Robert, and the group,
Although I have only limited experience (1 instance) with dithered clocks, I
thought that I might share that experience with the group. I had a product
that was failing radiated emissions at one particular frequency with a
vertical antenna orientation. I tracked the problem down to a particular
cable and circuit card. Use of a near field probe identified the oscillator
package as the source of the emissions (the second harmonic).
The device was located near the card edge, insufficient decoupling, etc. All
the textbook layout errors. A check of the oscillator specifications showed
that it was a standard TTL device with a fanout of 10 inputs. I had already
found some fixes that would reduce emissions, clamp-on ferrites, additional
shielding, etc., but thought it made more sense to reduce the emissions at
the source. I suggested to the design engineer that a low-power TTL
oscillator, with a fanout of 2 inputs, would reduce currents through the
offending etches. Someone else suggested a dithered clock device instead.
When we tested the dithered clock, emissions were actually worse. We had
simply spread the problem over a wider spectrum.
I have heard some success stories for these devices, but results in this
case were disappointing. I still think a lot depends on the layout. I
strongly suspect that real-world disruption to nearby devices is generally
going to be worse, since the idea behind these devices is to "fool"
quasi-peak measurements in order to pass.
Caveat Emptor!
Scott Lacey
-----Original Message-----
From: Robert Macy [SMTP:m...@california.com]
Sent: Friday, March 17, 2000 9:38 AM
To: emc-p...@ieee.org
Subject: Fw: Dithered clocks and EMC - BEWARE
Of interest, so I forward this to the group:
- Robert -
Robert A. Macy, PE m...@california.com
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: John Woodgate <j...@jmwa.demon.co.uk>
Newsgroups: sci.engr.electrical.compliance,sci.electronics.design
Date: Tuesday, March 14, 2000 11:55 PM
Subject: Dithered clocks and EMC - BEWARE
>About a year ago we had a thread on this subject, concluding that
some
>research was needed to see whether dithered clocks were better or
worse
>in terms of conforming to EMC requirements.
>
>I learned very recently that some reliable but as-yet unpublished
>research has found that digital TV receivers are some 40 dB (!!)
more
>sensitive to dithered clock emissions than to unmodulated carriers.
This
>is likely to lead to changes in EMC limits within maybe as little
as
>three years, since the authorities certainly don't want to be
deluged
>with complaints of interference from people who have just opted for
>digital TV.
>
>So, if you are thinking of using a dithered clock, think again!
>--
>Regards, John Woodgate, OOO - Own Opinions Only. Phone +44 (0)1268
747839
>Fax +44 (0)1268 777124. http://www.jmwa.demon.co.uk
>I wanted to make a fully-automated nuclear-powered trawler,
>but it went into spontaneous fishing.
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