TL;DR: Bob Smith termination is the R-C network inside an RJ45 jack that sinks common-mode currents to chassis ground so your Ethernet port passes EMC. In a magjack it's already inside the connector.
The 30-second version
- Ethernet jacks carry common-mode energy on the pairs and on the transformer center taps. Left floating, it radiates as EMI.
- Bob Smith termination = one ~75 Ω resistor per pair → a common star node → one ~1000 pF, ≥2 kV capacitor → chassis ground.
- Terminates the unused pairs (4-5, 7-8) in 10/100 too — they radiate if left open.
- Origin: Robert W. Smith, U.S. Patent 5,321,372 (1994).
Gotchas
- Low-voltage cap — it must be ≥2 kV (survives 1500 Vrms hi-pot + surge).
- Wrong ground — tying the star node to digital ground instead of chassis.
- Doubling up — adding an external network to an integrated magjack; it's already inside.
- Over-tuning — 75 Ω vs 52.3 Ω barely moves return loss.
Skip the discretes: use an integrated magjack
An integrated-magnetics RJ45 jack (母座) puts the transformer, common-mode choke and Bob Smith network inside a shielded socket, so termination is fixed and repeatable. VOOHU's integrated RJ45 series spans 100M–10G; e.g. the Gigabit SYT511Q293AC1A8D057: 1CT:1CT, 350 µH OCL, 2250 VDC isolation, insertion loss ≤1.0 dB, CMRR −30 dB, −40…+85 °C.
Full write-up (tables + FAQ): https://voohuelectronic-ux.github.io/voohu-rj45-guide/resources/bob-smith-termination-rj45-magjack.html
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