Why is Bob Smith termination for Ethernet recommended if it's wrong?

A presentation by Royce Bohnert clearly shows that termination has no effect.

Conditions of the experiment:

How much of an impact does this termination actually have on radiated emissions?

Marine VHF band (156 – 165 MHz) of particular interest

Evaluated by measuring CM current on cable

RF current probe place on ~20m long cable

Cable connected to a device on each end

Results viewed on spectrum analyzer

Results:

Conclusions:

CM chokes so effective at reducing CM currents that the effect termination is negligible.

Impedance is not constant along length of cable. Different twist rates on each twisted pair cause cancellation of field every few centimeters.

Picture about common mode choke integrated in Ethernet signal transformers:

All the pictures and text cited above are prepared by Royce Bohnert, the author of the referenced report.

I "Bob Smith" with 75 ohms and I believe you should too. You can find a few corner cases where this termination doesn't work as well as something else, but overall it is the most robust if you do enough testing. No matter how good your choke is, there can always be longitudinal modal conversion caused by cables, connectors, etc... and somehow this energy needs to either dissipate or radiate away.

You'll find many answers and opinions on this. I think I'm perhaps uniquely qualified to talk about this one compared to the causal engineer who's worked on a bunch of stuff and dabbles with Ethernet from time to time.

I independently arrived at the Bob Smith termination in the summer of 1993 and further refined it in 1996 before I became aware of his "patent" in 1998. I've spent a large part of my career on this kind of circuit (and derivatives) and have led some well-funded investigations into it. I have probably worked on this more than anyone else I know of but I could be wrong - I know many of the people working on these sorts of things. I've worked with most of the magnetics companies, PHY IC companies, connector companies, and data cabling companies on this. I've also talked to Bob Smith himself about this circuit.

There is a popular article about it having to do with the "binder" modes of between-pair coupling that claim that a better value is roughly 52 ohms. I believe this oft-cited article traces back to Cicada semi (an Austin-based GigE PHY company last active 15 years ago). While somewhat correct, I believe that this article misses the real purpose of the "Bob Smith". Bohnert's presentation also has some holes. Don't get me wrong, I really appreciate that these fellows have dug into this and were able to make their work public. Kudo's to them. However, I'm not sure that they got the whole story.

Here's some points I have learned:

1. You cannot measure real-world radiated emissions accurately with a cable clamp. It is a useful shortcut to study and reduce specific problems to a simpler setup outside a $500/hr chamber. But to really understand this termination you need a variety of physical cable configurations, and a large 10 meter or larger chamber to accurately assess the different techniques. Bohnert's presentation uses a clamp and the radiated emissions he's detecting with it won't match the real-world cabling plant.
2. A LOT depends on the cable and how it is crimped. In particular the Scd21 (L-TCL) of the cables and connections make a large difference.
3. The common mode impedance that is most important w.r.t. radiated emissions is vaguely defined to "ground" or "free space"
4. The "Bob Smith" termination is overall a substantial benefit in the most typical cases to reduce UTP cable emissions.
5. Shielded twisted pair cable is a neat idea but it doesn't really work better compared to UTP as the shield causes more problems than it solves in most cases and costs a lot more money
6. I have seen CM chokes saturate for a variety of but not so much due to RF energy

Wish I could say more. But in summary the 75 ohm Bob Smith is your best bet. Try to use it and get a "passing" result with it. If you have specific emissions needs for a particular installation, (like a Navy ship), use CAT6A cable that has been professionally terminated.

cdwilson, please take on all your questions:

... Bob Smith termination ... may not be ideal (or correct). If this is the case, why is Bob Smith termination still recommended in Ethernet design guides from Intel, TI, etc?

There is no conspiracy here because it's not the case. Auto-crossover (Auto MDI-X) function is popular now (and was popular even in 2000s) and it requires a symmetric CMC enabled transformer. Again, as Royce Bohnert shows, CMC solves the problem in its root making the need of the termination practically negligible. Also, CMC enabled transformer make possible to use various mediums with various impedances in the single design: for example, 100BASE-TX is intended to operate over 100-Ohm UTP and 150-Ohm STP (by IEEE Std 802.3). Repeating, there is no conspiracy and/or irrationality here.

Is the author of the article incorrect in his analysis?

IMO, the author looks at the problem narrower than it is: there are many variants of BS termination (e.g. Intel'2001, for another ones lurk by yourself :-) in the first, and the termination is intended to solve not only EMI but also ESD issues in the second. Looks over the past 15-20 years (with help of Google, of course :-) and you'll see how the termination evolves.

Or do people just not care that the return loss could be improved with a proper choice of resistor?

They care. But also they care about the cost (and other features of the design) in complex.

... do the magnetics always come with chokes built in?

CMC is a common feature now. During my 10+ yrs carrier i used only CMC enabled magnetics. Maybe it is because i like AutoMDIX :-) You could browse Pulse, PCA, Halo, and many others to make your own opinion for today.

If the proper termination was used, would it be possible to use magnetics without the chokes?

If you find such a transformer you could, but i think it is problematically now :-) PHY chip manufacturers (Micrel/Kendin, Intel/LevelOne, TI/National) recommend CMC enabled ones and specifies corresponding BS termination variant based on that.

Also, the Bohnert presentation addresses emissions but I'm also curious whether this applies the same to immunity? (i.e. imbalance in the PHY receiver can reduce tolerance to immunity)

Yes, it applies: less emissions -> less crosstalks/disturbances/imbalance. Bohnert states:

Impedance is not constant along length of cable. Different twist rates on each twisted pair cause cancellation of field every few centimeters.

If it's not about immunity, what is it about?