EMC compliance is a property of the complete PoE product and its test configuration—not of the Ethernet controller or cable in isolation. Würth Elektronik’s RD022 “GB PoE+ Ethernet USB adapter” is a useful Gigabit reference design because it combines a 1 Gigabit RJ45 interface with PoE+, USB Type-C and an adjustable 6–18 V DC/DC output rated up to 25 W. Its measurements show why load wiring, cable shielding, grounding, converter operating point and filter layout must be evaluated together.
What the RD022 reference design contains
The RD022 board is intended for exploring PoE operation and Ethernet transmission behavior. Its relevant interfaces and ratings are:
- 1 Gigabit Ethernet through an RJ45 connector with integrated PoE+ circuitry.
- USB Type-C used to operate the board and included in many EMC tests.
- An adjustable DC/DC output from 6 to 18 V, with a maximum output power of 25 W.
Those specifications identify the evaluated hardware; they do not establish compliance for another PoE product, a different cable, or a finished installation.
Why the attached hardware is part of the EMC design
A PoE interface connects a switching power converter, magnetics, Ethernet pairs, a shield system, USB wiring and an external load. Each connection can provide a path for common-mode current or become an antenna. The RD022 work therefore considered both a compact point-of-load arrangement and a remote load connected by longer output wires.
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| Load arrangement | EMC implication | Design action |
|---|---|---|
| Compact load close to the board | Short wiring generally reduces the antenna area and the coupling path from the converter. | Use this as a baseline, but do not treat a compact setup as representative of a product with remote wiring. |
| Long output leads or a larger external device | Output-switching currents can excite the leads and increase radiated or conducted noise. | Evaluate an output filter on the actual assembly and cable; verify both emissions and immunity. |
For the RD022 radiated-emission and immunity work, load lines were measured with 2–3 m cables. Adrian Stirn of Würth Elektronik recommends keeping EMC-relevant load cables as short as possible, preferably below 3 m. That is a recommendation for controlling the test and product configuration, not a universal cable-length limit for every PoE installation.
Test setup details that changed the result
USB shield and board ground
The USB interface was required to operate the board and appeared in many tests. In the RD022 setup, a direct connection between the USB-cable shield and board ground was required for stable operation at high immunity levels, including 20 V/m above 1 GHz. This is an observation about that board, cable and grounding arrangement. It should not be converted into a blanket rule that every USB shield must be bonded in the same way; the bond, enclosure, connector construction and safety architecture must be reviewed on the target product.
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Operating point
The article compared 12 V at 2 A with 18 V at 1.3 A and reported higher interference emission at the 12 V/2 A point. Filter development was consequently carried out at the 12 V operating point. Converter topology, switching frequency, control mode, load and thermal state all affect this relationship, so the result cannot be generalized to another DC/DC converter.
| RD022 state | Reported observation | How to use it in design |
|---|---|---|
| 12 V, 2 A | Higher emission than the reported 18 V/1.3 A state. | Include the maximum-noise operating state in pre-compliance scans and filter tuning. |
| 18 V, 1.3 A | Lower emission than the 12 V/2 A comparison state on this board. | Do not use a lower-emission state as a substitute for worst-case testing. |
Ethernet-cable shielding is a variable, not a guarantee
The companion RD022 report found that conducted-emissions performance was borderline with shielded Ethernet cable and exceeded the limit with unshielded cable. The difference demonstrates that the cable and its termination are part of the current-return network. It does not mean that shielded cable alone guarantees a pass, nor that unshielded cable always fails in another design.
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During development, record at least the cable category, shield construction, length, connector shell termination, chassis or board bond and the PoE switch or injector used. Retest after any of those items changes. Also verify whether a frequency-point change is caused by the device under test or by auxiliary equipment such as the PoE switch; a speed drop can alter the observed result.
An example filter for long output lines
For larger devices or designs with output cables, the RD022 article recommends evaluating a filter made from a ferrite bead, a capacitor and a signal-line common-mode choke. The cited values are starting points for that design, not a plug-in compliance solution.
