Ethernet Magnetics: Design Rules and Process Limits
An Ethernet port looks simple from the outside, but between the RJ45 jack and the PHY there is a small block of transformers, chokes and resistors that decides whether the link is reliable. The ethernet magnetics provide isolation between the cable and the board, convert the single ended cable signal into the differential signal that the PHY expects, and carry the bias that some PHYs inject onto the pair for detection and for power. Each of those jobs has a layout consequence that shows up later as a link that drops or a radiated emission failure.
What Ethernet Magnetics Do
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Isolation and the Common Mode Choke
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Bob Smith Termination and Chassis Ground
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Placing and Routing the Magnetics Module
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Differential Pair Routing to the Jack
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Power, Ground and Protection at the PHY
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Common Mistakes and How to Check the Layout
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Process Control and Verification
The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Checks Before Release
The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.
Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.
Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.
Verification and Records
Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.
The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.
FAQ
Do I need external magnetics if the RJ45 jack already includes them? Not normally, but you still need the Bob Smith termination network and the chassis capacitor if the integrated module does not provide them. Check the datasheet for the internal termination value before adding a second network, because two networks in parallel change the impedance.
How much does magnetics placement matter at 100 Mbps? Less than at gigabit rates, but the common mode behaviour that drives EMC is largely speed independent, so a poor layout can still fail a radiated emission test at 100 Mbps even when the link itself works perfectly.
Can I connect signal ground to chassis ground with a zero ohm resistor? It works on the bench, but a resistor gives a low impedance path at all frequencies and lets board noise onto the shield. A capacitor, or a resistor in parallel with a capacitor, sets a defined impedance at the frequencies that matter.
