Magnetics selection for VITESSE's Gigabit chip.

  

Application Note

1Scope of This Document
Many factors govern the proper selection of magnetics to achieve the best system performance from copper Gigabit Ethernet PHYs.
This Application Note offers guidance on selecting the best magnetics configurations for use with Vitesse¡¯s SimpliPHYTM Series
transceivers to achieve targeted EMI objectives at the lowest system cost. The resulting simplifications of the magnetics used with
Vitesse¡¯s SimpliPHYTM products lead to the concept of SimpliPHY¡¯d Magnetics.
2 Role of Magnetics in Gigabit Ethernet Systems
While not explicitly required by IEEE 802.3-2002, magnetics are the most commonly used method of meeting the requirements of the
10/100/1000BASE-T PMA electrical interface. However, there is no one standard configuration that meets all objectives for all designs
at the lowest cost. Magnetics offer a straightforward solution to many functions of this interface, including electrical isolation, signal
balancing, common-mode rejection, impedance matching, and EMI reduction. Following is a brief description of each of these areas.
2.1 Electrical Isolation
For safety, the IEEE specification requires a 10/100/1000BASE-T port to be able to withstand 1500 Vrms at 50 Hz to 60 Hz for 1 minute
between ports or from each port to the chassis ground. Transformers can easily and inexpensively meet this isolation requirement and
are commonly used for this purpose.
2.2 Signal Balancing/Common-Mode Rejection
Each 10/100/1000BASE-T network cable consists of four sets of twisted pairs connected in a balanced configuration. While there are
various circuit topologies that can provide the necessary balanced operation without magnetics, transformers simply and easily provide
the balanced connection to each pair of a cable and also provide very effective rejection of common-mode signals.
The common-mode rejection of a transformer functions in both of the signal directions of a port. This common-mode rejection
attenuates common-mode signals coming both from the cable to the PHY and also from the PHY (and its surrounding system) to the
cable. Reducing common-mode signals picked up by the cable from its environment improves the signal-to-noise ratio of the system
and allows the PHY¡¯s DSP to more easily recover the data signal and achieve the desired bit error rate. Attenuating common-mode
signals going onto the cable helps to reduce EMI. This is discussed further in Section 2.4.
2.3 Impedance Matching
Each twisted pair in a CAT5 cable is designed to have a characteristic impedance of 100 Ohms. In some situations this might not be
compatible with other electrical parameters of the transmitting and receiving circuitry. Should an impedance transformation be required,
transformers with winding ratios other than 1:1 can accomplish an impedance transformation according to the formula.




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