[SI-LIST] Re: 800MHz Radiated Emissions
- From: "Bashir, Shiraz (MED)" <Shiraz.Bashir@xxxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 12 May 2004 18:46:06 -0500
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.
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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