Re: [SI-LIST] : EMI due to fans

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From: Patrick Lawler (plawler@west.net)
Date: Thu Oct 12 2000 - 08:29:37 PDT


If a DC fan is being used, the pulsating current load current may be
feeding back through the power supply into the logic circuitry.
This applies whether the fan is brush or brushless.

If this is your situation, try powering the fan from a separate power
supply to see if the symptom goes away.

On Thu, 12 Oct 2000 11:41:55 +0500 (GMT+0500), Sunil Kumar
<sunilb@cdotb.ernet.in> wrote:
>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.

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