Four Key Facts About the Decibel
There are many assumptions about dB that together can make it difficult to understand the term without being explicitly stated. Here are the four essential facts:
1
dB is one tenth of a Bel. The prefix "deci" means one tenth — so a decibel is one tenth of the unit called the Bel.
2
It is a unit of power. Even when used to represent other units such as voltage or current, the ratio is always that for power.
3
It is a comparison on a logarithmic scale. We must take the log of the ratio of powers being compared, then multiply by 10 (because dB is one tenth of a Bel).
4
Derived units reference a fixed physical quantity. When applied to voltage, current, field strength, etc., the ratio comparison is with respect to a fixed physical unit such as volts, amps, or V/m.
Definition and the Core Equation
Generally, dB refers to the log of a ratio of power of two quantities. It is most commonly used for gain of an amplifier, such as output power (Po) with reference to input power (Pi):
Gain = A = 10 log (Po / Pi) dB — Eqn. 1
It can also be used to represent an increase or decrease in noise levels. If using this for voltage comparison, the assumption is that the load impedance Z remains constant, so:
A = 10 log (V2²/V1²) = 20 log (V2/V1)
because: log(X²) = 2 log(X)
In the EMC/EMI test and measurement field, dB is also used for current, E-field (electric field), and H-field (magnetic field). Since in each case the power is proportional to the square of the voltage, current, or field quantity, we multiply by 20 (not 10) for voltage, current, and field quantities. For power ratios directly, multiply by 10 as in Eqn. 1.
Such comparisons are just ratios — therefore they have no units such as voltage, current, etc., only dB. So what is meant by terms like "dB microvolt"? The ratio is with respect to microvolt as the reference quantity.
Microvolts in dB (dBµV)
When the units measured are with respect to a fixed quantity such as a microvolt, the ratio is referred to as dB microvolt (dBµV). The reference unit for the ratio is microvolt. As an example, measuring 100 microvolts:
20 log (100 / 1) = 20 × 2 = 40 dBµV
The reference can be millivolt, volt, or kilovolt. For the E-field it could be µV/meter, mV/meter, or V/meter. However, most EMI testing specifications — CISPR for example — used for non-military testing applications such as information technology, multimedia equipment, medical devices, and household appliances, have applicable limits specified in dBµV/m for E-field values and dBµV for voltage emissions. These units are used because the values are so small — it is easier to compare to microvolts. Part 2 explains why we express units in dB.
The same concept applies to current or H-field measurements. Consider 10 Amps:
20 log (10/1) = 20 × 1 = 20 dBA
20 log (10,000,000/1) = 20 × 7 = 140 dBµA
These units are used when measuring magnetic field emissions as in MIL-STD-461 or DO-160 requirements.
