EMI/EMC Academy ~35 min Interactive

Reading the Spectrum — Signatures & Root-Cause Analysis

Every emission has a fingerprint. A comb of evenly spaced spikes is a clock; a broad mound that shifts with load is a switching supply; a lump that moves when you wiggle a cable is a resonance. Learn to read the plot and you've already localized the source.

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Learning objectives. Distinguish narrowband from broadband emissions; recognize a clock harmonic comb and use its spacing to identify the source clock; spot SMPS switching signatures and sidebands; identify cable/enclosure resonances; and map a measured peak back to a physical part of the product.

Narrowband energy sits at discrete frequencies — clocks, oscillators, CW carriers. It appears as sharp lines whose height barely changes when you narrow the resolution bandwidth. Broadband energy is spread — switching edges, arcing, motor brushes. Its displayed level drops as you reduce RBW (less bandwidth captures less of the spread energy). That RBW test is the fastest way to classify a peak.

Quick test: halve the RBW. A line that stays put is narrowband; a line that falls ~3–6 dB is broadband. This one trick guides your whole fix strategy.

Toggle each source on/off and watch how it stacks up against an illustrative Class B limit line. Note the shapes: the clock is a comb, the SMPS is a broad hash, the cable resonance is a single moving lump.

Illustrative spectrum (dBµV/m vs MHz). Values are for teaching, not a real limit line.
Clock harmonic comb. Evenly spaced spikes at N × f. The spacing equals the clock frequency, so a comb every 100 MHz points to a 100 MHz clock (or its source). Odd harmonics dominate for ~50% duty square waves.
SMPS switching. A cluster around the switching frequency and its harmonics, often with a broadband hash from fast FET edges and diode reverse recovery. Shifts when load or line changes; strongest near the hot loop.
Cable/enclosure resonance. A single broad hump whose center frequency corresponds to a physical length (λ/4 or λ/2). It moves or changes when you reroute the cable — the tell-tale of a resonant radiator.
Ambient. Stable carriers (FM 88–108 MHz, broadcast TV, cellular, 2.4/5 GHz Wi-Fi) that are present with the EUT off. Subtract them mentally, or tent the setup.
Common mistake: "fixing" an ambient. If a 98 MHz spike is still there with the EUT unpowered, no ferrite on your board will remove it — it's the local FM station.

Turn the plot into a physical location: read the frequency, classify (narrow/broad), infer the likely source from the signature, then confirm with a near-field probe or current clamp (Module 3). For example, a comb at multiples of 25 MHz that gets louder when you clamp the Ethernet cable → the PHY clock is coupling common-mode onto that cable.

Key takeaways. Shape tells source: combs are clocks, broad hash is switching, moving humps are resonances, stable carriers are ambient. Classify with the RBW test, then confirm with a probe. Next: fixing the source you just found.
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