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
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Isolation and the Common Mode Choke
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Bob Smith Termination and Chassis Ground
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Placing and Routing the Magnetics Module
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Differential Pair Routing to the Jack
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Power, Ground and Protection at the PHY
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Common Mistakes and How to Check the Layout
function require(path) {
return mod.require(path);
}
[object Object]
G:\guobing\_a91041.cjs
G:\guobing
Process Control and Verification
On a design of this kind, ethernet magnetics is the item that decides how the rest of the board is arranged. 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.
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.
Process Control and Verification
On a design of this kind, ethernet magnetics is the item that decides how the rest of the board is arranged. 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.
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.
Process Control and Verification
On a design of this kind, ethernet magnetics is the item that decides how the rest of the board is arranged. 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.
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.
Process Control and Verification
On a design of this kind, ethernet magnetics is the item that decides how the rest of the board is arranged. 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.
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.
