor on the same side, no vias, then I try to make the lengths
the same.
I hope this helps you,
Brad
Greg Olson wrote:
> 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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