dBm ⇄ dBµV (50 Ω)
Convert between the RF power and EMC voltage dialects.
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The engineering
The RF world quotes power, the EMC world quotes microvolts at the receiver input; in 50 Ω they differ by exactly 106.99 dB. Your spectrum analyzer speaks dBm, the CISPR limit line speaks dBµV — this is the bilingual dictionary.
Only valid in 50 Ω. EMC work at other impedances (LISNs are 50 Ω, but field probes aren't) needs the long way around.
Where this math comes from
The split is institutional: receiver and field-strength people (CISPR, founded 1934 to referee radio interference) standardized on microvolts, while the microwave people standardized on milliwatts. Fifty-ohm coax — the postwar compromise between lowest-loss 77 Ω and highest-power 30 Ω — is what pins the two scales exactly 107 dB apart.
Every EMC test report is this card applied a few hundred times.
- 1934CISPR foundedInterference measurement standardizes on field-strength/µV language.
- 1946Coax industry50 Ω becomes the RF impedance — fixing the 107 dB offset.
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