Royce Bohnert Typical Ethernet Circuit Diff pairs from RJ45 connect to isolation transformer Diff pairs from transformer connect to Ethernet Phy IC 100 Ω diff pairs are terminated at the ID: 1031461
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1. Common Mode Termination Of Ethernet Twisted PairsRoyce Bohnert
2. Typical Ethernet Circuit Diff pairs from RJ45 connect to isolation transformer Diff pairs from transformer connect to Ethernet Phy IC 100Ω diff pairs are terminated at the Phy
3. Ethernet MagneticsMost Ethernet magnetics come with integrated CM chokesTransformer center taps provide access to CM signals
4. Robert Smith Termination MethodUS Patent 5321372, Published 6/14/1994The combination of pairs within a UTP cable form transmission lineCM currents can flow on these transmission lines, causing radiated emissions
5. Was Smith Wrong?Article on EE Times by Jim SatterwhiteAuthor asserts that Smith was wrong about the value that should be used for the termination.Presents measurements and claims that the appropriate termination resistance is 52.3Ω
6. Goal Of This ProjectHow much of an impact does this termination actually have on radiated emissions?Marine VHF band (156 – 165 MHz) of particular interestEvaluated by measuring CM current on cable
7. Measuring CM CurrentRF Current ProbeSolar Electronics Company, Type 9210-1300 KHz -500 MHzNominal ZT is 0.3-5Ω. Relates measured voltage to current.
8. Measuring CM CurrentSpectrum AnalyzerAgilent E7402A (EMC Analyzer)Agilent E4446A
9. Measurement SetupRF current probe place on ~20m long cableCable connected to a device on each endResults viewed on spectrum analyzer
10. CM Current - Results75 Ohm Termination
11. CM Current - Results52.3 Ohm Termination
12. CM Current - ResultsNo Termination
13. CM Current - ResultsBased on initial results, the termination does not appear to have a significant impact.
14. Possible ConclusionsCM 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.
15. References Updating the Bob Smith Termination Techniquehttp://www.eetimes.com/document.asp?doc_id=1277940 US Patent 5321372, Apparatus and method for terminating cables to minimize emissions and susceptibilityElectromagnetic Compatibility Engineering, Henry W. Ott