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

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From: Doug McKean ([email protected])
Date: Thu Oct 12 2000 - 11:25:50 PDT

Sunil Kumar wrote:
> Hello everybody.
> I am measuring cycle-to-cycle jitter in clock in a system, where clock
> (PECL, 50MHz) is going from one PCB (card) to other through twinax
> cable. In the second card clock is converted into TTL and then distributed
> to various ICs. I am looking at the clock just before the PECL-to-TTL
> convertor. Both the cards are in the same backplane. The system is
> equiped with a set of fans for cooling, which are kept below the card
> frame. Now what I have observed is, when the fans are not on, jitter is
> much much less in comparision of the jitter when fans are on. I think, the
> fans are generating EMI, which is disturbing the planes in the backplane
> and the cards, and hence jitter is increased. Any explanations?
> One more thing which I have observed is, the PECL-to-PECL clock
> fanout buffer (no PLL inside) is reducing jiiter (with fans off), i.e.
> jitter at input is more than that at output. Can anybody explain this?
> Is there any PECL-to-TTL clock fanout buffer which adds very low
> jitter? The PECL-to-TTL buffer which I am using, is adding lot's of
> jitter.

How much current are the fans drawing?

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