120 Ohms at 100 MHz, 1.8 A — the supply-line decoupling gate
The BLM15EX121SN1D is a single-line ferrite bead from Murata's EMIFIL® BLM15 series, packaged in 0402 (1005 metric) for surface-mount assembly. Its 120 Ohms impedance at 100 MHz targets noise suppression on power and signal lines where the DC resistance (75 mOhm max) and current rating (1.8 A) limit the voltage drop and thermal rise in the trace.
1.8 A continuous, 75 mOhm DCR — the voltage-budget check
Rated for 1.8 A continuous current, the 75 mOhm max DCR produces a 135 mV voltage drop at full load. The operating temperature range spans -55 °C to 125 °C, covering industrial and automotive underhood environments. The impedance curve shifts with temperature; at 125 °C the ferrite's permeability drops, reducing the effective suppression at the target frequency by roughly 20 % — a margin to budget for if the bead is the sole filter element in a high-temperature location.