(Last Updated On: February 10, 2020)
This is the Review Notes in Decibel (dB) as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.
Summary of Important Decibel (dB) Review Notes
The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, often power or intensity. One of these quantities is often a reference value, and in this case the decibel can be used to express the absolute level of the physical quantity, as in the case of sound pressure.
dBm or dBmW
dB(1 mW) — power measurement relative to 1 milliwatt. XdBm = XdBW + 30.
dB(1 W) — similar to dBm, except the reference level is 1 watt. 0 dBW = +30 dBm; −30
dBW = 0 dBm; XdBW= XdBm −30.
Note that the decibel has a different definition when applied to voltage (as contrasted with power). See the "Definitions" section above.
A schematic showing the relationship between dBu (the voltage source) and dBm (the power dissipated as heat by the 600 Ω resistor)
dB(1 VRMS) — voltage relative to 1 volt, regardless of impedance.
dBu or dBv
dB(0.775 VRMS) — voltage relative to 0.775 volts. Originally dBv, it was changed to dBu to avoid confusion with dBV. The "v" comes from "volt", while "u" comes from "unloaded". dBu can be used regardless of impedance, but is derived from a 600 Ω load dissipating 0 dBm (1 mW). Compare ambiguous use of dBu in radio engineering.
dB(1 mVRMS) — voltage relative to 1 millivolt, regardless of impedance. Widely used in cable television networks, where the nominal strength of a single TV signal at the receiver terminals is about 0 dBmV. Cable TV uses 75 Ωcoaxial cable, so 0 dBmV corresponds to −48.75 dBm or ~13 nW.
dBµV or dBuV
dB(1 µVRMS) — voltage relative to 1 microvolt. Widely used in television and aerial amplifier specifications. 60 dBµV = 0 dBmV.
dB (Sound Pressure Level) — for sound in air and other gases, relative to 20 micropascals (µPa) = 2×10−5 Pa, the quietest sound a human can hear. This is roughly the sound of a mosquito flying 3 metres away. This is often abbreviated to just "dB", which gives some the erroneous notion that "dB" is an absolute unit by itself. For sound in water and other liquids, a reference pressure of 1 µPa is used.
dB Sound Intensity Level— relative to 10−12 W/m2, which is roughly the threshold of human hearing in air.
dB Sound Power Level— relative to 10−12 W.
dB(A), dB(B), and dB(C)
These symbols are often used to denote the use of different weighting filters, used to approximate the human ear’s response to sound, although the measurement is still in dB (SPL). Other variations that may be seen are dBA or dBA. According to ANSI standards, the preferred usage is to write LA= x dB. Nevertheless, the units dBA and dB(A) are still commonly used as a shorthand for A-weighted measurements. Compare dBc, used in telecommunications.
dB HL or dB hearing level is used in Audiograms as a measure of hearing loss. The reference level varies with frequency according to a Minimum audibility curve as defined in ANSI and other standards, such that the resulting audiogram shows deviation from what is regarded as ‘normal’ hearing.
dB Q is sometimes used to denote weighted noise level, commonly using the ITU-R 468 noise weighting
dB(Z) – energy of reflectivity(weather radar), or the amount of transmitted power returned to the radar receiver. Values above 15-20 dBZ usually indicate falling precipitation.
Radio power, energy, and field strength
dBc — power relative to the power of the main carrier frequency; typically used to describe spurs, noise, channel crosstalk, and intermodal signals which may interfere with the carrier. Compare dB(C), used in acoustics.
dB(J) — energy relative to 1 joule. 1 joule = 1 watt per hertz,so power spectral density can be expressed in dBJ.
dB(mW) — power relative to 1 milliwatt.
dBµ or dBu
dB(µV/m) — electric field strength relative to 1 microvolt per meter. Compare the ambiguous use of dBu as a unit of voltage level.
dB(fW) — power relative to 1 femtowatt.
dB(W) — power relative to 1 watt.
dB(kW) — power relative to 1 kilowatt.
dB(dipole) — the forward gain of an antenna compared to a half-wave dipole antenna.
dBFS or dBfs
dB(full scale) — the amplitude of a signal (usually audio)compared to the maximum which a device can handle before clipping occurs. In digital systems, 0 dBFS (peak) would equal the highest level (number) the processor is capable of representing. Measured values are usually negative, since they should be less than the maximum.
dB(hertz) — bandwidth relative to 1 Hz. E.g., 20 dB-Hz corresponds to a bandwidth of 100 Hz. Commonly used in link budget calculations.
dB (isotropic) — the forward gain of an antenna compared to the hypothetical isotropic antenna, which uniformly distributes energy in all directions.
dB (isometric circular) — power measurement relative to a circularly polarized isometric antenna.
dB (overload) — the amplitude of a signal (usually audio) compared to the maximum which a device can handle before clipping occurs. Similar to dBFS, but also applicable to analog systems.
dB (relative) — simply a relative difference to something else, which is made apparent in context. The difference of a filter’s response to nominal levels, for instance.
dB above reference noise. See also dBrnC.
dB relative to carrier — in telecommunications, this indicates the relative levels of noise or sideband peak power, compared to the carrier power. Compare dBC, used in acoustics.
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