[SI-LIST] Re: EMC, Frequency or edge
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <cchalmers@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 3 May 2004 09:09:07 -0400
Chris,
The bandwidth that you calculate from the rise or
fall time determines the frequency point below which
the spectral envelope falls with 20dB/decade, and
above which it fall with 40dB/decade. Within the
signal bandwidth, the level of the Nth harmonic is
(at least) N times lower than the level of fundamental
component. If everything else stays the same, and
you change the clock frequency from 50MHz to
25MHz, the level of harmonics at a fixed frequency
will be two times lower, because to get to the same
absolute frequency, now you have to go to twice the
number of harmonics (N increases by a factor of two).
I hope this clears the confusion.
Regards,
Istvan
SUN Microsystems
----- Original Message -----
From: "Chris Chalmers" <cchalmers@xxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Monday, May 03, 2004 5:01 AM
Subject: [SI-LIST] EMC, Frequency or edge
> SI List,
>
> I am just having one of those moments which I
> have ended up confusing myself (Its not difficult).
>
> When dealing with high speed signals running
> across a board it is always the rise time or fall time you
> worry about because the faster it is, the higher the
> frequency compenents are in the edge. Is this
> still true from a radiated emissions perspective? Let
> me give an example,
>
> If you had a clock frequency of 50 MHz with a rise
> and fall time of 200ps. Would the emissions be less
> with a 25MHz clock with a rise and fall time of 200ps?
>
> Just take this simple case neglecting all the other factors
> that may make one clock frequency radiate better i.e. geometry.
>
> Please unconfuse me cos my head hurts
>
> Thanks in advance
>
> Chris
>
>
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