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Placing and Routing of Gigabit Ethernet

January 23rd, 2014

A Typical Gigabit Ethernet ( or for that matter a 100 Mbps Ethernet ) circuit has a Phy Integrated Circuit on the PCB board that connects to a transformer, also called magnetics in reference to the Ethernet Circuit. The Magnetics is followed by EMI chokes and finally, we have Ethernet connector.

We may also have ESD protection diode, placed typically at the Secondary side of the magnetics transformer ( The side of the transformer connecting to the Phy). Since the Phy is not directly exposed to the outside world, the chances of the damage to the Phy IC due to ESD is mininal. The chance that ESD will reach to the
secondary side of the magnetics is fairly small. Therefore, it may not be a bad idea to not install the ESD protection diode. It remains a topic of experiment and test to see if the ESD surge voltage at the Ethernet connector reaches the Phy in a given design. Infineonn's TVS3V3L4U is a good part to start with for ESD protection diode. For Gigabit Ethernet, the ESD protection diodes should be carefully chosen, as, the additional capacitance may lead to slight signal degradation.

ESD

The magnetics or the transformer provides three basic functions

(1) impedance conversion
(2) mode conversion,
(3) Low pass filtering.

The Trace running from the Phy to the Magnetics are differential signals and they typically have a differential impedance of 100 Ohms. The Phy side of the magnetics have the system ground as the return path. Therefore, these signal must be routed with Ground Signals beneath them. The Ethernet Connector side of the magnetics does not use the ground for the return signal. There is no use of the ground plane underneath the Signal running under on the connector side of
the Magnetics.

Transmitter and Receiver Coupling

In Gigabit Ethernet there are two differential pairs for each - the Transmitter and the Receiver. It is possible that some of the Transmitter Signal couples to the receiver signal, which in turn will degrade the signal at the receiver end. To eliminate this issue, the receiver differential pair should be spaced out from the Transmitter signal. The exact minimum required separation will require calculation of the magnitude of the crosstalk. If can not afford to accomplish this you may like to follow the recommendation pointed out by the Phy chip manufacturer. In the absence of ant recommendation, you may like to space these signal out as much as the layout density allows. A typical separation of 50 mils to 100 mils can be good guess. A separation of only 10 mils is probably too close.

Clean Power Supply on the Center Tap

The Center Tap of the Magnetics on the Phy side is required to supply DC voltage. Any noise on this power supply will affect the signal quality. The power supply should be clean, typically with a decoupling cap placed close to the magentics. A ferrite bead on the supply side could filter the noise coming from other parts of the circuit.

Integrated Magnetics and RJ45

A number of manufacturer's including Halo, manufacture the Phy and the magnetics is a single package. This not only saves space, but also reduced the worry of routing the signal between the magnetics and the RJ45 connector. As far as possible, the Integrated Magnetics and RJ45 should be used.

Gigabit Ethernet Transmission Rate

A 100 BaseT ethernet uses only two pairs - one transmitter and another receiver. The coding scheme is 8bit | 10 bit which means that for every 8 bits of data, 2 additional bits are added. Because of this, the actual data rate is 125 Mbps in place of 100 Mbps. To boost the data rate to 1000 Mbps, the Gigabit ethernet uses two novel ideas. First, in place of just +1V and -1V signals ( to represent 1 and 0), it uses 4 signal levels - +1.0V, 0.5V, -0.5V and -1.0V ( actually there is one more - 0 V, but let us ignore it for the moment to keep the things clear). With two bit signalling, the data rate boosts to 250 Mbps. Now, instead of 2 pairs, it uses 4 pairs in the Cat 5 Cable. Also, each of the 4 cables can transmit or can receive. We know that, is a typical, browsing patter, we are mostly receiving data. So all 4 pairs are used for reception. This boosts the transmission rate to 1000 Mbps.

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