[SI-LIST] Re: Gigabit Ethernet Magnetics
- From: "npischl" <npischl@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 01 Mar 2004 02:56:20 -0000
Martin,
The information contained in transformer vendors' data sheets is
related to the IEEE 802.3 spec only, which is usually not a real
problem (from the transformer point of view).
However, the right selection of the magnetics is also a key factor
that will determine EMI performance of a product. The relevant
information is not contained in any data sheets. Factors such as
common-mode (CM) choke impedance, balance (CM-DiffMode conversion),
inductance in center tap are key factors, but they are never
specified in the data sheets. The performance is also in part a
function of the system-level design, and therefore cannot be
completely specified by the magnetics vendors. Magnetics that works
excellent in one application may not be appropriate in a different
system.
Price vs. performance is sometimes a tradeoff, but more expencive
doesn't necessarily mean better. The EMI performance often does not
depend on the magnetics schematic-level configuration, but these
other factors.
Transformer's function is in the first place to provide high-voltage
isolation. However, there are many PHYs that require center-tap
connection to Vcc. Some do it to feed power to the Tx, some just to
provide bias to the differential pair (but no power to Tx). These
PHYs do not work properly without the transformers. However, some of
them may work even without the transformer over short distance, e.g.
if Ethernet is used only for board-to-board over a backplane
communication. There may be special design considerations in that
kind of application, but as long as Ethernet doesn't leave the
chassis, the high voltage isolation is not required.
Is your question also related to a specific PHY? I recommend that
you work with your PHY vendor to help you out.
Neven
--- In si-list@xxxxxxxxxxxxxxx, "SI List" <si-list@xxxx> wrote:
> Hello all!
>
> I have two questions about 1000BASE-T Gigabit Ethernet Magnetics:
>
> 1) Transformer configuration
> ----------------------------
> There are at least 3 configurations which are commonly used. Some
use
> one, two or even three transformers per pair.
>
> Is it a price/performance trade-off?
>
> Which configuration should be used in which application? My
application
> would be a Gigabit Ethernet switch. I couldn't find this
information
> from the manufacturers.
>
> 2) Transformerless Ethernet
> ---------------------------
> Why is a transformer actually required?
>
> I have read information that AC coupling with a 0.1uF ceramic
capacitor
> also works (as in Fiber Channel, Infiniband, etc.).
>
> The only reason I can see is the 1500V isolation. Is this a
regulatory
> requirement? Why is this requirement not imposed on Infiniband,
etc.?
>
>
> I am new to this field. Any information is appreciated.
>
> Is there a good book, or papers, or web pages which would be
helpful to
> me?
>
> Best regards,
> Martin
>
> --
> Enclustra GmbH Email: heimlicher // enclustra \\
com
> Martin Heimlicher Phone: +41 43 3431943
> Wibichstrasse 10 Fax: +41 43 3431944
> CH-8037 Zurich, Switzerland Mobile:+41 79 2798183
>
>
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