Re: [SI-LIST] : EMI

Scott McMorrow (scottmc@teleport.com)
Tue, 28 Oct 1997 23:41:37 -0800

Manix

There are several vendors that manufacture spread-spectrum synthesizers
to reduce EMI. I don't remrmber the particular vendors (someone jump
in and help me here.) Basically, they are a digital phase locked synthesizer
that introduces digital phase noise into the feedback loop and causes
pseudo-random jitter on the frequency of the output. This will cause
the EMI frequency spectrum to smear in a gaussian fashion about the
the center clock frequency, thus reducing the radiated energy at any
particular frequency.

However, there is a problem with this technique. If the jittered clock is
used to drive other devices which contain internal phase locked loops,
it is possible to introduce a high enough frequency slew rate to drive
a phase locked loop out of lock. THIS IS A VERY BAD THING.
If the jitter noise frequency is higher than the maximum input tracking
frequency of the receiving phase locked loop, then icky bad stuff happens,
and the loop can go out of lock. When the a phase locked loop goes
out of lock it's behavior is unpredictable, most likely causing internal
glitches in the clock output.

So what? Well, many, if not most, processors, bridge chips, asics and
large vlsi devices use phase locked or delay locked loops. And, guess
what, most manufacturers do not specify the frequency response of
their internal PLL's, or do not even acknowledge the existance of such
a device internally. THERE IS NO WAY TO PREDICT WITH
ACCURACY WHETHER A JITTER SOURCE WILL OR WILL
NOT WORK RELIABLY WITH A PARTICULAR DEVICE IN
MANY REAL CASES, ESPECIALLY WITH MODERN
MICROPROCESSORS. Be forewarned!

Regards,

Scott McMorrow
SiQual

Manix Velu wrote:

> Hi,
>
> A WEB source suggests that:-
>
> In order to reduce EMI, there should be some logic that introduces a
> jitter into the clock network. This results a pulse energy to spread
> horizontally in the frequency domain which inturn results in the
> quasi-peek energy reduced. This will cause the resultant average
> frequency reduced slightly.
>
> Could anybody please explain what does this mean? Will it help? If
> so, where do I get more deatiled information about this?
>
> Thanks for your attention,
>
> Regards,
> Manix Velu.
>
> E-Mail: manix@shuttletech.co.in
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