Paolo,
Thank you for the information. I suspect that you are correct as to
the critical nature of the frequency modulation method. The design engineer
picked the device, probably as the cheapest vendor. The device did not
behave as I expected, based on some Internet research of dithered clock
technology. We were very pressed for time, but I hope someday to spend some
more looking at this technology. Perhaps my experience explains the digital
TV set interference, since all such devices are obviously not the same.
Scott Lacey
-----Original Message-----
From: paolo.ronc...@compuprint.it
[SMTP:paolo.ronc...@compuprint.it]
Sent: Monday, March 20, 2000 5:21 AM
To: emc-p...@ieee.org
Subject: RE: Dithered clocks and EMC - BEWARE
I have a very succesfull experience with SSCG (Spread Spectrum Clock
Generation) that is a patented technology used to intentionally
spread the energy of oscillators and clock signals and any signal derived
from the clock. I am sure Keith Hardin knows it first-hand because he is one
of the developers ! We have been using an SSCG chip since 1995 on most
of the electronics of our printers - most of them class B devices, with
oscillator frequencies in the range 24 - 32 MHz. We even managed to
get significant cost reductions by switching from 4-layer to 2-layer boards
and still passing CISPR22 and FCC class B limits for radiated emissions.
One point that seems not to have come up in this discussion is
the kind of dithering we are talking about.
In SSCG the main clock frequency is modulated, but just ANY
frequency modulation is not enough ! Key is the wave-shape of the
modulating function, and that is the crucial point in the SSCG patent
(Keith, correct me if I'm wrong). The "trick" is to maximize the rate
of change of the function representing the frequency modulation, so that
the oscillator spends the minimum time on any given frequency within the
modulating range, thus avoiding additional peaks in the emission spectrum.
The modulating function in the SSCG technique does just that.
This is best explained in the first (to my knowledge) publication on
the subject (here, too, Keith knows much more !!) :
K.B.Hardin, J.T.Fessler, D.R.Bush (Lexmark Intl.): "Spread
Spectrum Clock Generation for the Reduction of Radiated Emissions"
- 1994 IEEE Intl. Symposium on EMC, August 1994 Chicago (Symposium
Record page 227)
I suspect an explanation to Scott's problem may be that he didn't
use the SSCG technique but another form of dithering that was not
controlled in such a way as to get the flat frequency distribution that
you get with SSCG. Also the amount of frequency deviation can be
important. The attenuation you get in quasi-peak (QP) readings is
dependent on that. I hope Scott can give us more details.
As to the problems with digital TV, I honestly don't have any
knowledge about that, but I remember the same inventors of SSCG did a
study on interference potential of this technique :
K.B.Hardin, J.T.Fessler, D.R.Bush (Lexmark Intl.): " A
Study of the Interference Potential of Spread Spectrum Clock Generation
Techniques" - 1995 IEEE Intl. Symposium on EMC Atlanta ( Symposium Record
page 624).
Hope this helps.
Paolo Roncone
Compuprint s.p.a. - Italy
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