[SI-LIST] Re: 800MHz Radiated Emissions

When you "removed the connector....  Did you plug the hole that was in =
the
chassis??
ap

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Bashir, Shiraz (MED)
Sent: Wednesday, May 12, 2004 6:46 PM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: 800MHz Radiated Emissions


interestingly we found that a serial 9-pin connector was floating i.e., =
it
was not connected to ground. When we connected it to ground we got like =
~7db
margin below class A.

Then we removed that connector (we dont need that for production), =
radiated
emissions become worse ..less than 1 db from Class A limit. then we =
removed
RS232 IC which was feeding signal to that connector (thought that lines =
may
be acting as antenna)..it become more worse...> 3 db from Class A limit =
..

Anybody has clue why it is happening...

also on separate experiement we changed termination scheme from series =
(33
ohm) to parallel (50 ohm to ground) at load..we supress 800MHz but =
900MHz
failed test...any idea why ? as parallel termination draws enormous =
amount
of current so we try 100 ohm to Vcc and 100 ohm to ground termination at
load..it matches 50 ohm transmission line but results at 800Mhz are =
worse...


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx =
[mailto:si-list-bounce@xxxxxxxxxxxxx]On
Behalf Of Bashir, Shiraz (MED)
Sent: Wednesday, April 28, 2004 7:37 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] 800MHz Radiated Emissions


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.=20
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 ?=20
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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