[SI-LIST] Re: EMI Question Related to Spread Spectrum Clock Generator
- From: Bill Owsley <wdowsley@xxxxxxxxx>
- To: khardin@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Wed, 5 May 2004 09:35:21 -0700 (PDT)
For those who may not know...
United States Patent 5,488,627 Hardin , et al. January 30, 1996
---------------------------------
Spread spectrum clock generator and associated method
Abstract
A clock circuit includes an oscillator for generating a reference frequency
signal, and a spread spectrum clock generator cooperating with the oscillator
for generating a spread spectrum clock output signal having a fundamental
frequency and reduced amplitude EMI spectral components at harmonics of the
fundamental frequency. The spread spectrum clock generator preferably includes
a clock pulse generator for generating a series of clock pulses, and a spread
spectrum modulator for frequency modulating the clock pulse generator to
broaden and flatten amplitudes of EMI spectral components which would otherwise
be produced by the clock pulse generator. The spread spectrum modulator
frequency modulates the clock pulses with specific profiles of frequency
deviation versus the period of the profile. Electronic devices including the
spread spectrum clock circuit and associated method are also disclosed.
---------------------------------
Inventors: Hardin; Keith B. (Lexington, KY); Fessler; John T. (Lexington, KY);
Bush; Donald R. (Winchester, KY); Booth; James J. (Nicholasville, KY) Assignee:
Lexmark International, Inc. (Greenwich, CT) Appl. No.: 160077Filed: November
29, 1993
khardin@xxxxxxxxxxx wrote:
To all,
The FCC class B radiated emissions limits are stepped above 30MHz.
The 3m limits are:
Frequency Range Limit Detector
30MHz - 88MHz 40dBuV/m Quasi-Peak Detector
88MHz-216MHz 43.5dBuV/m Quasi-Peak Detector
216MHz-960MHz 47dBuV/m Quasi-Peak Detector
>960MHz 54duBuVm Average Detector
>960MHz 74duBuVm Peak Detector
There are different limits for CISPR 22 (the rest of the world) that are a
bit more stringent.
Refering to previous appends on this thread.
1) To determine if your processor is compatible with Spread Spectrum
Clock Generation (SSCG), an analysis must be performed by the designer
of the PLL and system that is tracking the modulated clock. Many processor
manufactures are very aware of SSCG and design for some ranges of SSCG
operation. Some data sheets discuss SSCG specification. If not you must
contact the manufacturer.
For more information see: www.lexmark.com/sscg and see technical papers
link.
2) SSCG reduces the amplitude in any 120kHz given bandwith by distributing
energy uniformily over a much greater bandwidth. There has been much
discusion
over the last 10 years about the EMI implications. A number of studies
have been
performed with several communications methods and SSCG has been found not
to cause additional interference. There may be other communciation schemes
that may have a higher risk of interference but none have been found
significant.
See above link for interference studies.
Hope this helps,
Keith Hardin
|---------+---------------------------->
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| | Waldstein" |
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| Subject: [SI-LIST] EMI Question Related to Spread Spectrum Clock Generator |
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To all,
Here's another spread spectrum PLL question.
Assume you are taking a 33 MHz clock in generating
800 MHz from it with a spread spectrum PLL. ( 24x multiplier ).
From the 800 MHz you divide by 4 and 6 to get 200 MHz and 133 MHz.
Since the 800 MHz is the signal that has been modulated to spread
the energy, the 200 MHz and 133 MHz or even lower ( say 66 MHz ),
have different power densities. The 200 MHz would have a low power
density than the 133 or 66 MHz because it is divided by a smaller
amount.
So the question is....
Do FCC emission limit change as a function of frequency band to
allow a single high frequency clock to be divided down to several
different bands?
Does anybody know where I can find info on allowed emissions energy
and is it a function of frequency or a hard and fast maximum limit,
independent of frequency?
Thanks in advance for any help
Steve
swldstn@xxxxxxxxxxxx
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