How to sniff USB3 and Ethernet signals on the PCB?

Question

I want to sniff USB3 and Ethernet diff lines without affecting communication on my PCB. But I couldn't figure out how to do that with a small form factor.

Answer 1

Inductive pickup is one way to do it. It may introduce less of a discontinuity than a differential amplifier input would. Back when I was doing it, I did need the galvanically isolated pickup. You may not need it though. The signal degradation due to a galvanically connected high speed diff-amp may well be acceptable with modern parts. Decades ago, in my application, it wasn’t the case.

So, you could try with a pair of pogo pins touching exposed traces, and a fast diff—amp.

I’ve made such things in the past - like 30 years ago - and it was quite a bit of work to get usable signals. Maybe I was going the hard way about it, but in the end it worked.

I used a tiny inductive pickup – a solenoid with its axis perpendicular to the differential pair run. It was mounted very symmetrically on top of the gap between the differential traces. This was done under a microscope with an XY stage to position it prior to gluing.

The coils needed were tiny - grain-of-sand sizes. IIRC would wind 50AWG wire on a 40AWG “core”. That takes some custom tooling as the wire just rips if you try to handle it manually. It was like watchmaking on steroids – not very practical but work it did.

Right next to the solenoid was a high speed amplifier/buffer. I’d end up needing custom emphasis/“cleanup” after the preamp, to bring the signal back to a form that a PHY was able to deal with.

These days I’d try with a magnetoresistive head from a hard drive as a pickup. They have the feature sizes that should work well for this, and are made to handle high bandwidth signals. They also have ready-to-use analog preamplifiers close by. Those may be reusable as-is. The head gap would be oriented along the differential pair, centered between the traces. For highest signal there may be a need for slight shimming – or de-shimming.

If you are laying out the PCB to facilitate such probing, make a window in the soldermask at the probe points. The shim thickness may need to be smaller than the soldermask thickness. Non-conductive shims are probably best, although you’d need to experiment. Thin Mylar films would be probably a good shim material. Tweak the shim thickness for maximum signal amplitude.

Another way to shim that is to leverage the smooth surface of the head and put a couple of very small non-plated through holes centered between the differential pair conductors. Those would make a slight discontinuity, but would let you push air at adjustable pressure/flow from the opposite side of the board, and use the head float as a shim adjustment.