[SI-LIST] Re: EMI Question Related to Spread Spectrum Clock Generator

If I understand correctly, you are not dividing the original clock but =
making
a replica at a divided frequency.  The power still exists in the =
original
800MHz clock.  You have now introduced new clocks into the system which =
each
have their own spectra (albeit with jitter relationships to the =
original) and
power.

FCC does have different limits for different frequencies.   Generally it
looks like a step function for Class B along the lines of 39dBuv =
@1-80MHz,
44dBuv @80MHz-210MHZ, 46dBuV @210MHz - etc.  The numbers might be wrong =
above
but you get the idea.


Gareth.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Steven M. Waldstein
Sent: Tuesday, May 04, 2004 4:42 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] EMI Question Related to Spread Spectrum Clock
Generator


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?=20

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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