Using Magnetics
ENT-AN0098 Application Note Revision 2.1 9
Figure 7 • Return Loss Specification, Recommended and Measured
There are two key points to emphasize about the return loss plot:
• For a real system with output impedance contributions from both PHY output and a magnetic
transformer to satisfy the IEEE specification, magnetics with a return loss margin of at least 2 dB
(versus the IEEE 802.3 specification at the MDI) are recommended. The red curve in the plot
reflects a 2 dB back-off from the IEEE 802.3 limit.
• The measured return loss is flat to within 3 dB up to 40 MHz–50 MHz. That flatness is a key aspect
in maintaining signal integrity.
2.5 Suggested Magnetics
This section lists the suggested characteristics when selecting a magnetics product compatible with
Microsemi 1000BASE-T Ethernet PHYs. Use the magnetic vendor's datasheet or lot-sample test data to
check for the following characteristics.
• Return loss as specified in IEEE Return Loss Data, page 8
• Turns ratio tolerance: (1:1) ± 2%
• Common mode choke on the line side for all applications that permit line-side CMC
• Center-tap isolation between each pair A, B, C, D of the magnetic
• Line-side center-taps that can be individually terminated with Bob Smith termination
Contact your Microsemi representative for more information about specific magnetics proven successful
with Microsemi Ethernet PHYs.
