Measuring
dBm converter: level, power, voltage
For spectrum analyzers, signal generators and oscilloscopes: a value in dBm, mW, W, Vrms, Vpp or dBµV is converted into all the others, at a selectable characteristic impedance.
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Abbreviations and symbols on this page (6)
- dBm
- Power on a logarithmic scale, referenced to 1 mW: 0 dBm = 1 mW, +10 dBm = 10 mW, +30 dBm = 1 W, −30 dBm = 1 µW.
- dBW
- Power referenced to 1 W: dBW = dBm − 30.
- dBµV
- Voltage referenced to 1 µV: 20 · log10(U ÷ 1 µV). Into 50 Ω, dBµV ≈ dBm + 107.
- Vrms (RMS)
- RMS value of an AC voltage; for a sine wave = peak value ÷ √2.
- Vpp
- Peak-to-peak voltage: from the lowest to the highest point; for a sine wave = 2 √2 × RMS value. This is what an oscilloscope shows.
- 50 Ω
- Common characteristic impedance in radio and test equipment; levels in dBm usually refer to 50 Ω.
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dBm
Ω
| dBm | 0 |
| dBW | -30 |
| Power | 1 mW |
| RMS voltage | 223.6 mV |
| Peak voltage | 316.2 mV |
| Peak-to-peak | 632.5 mV |
| dBµV | 106.99 |
| Current | 4.472 mA |
Formula
P(mW) = 10^(dBm ÷ 10); dBm = 10 · log10(P ÷ 1 mW); dBW = dBm − 30. Vrms = √(P · R), Vpeak = √2 · Vrms, Vpp = 2 √2 · Vrms. dBµV = 20 · log10(Vrms ÷ 1 µV); at 50 Ω ≈ dBm + 107.
Example
0 dBm at 50 Ω = 1 mW = 0.224 V rms = 0.632 V pp = 107 dBµV. +10 dBm = 10 mW = 0.707 V rms = 2 V pp. +30 dBm = 1 W.
Notes
- The voltages apply to sine waves. For other waveforms, peak and RMS value are related differently.
- Spectrum analyzers and generators are usually designed for 50 Ω; a high-impedance probe shows double the voltage on an open line.
- Levels add in dB: a 20 dB attenuator turns +10 dBm into −10 dBm.
A calculation aid for planning. The result does not replace a measurement on your own build.