RBW, Not Bandwidth Alone, Decides If Your Near-Field Probe Reading Is Narrowband or Broadband
How a spectrum analyzer's resolution-bandwidth setting — not just the emission itself — decides whether a near-field probe reading reads as narrowband spikes or broadband noise.
Reviewed by Mike Kwak, EMI/EMC shielding · POCONS USA
The finding
According to Kenneth Wyatt, writing in In Compliance Magazine's EMC Bench Notes column, the first move in any board-level EMC troubleshooting session is picking the right near-field probe: circuits with fast voltage transitions tend to be higher-impedance and radiate a dominant E-field, while fast-current circuits (clock traces, switching regulators, digital buses) radiate a dominant H-field, so H-field loop probes of varying diameter are the workhorse for locating noise sources. Wyatt then walks through how to read what the probe picks up on a spectrum analyzer: whether a harmonic shows up as a discrete narrowband spike or smears into a broadband noise floor depends on whether the harmonic's energy fits inside the analyzer's resolution bandwidth (RBW) — and EMC test standards fix that RBW at 9 kHz below 30 MHz, 120 kHz from 30 MHz to 1 GHz, and 1 MHz above 1 GHz, so the same physical signal can present differently depending on which band it falls in. He illustrates three signatures engineers will recognize on the bench: clean harmonic combs from digital clocks, broadband hash from parallel data buses (often with narrowband spikes riding on top), and broad resonant humps from DC-DC converter switching noise coupling into cables or board structures. Matching a measured profile to its likely circuit source, Wyatt argues, is what turns a probe sweep into an actual root-cause diagnosis.
Why it matters for RF/EMC design
Distinguishing a converter-driven broadband hump from a clock-driven narrowband comb changes the fix: broadband resonance-coupled noise usually calls for board-level filtering, better DC-DC layout, or absorber material to damp cavity/cable resonances, while narrowband clock harmonics are better addressed with a shield can or gasketed enclosure seam tuned to the specific harmonic frequencies causing the failure. Doing this triage with a near-field probe before committing to a shielding or gasket strategy avoids over-specifying (or under-specifying) can coverage and absorber placement later at full compliance test, when changes are far more expensive.
→ Read the original at In Compliance Magazine
Source: Kenneth Wyatt, "EMC Bench Notes: Interpreting Emissions Using a Near-Field Probe", In Compliance Magazine, 2026-06-11.