Neven,
If you are using spread spectrum modulation it will help to set the spread to
maximum.
You can tell if it is the clock or the DQs by seeing if the emissions increase
during the exercise program.
The 2-layer PCB sounds like an EMC challenge in a plastic enclosure. A single
SDRAM package can radiate enough by itself to fail class B near 1 GHz. PCB
traces that act as antennas can cause the system to fail at lower frequencies.
It does sound like you would like to fix things by changing the SDRAM. The DQ
edge shape isn’t very important below 500 MHz (or even higher). Reducing the
DQ output current via series resistors (generally 22 ohms) can help a little
bit. SDRAM DQ driver crowbar current can be an issue. If it is, output
resistors will have no affect.
Dave Cuthbert
Linear Technology
_____
From: [email protected] [mailto:[email protected]] On Behalf Of
[email protected]
Sent: Friday, February 09, 2007 2:20 PM
To: [email protected]
Subject: Best SDRAM for Low-Emission?
Hello everyone,
I have a 2-layer PCB design, plastic enclosure, using 120 MHz SDRAM, two
stuffing options: 2MB (16 Mb) in 50-pin tsop package or 8MB (64 Mb) in 52-pin
tsop.
It is slightly above class B radiated emission at the multiples of 120 MHz. I
troubleshot it and I determined the SDRAM is the only source of the failing
frequencies, everything else is quiet.
I wonder if anyone has looked in how various manufacturers' memory chips
perform emission-wise, particularly the type that I am using. I hope to find a
manufacturer who makes "quieter" modules, so I do not need to change anything
else on my board.
If anyone has done such a comparison/investigation, I would appreciate if you
would be willing to share, on-line or off-line, the results - which
memory/manufacturer works best EMI-wise in such conditions.
Regards, Neven
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