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How to build an etherProbe (and why/how to use it)

Motivation

One of my projects operates several nucleo boards interconnected via an Ethernet LAN. This system is to have hard real time characteristics; so it is instrumental to master scheduling. Hence, one of the aspects was to check what packets were actually present on the network, and when they occurred.

The most straightforward idea is to connect oscilloscope probes on the RX and TX twisted pairs via a network derivation.

Unfortunately, the hobbyist’s oscilloscopes generally haven't enough bandwidth to analyze a 100BASE-TX signal (please look carefully at the time scale on the linked image!).

A possible workaround should be to detect the signal in the sense of an RF signal detection.

Engineering

It is important to preserve the galvanic isolation of the bus. Hence the detector schematic:

One shall notice that most of the available 10/100BASE-T ethernet magnetics have 2 lanes. Considering that we target to analyze both RX and TX of an Ethernet link, this is good news.

Cherry on the cake, why not to layout a 2-sided PCB. The online vendors are so cheap that it would be stupid not to afford a professional device: