7 Common Mistakes in dB Conversions

Most EMC data is expressed in dB, so a small conversion slip turns directly into an error in the test result. Here are seven that come up all the time.

1. Using 10·log₁₀ for voltage

Voltage, current and field are amplitude quantities and use 20·log₁₀. 10 mV is 20·log₁₀(10 mV / 1 µV) = 80 dBµV. Using 10·log₁₀ would give 40 dBµV, off by 40 dB.

2. Memorising the dBm ↔ dBµV offset for 50 Ω only

dBµV = dBm + 90 + 10·log₁₀(Z), so the offset is +107 at 50 Ω but about +108.75 at 75 Ω. A different system impedance means a different offset.

3. Applying the preamp gain with the wrong sign

The preamp enlarged the signal, so you subtract its gain to get the true field strength. Cable loss and attenuators are added. The most reliable check is that the converted result is identical with the preamp on and off.

4. Converting 3 m ↔ 10 m limits with the computed value

The computed correction is 20·log₁₀(10/3) ≈ 10.46 dB, but standards usually tabulate a rounded value (for example 10 dB). For compliance decisions always use the tabulated limit for the actual distance.

5. Adding absolute levels in dB

Do not compute "40 dBµV + 37 dBµV" as 77. The sum of two uncorrelated signals adds power (or squared voltage) and gives about 41.8 dBµV. Adding relative values (dB) is fine, but absolute levels are not: dBm + dB works, dBm + dBm does not.

6. Mixing V/m and dBµV/m

1 V/m = 120 dBµV/m and 10 V/m = 140 dBµV/m. Bring emission limits (for example 40 dBµV/m = 0.1 mV/m) and immunity levels (for example 3 V/m) into the same unit before comparing.

7. Adding uncertainties linearly

Independent uncertainty components combine as the root of the sum of squares (RSS). Do not add 0.8 dB and 0.6 dB to get 1.4 dB; compute √(0.8² + 0.6²) = 1.0 dB. Also convert ± limits to standard uncertainties with the divisor for the right distribution (√3 for rectangular) before combining.