RE: [SI-LIST] : Micro Noise

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From: Chris Rokusek (crokusek@viewlogic.com)
Date: Wed Nov 03 1999 - 13:29:28 PST


Derek,

You may also want to rule out noise induced from said IC onto the VDD/GND
network by taking a conducted V(f) measurement across the VDD/GND at IC
location. This noise results primarily from "shoot-through" currents of the
CMOS drivers and can be reduced by improving power bus decoupling.

Also, of course large near fields don't ALWAYS translate to large far
fields.

If you can replace uP with equivalent from another MFG and radiation goes
down then cause is likely power/gnd routing on the die so you can try to
pass the buck to IC MFG.

Chris Rokusek
Viewlogic Systems

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Lfresearch@aol.com
> Sent: Wednesday, November 03, 1999 7:31 AM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : Micro Noise
>
>
> Hi all,
>
> I have a problem with a microprocessor which appears to be radiating very
> strongly from the package. I've deactivated all other circuits ( pulled
> crystals, pins etc ) on the board. Additionally, the micro is
> held in reset,
> PROM's are pulled, but it's a noisy as heck. Close field probes

> both E and
> H field ) show the package to be the dominent source of radiation.
>
> The device has a maximum clock rate of 50 MHz, this is where
> we're using it.
> Clock it at 32 MHz and the emissions fall about 10 dB up to about
> 300 MHz,
> more above 300 MHz. Yes it's a CMOS device.......
>
> Am I stuck with putting a can over the chip? The product has a plastic
> package, so there is no shielding available at all.....
>
> Thanks,
>
> Derek
>
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