[SI-LIST] 800MHz Radiated Emissions
- From: "Bashir, Shiraz (MED)" <Shiraz.Bashir@xxxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 28 Apr 2004 07:37:06 -0500
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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
