Also known as: decibel, dB
The decibel (dB) is a logarithmic unit expressing the ratio between two power levels: dB = 10·log₁₀(P₁/P₂).1 Radio relies on it because signal powers span an enormous range — from watts at a transmitter to attowatts at a receiver — and because in the log domain the multiplications of gain and loss along a chain collapse into simple addition.
How it works
The decibel is one tenth of a bel, a unit named for Alexander Graham Bell and rooted in early telephone loss measurements. The factor of ten in the definition converts bels to the more convenient decibel.2 Because the logarithm of a product is the sum of the logarithms, a chain of components that multiply power — an antenna, cable, amplifier, filter — can be tallied by adding their dB figures instead of multiplying raw ratios.
Two anchors cover most mental arithmetic: +3 dB ≈ double the power, and +10 dB = ten times. From these you can reconstruct almost any value: +6 dB is ×4, +20 dB is ×100, −3 dB is half. Negative decibels mean the ratio is less than one, i.e. a loss.
A crucial subtlety is power versus amplitude (field) quantities. The 10·log₁₀ form applies to power. When the quantity is a voltage, current, or field strength, power goes as the square of amplitude, so the same ratio is written dB = 20·log₁₀(V₁/V₂). The factor of 20 is not a different decibel — it falls straight out of log(V²) = 2·log(V). Getting this wrong is a common source of 2× errors: a voltage that doubles is +6 dB, not +3 dB.
The decibel is inherently relative — it needs a reference. Attach a reference and it becomes an absolute unit: dBm references one milliwatt, dBFS references digital full scale, dBW references one watt, and dBc references a carrier. The suffix names the reference; the “dB” part is always the same logarithm.
In practice
Almost every RF budget is kept in decibels. A link budget starts with transmit power in dBm, adds antenna gains, subtracts path loss and cable attenuation, and lands on a received power that is compared against receiver sensitivity — all by adding and subtracting. Working the same sum with raw watt ratios would mean multiplying numbers that differ by twelve orders of magnitude.
Decibels also let a single meter span that range. A spectrum display marked in dB shows a −120 dBm noise floor and a −20 dBm strong signal on one screen; a linear scale could not resolve both. The same logic drives figures like dynamic range and signal-to-noise ratio, which are naturally expressed as differences in dB.
Relevance to SDR
Every level in an SDR toolchain is a decibel quantity. Absolute RF power is dBm; digital headroom is dBFS; antenna gain, path loss, noise figure, and SNR are all reported in dB. GopherTrunk surfaces demodulator SNR and error metrics in decibels precisely because they add cleanly along the receive chain and compress a huge range into readable numbers.
Sources
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Decibel — Wikipedia, definition of the logarithmic ratio unit, the power/field distinction, and reference suffixes. ↩
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NIST Special Publication 811 — U.S. National Institute of Standards and Technology, guide to SI units covering correct use of the decibel and logarithmic quantities. ↩