I never used SSCG, but following the thread, my humble opinion is the following:
If the interference just comes from the oscillator, it is feasable that the use
of a SSCG will even increase the interference, as the circuit elements inside
the SSCG that create the clock signal will also have a current consumption that
will contribute to the interference emanating from the clock generator.
You will probably get better results from the use of a SSCG, if the interference
not just comes from the oscillator itself, but mainly from the circuits driven
by the SSCG.
-------------------------
Keith Hardin (in the mail shown below) wrote:
... The output spectrum is a series of stationary harmonics spaced at 30kHz
apart....
To my understanding, SSCG produce a FM modulated clock. I dimly remember that FM
does not produce a series of stationary harmonics spaced at the modulation
frequency, but the harmonics change dependent the modulating signal.
-----------------
I think the SSCG does not fool the QP Detektor, it fools the bandwidth of the
test receiver. The effiencency of fooling depends not only on the modulation
frequency, but also on the frequency offset. In the example, where the use of
the SSCG did not help to reduce interference from the second harmonic, the
frequency offset was probably less than 60kHz ("peak to peak")
-----
For analog TV you will minimize the perception of interference on the screen if
the modulation frequency of the SSCG is 39kHz or 55kHz, as these frequencies
have the maximum offset from the line frequency harmonics.
Furthermore, for analog TV the assessment of the level of acceptable
interference is decided by the eye and the brain of the beholder.
In digital TV it is decided by the software on the chip, and that these days is
still inferior to the brain of a beholder.
Where I live, there is no digital TV available. Maybe digital TV is not suitable
for a terrestial air link.
Dipl.-Ing. Rene Charton
Manager
EMC Services
_____________
TUV Rheinland Taiwan Ltd.
TAIPEI HEAD OFFICE
Spring Plaza Building
14F, No.6, Min Chuan E. Rd., Sec. 3,
Taipei 104, Taiwan, R. O. C.
Tel. (02) 2516 6040 Ext. [ 086 ] e-mail: r...@twn.tuv.com
khar...@lexmark.com on 03/18/2000 03:30:51 AM
Please respond to khar...@lexmark.com
To: emc-p...@ieee.org
cc: m...@california.com, sla...@foxboro.com (bcc: Rene Charton/TUV-Twn)
Subject: RE: Dithered clocks and EMC - BEWARE
Scott,
Sorry to hear about your experience but we have had great success using Spread
Spectrum Clock Generation (SSCG) since 1994 and have shipped many millions of
units.
Most PC's built over the last 2 to 3 years also use this technology.
For back ground information, see http://www.lexmark.com/sscg/ and
http://developer.intel.com/ial/scalableplatforms/sdt.htm. Included in this
information
is what SSCG is and its effects on some other devices. I suggest one read these
references as a basis for further discussions.
Experimental data has clearly shown is that the QP and Peak readings are
exactly the same for SSCG signals (No Fooling). An SSCG output properly
designed will create
a clock that is frequency modulated at ~30kHz. The output spectrum is a series
of
stationary harmonics spaced at 30kHz apart.
Finally, this method will attenuate the clock and system as compared to the same
system
with the modulation turned off. All other EMI reduction techniques will also
reduce emissions further.
Our design philosophy is to bring all good low cost techniques to bare on
reducing the emissions
and SSCG is just one of them.
Keith Hardin
Lexmark International
For the last time this came up, please see the append on the emc-pstc archives
at http://www.rcic.com/ with subject "Clock Oscillator &Re: Spread spectrum
clock oscillator (1) 20-Feb-97 ".
slacey%foxboro....@interlock.lexmark.com on 03/17/2000 11:18:46 AM
Please respond to slacey%foxboro....@interlock.lexmark.com
To: macy%california....@interlock.lexmark.com
cc: emc-pstc%ieee....@interlock.lexmark.com (bcc: Keith Hardin/Lex/Lexmark)
Subject: RE: Dithered clocks and EMC - BEWARE
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.
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
-------------------------------------------
This message is from the IEEE EMC Society Product Safety
Technical Committee emc-pstc discussion list.
To cancel your subscription, send mail to:
majord...@ieee.org
with the single line:
unsubscribe emc-pstc
For help, send mail to the list administrators:
Jim Bacher: jim_bac...@mail.monarch.com
Michael Garretson: pstc_ad...@garretson.org
For policy questions, send mail to:
Richard Nute: ri...@ieee.org
