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

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From: Patrick Lawler ([email protected])
Date: Thu Oct 12 2000 - 08:47:25 PDT

How about vibration-induced problems? Can you mechanically isolate the fan
and check the result?
I remember working on RF transmitters & receivers that were microphonic.

But now this is getting out of my area of expertise ...

On Thu, 12 Oct 2000 21:05:00 +0500 (GMT+0500), Sunil Kumar
<[email protected]> wrote:
>Yes, DC fans are being used. I have already tried that. I had powered up
>fans from separate power supply. But still I did see increase in jitter.
>On Thu, 12 Oct 2000, Patrick Lawler wrote:
>> 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
>> <[email protected]> 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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