Wed, 29 Oct 1997 10:57:28 -0500

Scott, Manix, and Derek
The two vendors that make Spread Spectrum Clock Generators (SSCG)
are International Microcircuits Inc (IMI) and IC Works. These work by
frequency modulating the clock to a particular profile that is not random.
The frequency deviation ranges from +/- .5% and 2.0% and the
modulating frequency is greater than 30kHz. By modulating at a
frequency where all components are greater than the audio band,
most communications will filter the signal out. There have been
several publications on the subject. These should address the
concerns of microprocessors with PLL/DLLs (g), FCC (c,d,e) and
interference (c) issues (if you get (d), make sure read (e)).

The IEEE symposium papers are available for download from

a) Digital Circuit Radiated Emission Suppression with Spread Spectrum
Techniques, .ITEM 1994.
b) Spread Spectrum Clock Generation for the Reduction of Radiated
1994 IEEE EMC Symposium.
c) A Study of the Interference Potential of Spread Spectrum Clock
Techniques, 1995 IEEE Symposium.
d) EMC New watch pages 15-17, Compliance Engineering, May/June 1995.
e) EMC New watch pages 74-75, Compliance Engineering, July/August 1995.
f) Electromagnetic Interference Control Using Solid-State Technology,
pages37-39, Compliance Engineering, May/June 1995.
g) Design Considerations of Phase-Locked Loop Systems for
Spread Spectrum Clock Generation Compatibility, 1997 IEEE EMC Symposium.

If there are any more questions on the subject, I would be glad to attempt
an answer.

Dr. Keith Hardin
Senior Engineer
Lexmark International Inc.
740 New Circle Rd NW
Lexington, KY 40511