Ethernet PCB Layout Methodologies

Today, when there is so much talk about uplinks, downlinks, and IoT, it is easy to forget that networking is still over cables. Servers, switching stations, and business computer centers still use network cables. Ethernet is the most utilized network cable, and it has become recognizable to virtually everyone hooked up to a computer. Designing for Ethernet PCB layout requires an understanding of the varying capabilities of Ethernet according to the various communication standards under the Ethernet umbrella.

What is Gigabit Ethernet?

Ethernet is the primary means of connecting computers relative to each other, including local area networks (LANs), as shown below, and wide area networks (WANs).

As shown in the figure above, all network devices use an Ethernet cable connected to an Ethernet card—often referred to as a network interface card or network interface controller (NIC) card. The type of NIC and cable determines what networking protocols or standards your device can utilize. The Ethernet standards chart below indicates the transfer speed and range of some of the most common Ethernet protocols.

Ethernet Standards Chart
Ethernet Type	Bandwidth (Mbps)	Cable Type	Maximum Distance (m)
10Base-T	10	Cat 3/Cat 5 UTP	100
100Base-TX	100/200	Cat 5 UTP	100
100Base-FX	100 / 200	Multi-mode fiber	400 / 2000
1000Base-T	1000	Cat 5e UTP	100
1000Base-TX	1000	Cat 6 UTP	100
1000Base-SX	1000	Multi-mode fiber	550
1000Base-LX	1000	Single-mode fiber	2000
10GBase-T	10000	Cat 6a/Cat 7 UTP	100
10GBase-LX	10000	Multi-mode fiber	100
10GBase-LX	10000	Single-mode fiber	10000

As the list above illustrates, several Ethernet standards can use the transmission BW or speed. The rows colored in green represent the gigabit Ethernet or GbE standards. As shown, cables explicitly designed to carry speeds at the Gbps level are either unshielded Cat 5e or Cat 6 copper or single or multi-mode fiber for longer distances.

Designing Boards for GbE Networking

When designing boards for GbE networking, the board must include a connector that will accept an RJ45 plug. Fortunately, all Ethernet cables have these end connectors. As GbE networking is high-speed, there are several issues to consider, which are summarized below.

Challenges for Gigabit Ethernet PCB Layouts

Significant challenges for routing GbE Ethernet include the following.

• Maximizing signal strength
  Whether using a serial gigabit media-independent interface or serializer deserializer (SGMII) and (SerDes), respectively, to connect the media access control block or MAC interface to the physical layer (PHY) of the NIC, maximizing signal strength is essential.

• Ensuring spacing between differential pairs
  Differential pair routing has special requirements to avoid common-mode noise resulting from imprecise matching on the pair. One of the best ways to prevent this is by ensuring adequate spacing to avoid stray current leakage.

• Achieving electromagnetic compatibility
  Electromagnetic compatibility, which achieves minimal EMI between on-board components and nearby electronics, is critical to optimizing signal integrity.

• Isolation of different signal types
  GbE routing requires both digital and analog signal routing, which requires isolation to avoid performance degradation.

• Accounting for depanelization
  Ethernet connectors are often mounted on the edge of PCBs to match enclosure designs. Boards, however, are manufactured in panels requiring depanelization, which needs consideration during design for efficient board builds.

Gigabit Ethernet PCB Layout Guidelines

By following the guidelines listed below for your gigabit Ethernet PCB layout, you can circumvent most issues related to Ethernet layout.

• Isolate the PHY from magnetics–at least 25 mm, if possible.

• Ensure differential pairs between the PHY and magnetics are routed in parallel as precisely as possible.

• Place pull-up resistors on the PHY differential pair within 10 mm from associated traces.

• Do not route PHY differential pairs near the board edge.

• Link routing between PHY and MAC should be the same signal strength and in parallel.;

• Ensure a solid ground plane is present beneath the differential signals for the return path.

• Do not use a split ground plane or vias for differential pairs.

• Place decoupling capacitors as close as possible to power supply pins.

By following the PCB layout guidelines above, you should avoid the challenges that may threaten the operation of your gigabit Ethernet communication board.

Your Contract Manufacturer Connects Designs to Devices

Ethernet PCB layout requirements are similar to those of other high-speed constraints: designers must adhere to best layout practices to minimize EMI and maximize EMC. Keeping track of the many high-speed design and manufacturing aspects may be challenging, but VSE is here to help. Our engineers thoroughly review your design before entering production to ensure its quality, reliability, and cost-efficiency. We’ve been realizing life-changing and life-saving designs for over forty years with our valued manufacturing partners.