I came across this article today (more detailed paper), which claims that the widely recommended Bob Smith termination (75-ohm resistor + 1000pF to chassis GND) 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?
Is the author of the article incorrect in his analysis? Or do people just not care that the return loss could be improved with a proper choice of resistor?
Answer
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.
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