1 The Decibel
The decibel is defined as a ratio of two quantities, typically power, voltage, or current. In the field of EMC, the decibel must be understood. Here are the most important points:

Gain and Loss [dB]

Absolute Levels [dBm, dBµV, dBµA]

Unit Conversion [formulas, tables]
Gain [dB] and Loss [dB].
Let's have a look at the amplifier or damping network below.
The power, voltage, and current gain of this network can be expressed in [dB] as [1.1]:
If R1 and R2 are equal (typically 50Ω), then the following term is 0:
And we can write the following for power/voltage/current gain:
Points to remember when it comes to the calculation of gain and loss in decibel:

Amplification. If P2 is bigger than P1, the gain value in [dB] is positive. This means if there is amplification, the power gain in [dB] is positive.

Damping. If P2 is smaller than P1, the gain value in [dB] is negative. This means if there is a power loss, the power gain in [dB] is negative.

Cutoff Frequency. At the cutoff frequency, the output power (P2) is half the input power (P1). And the power/voltage/current gains are all 3dB.

Ratio to [dB]. If power increases by factor 2, the power/voltage/current gains increase by +3dB. If power increases by factor 10, the power/voltage/current gains increase by +10dB. To get an overview, have a look at our power/voltage/current ratios table below.
Absolute Levels [dBm, dBµV, dBµA].
The most common absolute power, voltage and current levels in EMC are [dBm], [dBµV] and [dBµA]. They are calculated like this [1.1]:
The most common absolute power, voltage and current levels in EMC are [dBm], [dBµV] and [dBµA]. They are calculated like this:
For example, a negative dBmvalue means that the power level is <1mW. 0dBm = 1mW and a dBmvalue bigger than 0 means that the power is higher than 1mW. The same for dBµV and dBµA: 0dBµV = 1µV (0dBµA = 1µA), a negative dBµVvalue means <1µV, and a positive dBµVvalue means >1µV.
Conversion formulas and tables.
Find below tables and formulas to convert between the different dBunits and from linear ratio values to dBvalues and vice versa. In addition, you can download an Excel sheet, which contains conversion calculations and more [1.2].