Magnetic-less 1GB ethernet


I have a design where I need to connect two 1Gb Ethernet devices on one card. To save space & cost I want to capacitively couple the four pairs instead of using transformers.

I've done point-to-point 100Mb Ethernet like this and it works flawlessly. The technique was to couple the lines with 100nf caps and then add appropriate pull up/down resistors to properly bias the drivers/receivers.

I saw one reference to the fact that this won't work at 1Gb, but I can't find the reference again. I have found several references to doing it at

100Mb, but they are all several years old.

Has anyone done this or heard about it?



Reply to
Steve Weingart
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As long as you can pass the signal, without distortion, what's the problem?

Reply to
James Knott

Hi Steve,

I came across your article explaining your work

with point-to-point Ethernet using capacitive coupling.

I have tried the same thing in a project, using two

National Semiconductor DP83847 Ethernet PHYs.

The PHYs are on a single PC board and located just

3 centimeters apart (the use of Point-to-point Ethernet

here was just because those are the most convenient

spigots between two of my processors). I am capacitiely

coupling them exactly as shown in Intel's app note

on Magneticless Ethernet.

However, I'm seeing flawless transmission on some boards,

and loads of dropped packets on others. I don't know if

I have a hardware problem, but it sure seems like it.

I emailed National Semiconductor, and the only suggestion

they had was to change some values in the PHY registers

to "improve the timing recovery."

Do you have any ideas? My link is literally 3 centimeters

on a PCB, through some capacitors with resistors tied to

power or ground for biasing. Is there such a thing as "too

short" of a link?

Would appreciate any ideas you might have, since you

have done this successfully!



mohan - at -

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