[SI-LIST] Re: (EMI)spread spectrum clocking
- From: "Michael Poimboeuf" <Michael_Poimboeuf@xxxxxxxxxxxxxx>
- To: <istvan.novak@xxxxxxxxxxxxxxxx>, <kbagga31@xxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Fri, 5 Sep 2003 09:49:00 -0700
Karan,
Your definition is accurate.
I disagree with your comment about spread spectrum clocks being "less =
disturbing
subjectively for instance to broadcast audio and video programs."
In my experience, spread spectrum clocks create problems for audio and=20
video conversion, transmission and reception. For instance, when =
designing a
delta-sigma converter with an oversampled ratio of 256/1 (such as =
12.288MHz/48kHz
as used for broadcast audio), the band +-20kHz relative to 12.288MHz is =
very=20
sensitive to interference, especially modulated frequency interference. =
This
interference will appear in the audio band as heterodyned noise =
"birdies." Having
modulated clocks sweep in and out of this band is a problem. In essense, =
we=20
do frequency allocation when we design these systems and if some =
components
sweep in and out of an allocated frequency band they can create =
intermittent
interference.
I have many other examples. In fact, in broadcast quality A/V systems, =
one often
goes to great lengths to insure that all clocks in the system are =
synchronous to
a low phase noise reference. Broadcast and post production plants run =
references,=20
such as NTSC or PAL video black, or tri-level 1920x1080@24p sync and all =
systems
genlock to that clock.
While using spread spectrum techniques sometimes helps meet compliance =
limits by=20
splattering the phase of radiated and conducted emissions around below =
the fixed
bandwidth limits of the measuring equipment, it causes problems for =
audio and=20
video designers of broadcast quality systems.
---
mkp=20
-----Original Message-----
From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
Sent: Friday, September 05, 2003 4:37 AM
To: kbagga31@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: (EMI)spread spectrum clocking
Karan,
The spread spectrum solution puts up to a few percent of frequency
modulation on the clock signal, with a low (10-100kHz) repetition =
frequency.
As long as the clock PLLs/DLLs in the system dont remove this =
modulation,
and assuming that the system timing allows for this modulation, it =
spreads
the energy of the clock over a wider spectrum and it becomes less =
disturbing
subjectively for instance to broadcast audio and video programs. The
measured peak interference also goes down, because legal compliance
measurements have to be taken with a pre-determined bandwidth: if you =
spread
the spectrum of the clock over a wider frequency band, the energy =
falling
into the measurement bandwidth will also be less.
Regards,
Istvan
----- Original Message -----
From: "karan bagga" <kbagga31@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Friday, September 05, 2003 6:09 AM
Subject: [SI-LIST] (EMI)spread spectrum clocking
> Hi Experts
>
> I want to know the fundamental physics behind spread spectrum =
clocking.
> How does modulating the clock frequency helps reducing EMI ?
>
> Thanks & Regards
> Karan
>
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