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Old October 21st, 2011, 08:17 PM   #5
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danw is offline
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Join Date: Oct 2004
Location: midwest, USA
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The reason everyone loves ethernet is that ethernet's hardware layer is so well designed. Every single NIC card incorporates a 'magnetic' transformer that provides incredibly good isolation and common mode rejection.

It's my understanding that the 'magnetics' transformer was orginally it intended to prevent emitted noise (FCC), but the side benefits of common mode rejection and isolation for those of us who have been burdened with serial comm links, which are not as well designed, are fantastic.

Here's what Lantronix says about the magnetics in a blurb for those using Lantronix's embedded network interfaces where the remaining design is the connection to the outside:

Selecting LAN magnetics
For communications on a local area network, a magnetic filter module is required. The magnetics provide impedance matching, signal shaping and conditioning, high voltage isolation and
common-mode noise reduction. 10/100 Base-T Ethernet utilizes Unshielded Twisted Pair (UTP) transmission cable. The UTP wiring is prone to noise pickup that leads to conducted and radiated noise emission. The magnetics help to filter out the noise, provide good signal integrity and good electrical isolation.

The recommended Magnetics has a 1:1 transmit turns ratio, 1,500 volts isolation, center taps on both sides of the transformer, a choke on both the transmit/receive channels and has been designed for use with 10/100 Base-T Ethernet networks. The following are some additional factors to consider when selecting the Magnetics for LAN communications.
Common-Mode Choke.

For 10/100 Mbps communications, a common-mode choke is necessary for a system to pass FCC testing. The choke presents a high impedance to common-mode noise but a low impedance for differential-mode signals. For a WiPort application, both transmit and receive channels must have a choke and both chokes should be wired directly to the RJ-45 connector (see figure 1).

Common-Mode Rejection
Common-mode rejection is the ability to reject a signal which, referenced to ground, has the same amplitude and phase on both inputs. This signal, which is usually the result of noise or a small impedance mismatch, produces a small differential error voltage at the input terminals of the Ethernet. Subsequently, this error gets amplified right along with the desired communications signal. The magnetics plays a major role in knocking down this common-mode noise down to an acceptable level.

Lantronix Application Note: How to Connect a Lantronix Embedded Module to a Wired Ethernet Port

My experience with ferrite cores was dramatic in the DC world, when signals from field transmitters would peg off scale when the tech's walkie talkie was keyed. The installation of ferrite cores dramatically reduced the impact of the RF on the DC, but that was an internal addition, not an external one.

I suspect that the ethernet packet is being swamped by normal mode noise and I'm just not sure how a ferrite coil at one end or the other can ameliorate that. I'd be glad to be proven wrong, though.

Last edited by danw; October 21st, 2011 at 08:31 PM.
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