dBm to dBu Conversion Calculator

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This tool converts from dBm to dBu.

dBm is deciBel referenced to 1 milliwatt. dBu is deciBel unloaded. Both are absolute values and can be converted to actual voltage values.

๐Ÿ” dBu to dBm

Formula

dBu = dBm – 27.78 + 10*Log10(Z)

where Z is the impedance

If the power is specified in Watt, convert the number to dBm first.

Example Calculations

  • A value of 30 dBm (1 Watt) is equivalent to 19.2 dBu when the impedance is 50 ohm.
  • 30 dBm is equal to 30 dBu when the impedance is 600 ohm.

dBm and dBu are numerically equal when the source impedance is 600 ohm.

Background

What is dBu?

dBu is a unit of measurement used to quantify the voltage level of an audio or electrical signal, referenced to a specific standard voltage. It is commonly used in audio engineering and telecommunications to describe signal levels in decibels.

Definition

The unit dBu is derived from:

  • dB: Decibel, a logarithmic unit that represents a ratio.
  • u: Refers to an unloaded circuit, meaning no impedance is specified (typically 600 ohms in older audio systems).

The reference voltage for dBu is 0.7746 volts RMS. This value was chosen because it corresponds to 1 milliwatt of power in a 600-ohm impedance system, a standard in early telecommunication systems.

dBu is the RMS voltage relative to 0.7746 Volt.

1 VRMS is therefore equivalent to 20*Log10(1/0.7746) = 2.218 dBu.

The inverse relationship or conversion from dBu to VRMS is defined by the formula VRMS = 10(dBu / 20) x 0.7746.

Notes:

  • dBu is different from dBV which is the RMS voltage relative to 1 Volt.
  • dBu is also not the same as dBuV or dBยตV which is the RMS voltage relative to 1 microvolt.

Where does the number 0.7746 come from?

It is the value of voltage that dissipates 1 milliwatt (mW) into a 600 ohm load. Reference: Wikipedia article on dB Voltage

Image Credit: Omegatron

V = โˆš(600ฮฉ * 0.001W) Volt = 0.7746 V

dBu was originally dBv, however it was changed to what it is today to avoid confusion with dBV. Wikipedia Reference.

What is dBm?

dBm stands for deciBel referenced to 1 milliwatt. It is an absolute unit of power. The following formula is used to convert Watt to dBm:

PdBm = 10*log10(1000*PW)

where

  • PW is the power expressed in watt.
  • PdBm is the power expressed in dBm