[SI-LIST] Re: EMI Question Related to Spread Spectrum Clock Generator
- From: "Gareth Baron" <Gareth.Baron@xxxxxxx>
- To: <swldstn@xxxxxxxxxxxx>
- Date: Thu, 20 May 2004 16:37:01 -0700
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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=20
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