FreeLists / si-list / [SI-LIST] Re: EMC Issue at 266 MHz

["Charles Grasso" <cgrassosprint1@xxxxxxxxxxxxx>]

phooey - its a power cord problem - If you yank the
power cord I'll bet the emissions vanish! :-D

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of npischl
Sent: Tuesday, March 02, 2004 9:59 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: EMC Issue at 266 MHz


Using a probe to sniff out the board will not probably lead to 
anywhere, because 266MHz will most likely be seen at every clock 
component/device.

A quick way to figure out if and which segment of the clock 
distributin is a problem is to systematically shut down different 
clock devices and traces (usually, the device doesn't have to be 
operational for clock-related emission to exist). It can be done by 
e.g. removing series termiation resistors, lifting clock output or 
input pipns, removing ferrite beads and/or lifting power pins, 
citting traces.

Monitor emission while doing it (depending on the radiation 
mechanism, you may use a bendh-method, e.g. current clamp, or you 
may have to use the EMI chamber).

If you are lucky, shutting down only one clock will have most 
significant impact, in which case you may be able deduce what is the 
noisiest part of the clock distribution. That may lead to a simple 
solution. If it turns out that all or many of the clocks are 
contributing equally, then you'll have to figure out something else. 
That would be, either a significant circuit change, or you may be 
able to block emission by tackling the radiation mechanism rather 
than the source.

Good luck, Neven






--- In si-list@xxxxxxxxxxxxxxx, "Ken Cantrell" <Ken.Cantrell@xxxx> 
wrote:
> Jean-Pierre,
> I think Brent DeWitt's approach is the first step, try and find 
out where
> it's coming from.  If this proves unproductive, don't forget about 
enclosure
> and/or cable length resonances.
> Ken
> 
> -----Original Message-----
> From: si-list-bounce@xxxx
> [mailto:si-list-bounce@xxxx]On Behalf Of Ingraham, Andrew
> Sent: Monday, March 01, 2004 10:53 PM
> To: si-list@xxxx
> Subject: [SI-LIST] Re: EMC Issue at 266 MHz
> 
> 
> Even if the clock waveforms were perfectly symmetrical ...
> 
> If on-chip circuits are clocked by both rising and falling edges 
of the
> input clock, then you've got a strong even harmonic.  There could 
be
> radiation from the on-chip circuits, or from the supply lines 
to/from the
> chip.
> 
> Also, keep in mind that there could be small amounts of 
intermodulation
> between the various frequencies present, which include not only 50 
and 66.7
> MHz, but also 16.7 MHz or something lower at the PLL's phase 
detector.  This
> lower frequency may not appear on any of the PLL's pins, but it 
could be
> causing a weak modulation of the VCO output.
> 
> Regards,
> Andy
> 
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