220 Ohm @ 100 MHz — board-level noise suppression
The BLM15AX221SN1D: Part of Murata's EMIFIL® BLM15 series, this single-line ferrite bead delivers 220 Ohms impedance at 100 MHz — the frequency where most conducted EMI from switching regulators and digital clocks lands. At that impedance, it attenuates the noise peak by roughly 20 dB into a 50-Ohm system, enough to knock down a failing radiated-emissions margin without a board spin. Rated for 600 mA continuous with a maximum DC resistance of 180 mOhm. At full rated current, the self-heating from I²R loss is about 65 mW — negligible in a 0402 body, but the voltage drop across the bead (108 mV at 600 mA) matters if the downstream rail has tight regulation. Spans -55°C to +125°C, so it stays in the circuit through thermal cycling from cold-soak to under-hood ambient. The 0402 footprint (1.00 mm x 0.55 mm) fits dense layouts where a larger 0603 would crowd the bypass caps.