Electronic – How to this layout be improved? (Gigabit Ethernet with discrete magnetics and POE)

ethernetlayoutmagneticspcb-designrouting

Answer:

No there is nothing majorly wrong with the layout, it turns out that the ethernet transformer was out of spec by 0.2dB on Insertion loss, when paired with the PHY IC we are using.

Question

Is there anything noticeably wrong with the PCB routing of the gigabit ethernet?

Gigabit Ethernet has many design constraints, due to the layout of components on the PCB it is at times impossible to follow all the design rules. This design is required to perform Gigabit speeds, and feed a POE supply.

It must also pass FCC EMC/EMI and ESD testing.

I have read through almost all application notes available (TI, Intel..etc). I have, to the best of my knowledge, followed them as best I can. Traces are routed as diff pairs, and with the best possible spacing to prevent cross talk. Minimum use of vias/stubs of 2 per a segment. They are symmetrical as possible, and post magnetics each pair is matched to within 1.25mm, pre magnetics they are matched to within 2mm. Traces are routed on the bottom layer to avoid crossing multiple power planes as a reference.

However this design presents some challenges which I am too inexperienced to assess. Ie When do you choose to violate design rules, and to what extent can you get away with it.

Specifically

  1. The RJ45 and Magnetics have to be positioned as they are. The traces from the RJ45 to the Magnetics are length matched to within 2mm and are all laid as differential pairs. However is is a bit of a muddle – will this cause an issue with the GBE performance?
  2. Due to constraints the magnetics has two center tap traces laid underneath it (For the POE) – would this become an EMI issue? (Application notes suggest avoiding the area below magnetics)
  3. Post magnetics there are two features to be wary of – a crystal oscillator, and a transformer (in a cutout) which may add noise to the signal.How can this be avoided?
  4. Are the VIAs/Stubs at the phy end laid out in an acceptable manner?

Are there and obvious shortcomings of this layout that I am missing?
GBE and POE routing

RJ45 to Magnetics

Best Answer

Things that spring to mind:

  • usually, you'd model your PCB trace as transmission line that has exactly the same characteristics on the top, as on the bottom layer. As such, it doesn't make much of a difference where on the length of a trace you put the via; so instead of having these "looking like boobies" vias right next to each other, I'd just offset them enough to keep them in the middle of your trace
  • R51, C5 could as well be on Top Layer
  • I don't know the frequencies of your xtal or CPU, but chances are that the 125 Mbaud of Gigabit ethernet won't be much impressed :) however, if you're nervous about coupling, you might want to consider the classical star-like multiple ground-plane architecture. I don't think this'll be necessary here – Gigabit Ethernet Network PHYs aren't exactly bleeding edge in 2016, so even with some interference, they should work.
  • just looking at the part of the layout I see, I'd say it might be easier to route if you just rotated the PHY by 90° – but that might break down the moment the complexity on the "processor side" of the phy comes into play.
  • I think your RJ45-magnetics layout is OK; I'd probably been lazy and just routed the two diff pairs that are at the right half of the transformer "down" from the connector's pins, and the left half "up"; but that wouldn't have saved you from the one pair that crosses the other if you're supposed to access the magnetic's pads only from one side (unless you fit two traces between adjacent RJ45 pins...) . Topology is not always your friend :/

Notice: 1GE has a baudrate of 125 MBaud, i.e. even if considering the first two sidelobes, you really shouldn't be worried about frequencies above 375 MHz. With FR4 (with specific epsilon), and a lot of laziness approaching formulas, the wavelength of that frequency is roughly \$\frac15 \frac{c_0}{375\text{ MHz}}= \frac 15 \frac {3\cdot 10^8 \frac{\text m}{\text s}}{3.75 \cdot 10^8\frac1{\text s}}\approx \frac4{15}\approx 0.27\text{ m}=270\text{ mm}\$, so a 2mm trace length difference is but 2.7° phase error ... I think you'll be fine, even with a bit of unelegant routing.

Related Topic