[SI-LIST] Re: 800MHz Radiated Emissions

Bashir, the difference between:

1) Your simulation and
2) The old board
3) The new board

Could be due to several factors working together.  I assume you are series 
terminating these clocks.  Off-hand the usual suspects list should include:

Underterminating.
Traces too close to edges, or bodies above the board.
Unfortunate choice of track length.
Long distances to return vias when switching layers
PCB cavity resonance
Unfortunate choice of routing layers

I suspect that your problem can be fixed in copper alone.

In your position, I would buy a couple hours consulting from whatever EMC 
consultant is local to you.  They can take a look at your specifics and 
should be able to pare the list down quickly, give you a verification plan 
and get this resolved quickly.

Regards,


Steve.
At 07:37 AM 4/28/2004 -0500, Bashir, Shiraz (MED) wrote:
>Hi,
>
>We are using a MPC9448 Clock Fanout buffer (1:12) whose input is 100MHz +/-
>50 ppm from a crystal osc.
>- 8 x outputs of clock fanout buffer are going to 8 x SDRAMs.
>- 9th output is going to microprocessor.
>-10th ouput is going to 74LVCX74 to get 25Mhz output for LVDS105 (whose one
>output goes to Ethernet Switch and same ouput goes to Ethernet Transceiver)
>-Last two outputs are Not Connected
>
>When we ran Radiated Emissions test for Class A compliance we saw 800, 900
>MHz freq above Class A limits.
>Using near field probe we found that Emissions is coming from clock fanout
>buffer and at front of device where there was a wide slot.
>
>By adding more 'aggressive' decoupling caps .001, 1 & .1 uF & "simulated"
>slots using copper braids on old enclosure, we pass class A limits.
>
>We designed new board & casing adding above mentioned changes, ran the test
>again. all freq are well below Class A, only 800 MHz is like 2-3 dbs above
>Class A limits.
>
>Old board and old enclosure with decoupling modifications and simulated
>slots shows 800Mhz having 14 db less than Class A.
>But when we put new board in old enclosure we see loss of 9 dbs at 800Mhz
>freq.
>
>Does somebody have any idea why it is happening ?
>What electrical characteristics should I be looking at MPC9448 Clock Fanout
>buffer while calculating its decoupling requirements.
>
>Note: it is a 12 layer board.
>
>Thanks,
>Shiraz Bashir
>
>
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