[SI-LIST] Re: Ferrite bead question
- From: kfrobinson@xxxxxxxxxx
- To: <ericg@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 10 Apr 2006 12:52:27 -0600
Eric,
The frequency components in a digital system can be estimated from DC to
.35/(rise time). The IC manufactures do give 10-90 rise time. Pick the
fastest rise time IC in your system.
Ken=20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Eric Goodill
Sent: Monday, April 10, 2006 12:16 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ferrite bead question
Howdy,
One thing I've never understood about designing the frequency response
of a power delivery system is what are the frequency requirements? That
is, where does the vendor give me the spectrum of the current demand for
the part? They don't (I'm skipping over they it'd be tough). So the only
solution I can see is to pick a probable maximum frequency and say from
DC to that frequency we need less than X ohms impedance based on some
assumptions about the peak current needs (related to lack of current
spectrum). This is probably over designing, but I don't have any other
data to use. Do others feel the same way?
-Eric
Scott McMorrow wrote:
> Joel,
> I think Lee is being a little bit black and white, but not without=20
> good reason.. Whether or not ferrites "work" in a power filtering=20
> design is a matter of whether the engineering was performed. What I=20
> think Lee often sees are systems where no engineering has gone into=20
> the design of ferrite, and they have been thrown into a circuit=20
> without thought. Often because "that's the way we've always done it"=20
> or "that's that way the competitors do it" or because "that's the only
> ferrite we have in our parts system." The inductance of a ferrite can
> interact with the power system and capacitors to form a pretty nasty=20
> resonance. This resonance often sits in the low frequency region,=20
> around the VRM output transition region, and has a strong tendency to=20
> cause peaking in the 100 kHz to 1 MHz range. This peaking can cause=20
> noise modulation of the Serdes, which Lee has often observed. If the=20
> PCB power delivery network has a lower impedance in the frequencies=20
> that affect the Serdes, than they ferrite filter does, then shorting=20
> the ferrite will help. But if a ferrite filter network is correctly=20
> designed, it is well matched and does not cause peaking.
>=20
> Whether or not noise injected into a Serdes affects it's output is a=20
> function of the internal PLL/DLL design. This is hardly ever=20
> specified, but can be measured.
>=20
> Scott
>=20
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>=20
> http://www.teraspeed.com
>=20
> Teraspeed(r) is the registered service mark of Teraspeed Consulting=20
> Group LLC
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