Today, when there is so much talk about the uplinks and downlinks and IoT, it is easy to forget that networking is still done over cables. Servers, switching stations, and business computer centers all still use network cables. The most utilized network cable is Ethernet, which has become recognizable to virtually everyone who has ever hooked up a computer.
Ethernet started as an in-house networking solution at Xerox’s Palo Alto Research Center (PARC). However, thanks to the work of Dr. Bob Metcalfe and others, Ethernet was created and has been a networking standard for nearly 40 years. The first Ethernet speeds were around 3 MB/s, which seems excruciatingly slow today, with the availability of gigabit Ethernet.
What is Gigabit Ethernet?
Ethernet is the primary means of connecting computers in relative proximity 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 are connected via an Ethernet cable 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 that your device can utilize. Options for Ethernet standards are listed below.
As the list above illustrates, several Ethernet standards can be broadly classified by 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 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 important.
- 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 is the achievement of 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 signals routing, which must be isolated 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 that require depanelization, which must be taken into account during design for efficient board builds.
Gigabit Ethernet PCB Layout Guidelines
The above issues can be successfully addressed by following good guidelines for your gigabit Ethernet PCB layout, as listed below.
- 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.
- Decoupling capacitors should be placed as close as possible to power supply pins.
By following the PCB layout guidelines above, you should be able to avoid the challenges that may threaten the operation of your gigabit Ethernet communication board.
Building GbE PCBAs
Most GbE boards are utilized for computer networking, which is a competitive industry. Therefore, in addition to functionality and reliable operation, it is also important that your PCB layout be designed for efficient manufacturing and the highest quality.
