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VSC8530XMW-03

P1-P3P4-P6P7-P9P10-P12P13-P14
Using Magnetics
ENT-AN0098 Application Note Revision 2.1 5
Figure 2 • Intrinsic Characteristic Impedance Between Each Pair of the Four Pairs
Providing those pair-to-pair transmission lines with termination matching their characteristic impedance
in order to absorb the reflected wave and prevent standing wave from occurring significantly lowers
parasitic interference radiation.
The high voltage capacitor between the common point of all the four resistors and the Chassis ground is
not a part of Bob Smith termination but helps in filtering the noise residue managing to leak out through
the transformer.
2.3.1.3 8-Core Magnetics
The 8-core magnetic only has a transformer and a CMC placed on either the cable side (see Figure 3,
page 5) or the PHY side (see Figure 1, page 4 and Figure 5, page 6) of the transformer.
Placing the CMC on the cable side has the following advantages:
• It also attenuates any common-mode noise that is generated by either the center taps noise of the
transformer,
• Or by the transformer windings imbalance,
• Filters the noise in the systems where the Chassis ground is connected to the digital ground (which
is often the case for computer motherboards) and, even being connected to Earth ground through
the power supply circuit, is still noisy enough to inject noise through the high voltage capacitor (in
this case removing the capacitor might be better EMC-wise).
The disadvantage, though, is that the Bob Smith termination becomes less effective for common mode
noise due to the high impedance inserted between the cable and the impedance-matching resistor. The
12-core version and the 8-core version with CMC on PHY side do not have this disadvantage but require
really quiet Chassis ground.
Figure 3 • 8-Core Magnetic with CMC on Cable Side (One Pair Shown)
Using Magnetics
ENT-AN0098 Application Note Revision 2.1 6
Using an 8-core magnetic with the classic 2-wire CMC on the PHY side (see Figure 1, page 4) is also not
ideal due to the possible noise coupling from the ground plane to the cable through the transformer’s
center tap capacitor.
In addition, based on a limited tests conducted with transformers having CMC on the PHY side, the
margin within the template has been somewhat reduced. We therefore recommend that future designs
not to use this transformer configuration (that is, with CMC on the PHY side).
For older designs where this configuration has been used successfully, and passed all system level
applicable standard tests, they may continue using these transformers.
Figure 4 • 8-Core Magnetic with Classic 2-Wire CMC on PHY Side (One Pair Shown)
Figure 5 • Modern 8-Core Magnetic with 3-Wire CMC on PHY Side (One Pair Shown)
2.3.1.4 8-Core Versus 12-Core
Generally speaking, there is no universal advice on what is the best, but the right magnetic selection
should be made depending on conditions in every specific case.
The 8-core magnetics are cheaper, thus more attractive, but riskier in many cases and cautiously
recommended to skilled. The 12-core ones make the design process much easier without demanding
extreme experience in the layout art, especially if the Ethernet module is a part of a system with a big
digital section generating a lot of interference noise.
2.3.1.5 Other Factors
Although the schematic representation of different magnetic modules looks alike, there might be a lot of
difference in their electrical and EMC performance. The coupling between the signals on each side of the
transformer and common-mode chokes can couple noise and thereby reduce the effect of the common
mode chokes.
2.3.2 Integrated or Discrete?
To minimize the component count and the product size, some manufacturers deliver magnetic modules,
where the magnetic block is build into the RJ45-type connector block. As any compromise, this has both
advantages and disadvantages.
Advantages:
• Lower component count -> lower production (assembly) cost,
• Potentially better EMC shielding of the sensitive cable side signals by the metal shield,
Using Magnetics
ENT-AN0098 Application Note Revision 2.1 7
• Smaller footprint than separate magnetics and a connector.
Disadvantages:
• Harder to rework if a magnetic/connector is failing during production test -> Higher production cost,
• Due to the space limitations, the magnetics cores are smaller and closer to each other thus
degrading crosstalk and EMC characteristics, increasing nonlinear distortion and losses.
Especially the electromagnetic emission performance can be either better or worse, depending on the
product they are used in. The integrated magnetic/connector modules have a metal shield placed around
the whole part; it effectively screens the part from any noise that is present inside the system box.
However, due to the small size of the magnetic part of the module, there is a high risk that the noise is
coupled between the cable side and the PHY side of the magnetics, thereby limiting the common-mode
choke effect. The sizes of the magnetic cores used in the magnetic/connector combinations are also
normally smaller than those used in separate magnetics, and the performance is thereby lower.
Using separate magnetics and connectors has an advantage of lower coupling between the different
parts inside the magnetics, and this results in better common-mode filtering. The disadvantage is that,
once the signals have been filtered by the magnetics, they are routed to the connector on the PCB, and if
the box contains a lot on noisy logic, this can couple to these lines and create EMC problems.
The designer needs to balance these advantages and disadvantages against each other when deciding
what to use in his board design but much more often the best EMC performance is reached by using 12-
core discrete magnetics.
2.4 Test Data
2.4.1 EMC Test Data
The table below summarizes the different magnetics and magnetic/connector combinations that have
been tested with the SparX customer evaluation boards (EVBs).
Experiment has been conducted with and without a closed metal chassis and, as no noisy circuits exist
on the SparX-G8 (VSC7388) and SparX-G5 (VSC7385) EVBs, the EMC results did not differ significantly.
Table 1 • EMC Test Data
Vendor Part Number Configuration Test Setup
EMC Class
(FCC, Part 15) Margin Comment
Pulse H5008 Single, 12-core SparX-G8 Managed EVB B 9.2 dB Without chassis
Pulse H5009 Single, 8-core SparX-G8 Managed EVB B 1.3 dB Without chassis
Pulse H5007 Single, 12-core VSC8641/VSC8601 EMC Board B 2.1 dB Without chassis
Pulse H5004 Single, 8-core VSC8641/VSC8601 EMC Board B 1.0 dB Without chassis
Pulse LF9207A Single, 12-core SparX-G8 Managed EVB B 2.0 dB Without chassis
P1-P3P4-P6P7-P9P10-P12P13-P14

VSC8530XMW-03

Mfr. #:
Buy VSC8530XMW-03
Manufacturer:
Microchip / Microsemi
Description:
Ethernet ICs 1Port Cu GE PHY iTMP SyncE, RGMII/RMII
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
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