RE: [SI-LIST] : EMI due to fans (fwd)

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Fri Oct 13 2000 - 09:26:15 PDT


> If probe is picking up noise then I whould see same amount of increase
> everywhere, I mean in the whole clock tree. But I am getting almost
> negligible increase at the top of the tree. While before PECL-to-TTL
> buffer, the increase is quite a lot. That's why, I think probe is not
> picking noise, it's due to noise generated in the planes.
 
If there is some physical distance involved and your probes are picking up
noise (perhaps due to grounding), the noise CAN vary depending on where you
look.

I am still curious about the common-mode noise. You say there is twinax, so
presumably there are differential signals. How are you probing them?

This could explain your observation that the output of a non-PLL fanout
buffer seems to have less jitter than there is on its input. It might
simply be that your probe sees common-mode noise at its input as jitter, but
the PECL buffer has better high-freq. CMR and rejects it.

Andy

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