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| Part in the example | Referenced value or type | Purpose and qualification |
|---|---|---|
| Ferrite bead | 780 Ω at 100 MHz, 1812 package; Würth Elektronik 742792515 is named. | Suppresses high-frequency differential/common-mode energy in the selected path. Impedance depends on bias, frequency and layout. |
| MLCC | 4.7 µF, X5R, 50 V | Provides a local high-frequency return with the bead. DC-bias derating and placement change its effective capacitance. |
| Signal-line common-mode choke | 17 µH | Attenuates common-mode current on the output wiring. The choke must meet the required current, voltage, insulation and signal-integrity conditions. |
Place the network according to the current paths it is intended to interrupt, keep the high-di/dt loop small and check the converter’s stability requirements. Validate insertion loss, temperature rise, conducted emissions, radiated emissions and immunity on the populated product with its final cable and load. Buying the named components does not ensure compliance.
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PoE and Ethernet interoperability
IEEE 802.3 defines Ethernet and PoE behavior. For example, IEEE 802.3af supports optional power for 10BASE-T, 100BASE-TX and 1000BASE-T devices. PSE detection and maintain-power-signature monitoring determine whether a power-sourcing equipment (PSE) continues to deliver power to a powered device (PD). These mechanisms answer whether the equipment can negotiate and operate correctly; they are not an EMC test result.
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The Ethernet Alliance certification program distinguishes Gen 1 certification based on IEEE 802.3 Clause 33 from Gen 2 based on Clause 145, and uses different marks for PSE and PD products. A compatibility mark must not be read as certification of EMC, the cable or the completed installation.
Product EMC requirements
The RD022 article identifies CISPR 32 emissions and CISPR 35 immunity as common context for PoE equipment. The controlling requirement depends on the product category, market and applicable edition; a dedicated product or product-family standard takes precedence where one applies.
IEC 61000-6-1:2016 is a generic immunity standard for equipment intended for residential, commercial, public and light-industrial locations when no relevant dedicated product or product-family immunity standard exists. Confirm the current edition and jurisdiction before selecting it for a compliance plan.
Installation requirements
IEC 60364-7-716:2023 addresses the design, erection and verification of extra-low-voltage DC distribution over ICT cable infrastructure, including IEEE 802.3 PoE systems. It is installation guidance and verification context, not the EMC test standard for a finished product. The Ethernet Alliance also states that its PoE program does not certify or label cables; its FAQ identifies Category 5 or better cabling as suitable for PoE subject to applicable cabling and safety standards.
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Quick Recap
A practical RD022-style EMC workflow
- Freeze the complete configuration. Record the PSE or injector, Ethernet cable type and shield termination, USB cable, load cable length, load, enclosure and grounding connections.
- Define operating states. Include the highest output current and the converter state that produces the greatest noise; for RD022 work, that included 12 V/2 A as well as 18 V/1.3 A.
- Test compact and remote loads. Keep EMC-relevant load lines below 3 m where the product allows, then deliberately test the longer-line arrangement if that is how the product will be installed.
- Compare cable shielding configurations. Measure shielded and unshielded cables only when both are permitted by the intended installation, and document the connector and chassis bonds.
- Check immunity while monitoring function. Confirm power regulation, USB operation, Ethernet link state, data errors and recovery. The RD022 observation at 20 V/m above 1 GHz shows why a pass/fail status alone is insufficient.
- Add and tune filtering at the source of the problem. Evaluate the ferrite, 4.7 µF capacitor and 17 µH common-mode choke example only after checking current, voltage, insulation, signal integrity and converter stability.
- Retest the final assembly. Repeat conducted and radiated measurements after changing the filter, cable, grounding or auxiliary equipment; retain the exact operating and test conditions.
What this reference design can—and cannot—prove
- It can show how a real Gigabit PoE board behaves when output wiring, USB grounding, cable shielding and operating point are varied.
- It can provide candidate component values and a test strategy for a design with long load lines.
- It cannot prove that arbitrary Gigabit PoE products, cables or installations comply.
- It cannot replace the product-specific standard assessment, safety review, cabling requirements or accredited compliance testing required for the target market.
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