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# Decibels Relative to c、mv、d、i&r

Date:2019/10/10 15:55:17 Hits:

decibels relative to carrier (dBc)
dBc (decibels relative to carrier) is a measure of the strength of an instantaneous signal at radio frequency . The dBc increment is based on the decibel , a logarithm ic measure of relative signal strength. Suppose a signal has an unmodulated-carrier power of P watts and a modulation signal power of P watts at some specified instant in time. Then the instantaneous modulated signal strength in dBc, symbolized S dBc ,is:

S dBc = 10 log 10 P / P

Signal strength in dBc can also be calculated in terms of the effective signal voltage if the impedance remains constant. Suppose a signal has an unmodulated-carrier root-mean-square ( rms ) voltage of V volts and a modulated rms signal voltage of V volts at the same impedance. Then the instantaneous modulated signal strength in dBc is:

S dBc = 20 log 10 V / V

If the dBc figure is positive, then the instantaneous modulated signal strength is greater than the unmodulated-carrier signal strength. If the dBc figure is negative, then the instantaneous modulated signal strength is less than the unmodulated-carrier signal strength.

decibels relative to one millivolt
dBmV (decibels relative to one millivolt) is a measure of the signal strength in wires and cables at RF and AF frequencies. A millivolt is 1/1000 of a volt (0.001 V or 10 -3 V). This unit is defined in terms of root-mean-square ( rms ) alternating current ( AC ) signal voltages in circuits in which the impedance is a pure resistance of some specified value.

The dBmV increment is based on the decibel , a logarithm ic measure of relative signal strength. Suppose a signal has an rms voltage of E mV. Then the signal strength in dBmV, symbolized S dBmV , is:

S dBmV = 20 log 10 E

A 1-mV signal across a pure resistance has a level of 0 dBmV. Signals weaker than 1 mV have negative dBmV values; signals stronger than 1 mV have positive dBmV values, assuming the impedance remains a pure resistance and remains constant.

decibels related to dipole antenna
dBd (decibels related to dipole antenna) is a measure of the gain of an antenna system relative to a dipole antenna at radio frequency.

The dBd specification is based on the decibel, a logarithmic measure of relative power. Suppose an antenna A produces an electromagnetic field of intensity IA microwatts per square meter (µW/m2) in its favored direction at a point located a defined distance away. Also, suppose a half-wave dipole antenna D produces an electromagnetic field of intensity ID µW/m2 in its favored direction at the same distance. Then the gain G of antenna A, in dBd, is:

G = 10 log10 (IA / ID) The expression dBi is used to define the gain of an antenna system relative to an isotropic radiator at radio frequencies . The symbol is an abbreviation for "decibels relative to isotropic."

The dBi specification is based on the decibel , a logarithm ic measure of relative power . Suppose an antenna A produces an electromagnetic field of intensity I A microwatts per square meter ( I A µW/m 2 ) in its favored direction at a point located some distance away. Also, suppose an isotropic antenna Q produces an electromagnetic field of intensity I Q µW/m 2 at the same distance. Then the gain G of antenna A , in dBi, is:

G = 10 log 10 ( I A / I Q )

A dipole antenna has a gain of 2.15 dBi. An isotropic radiator has a gain of 0 dBi. decibels relative to reference level
The expression dBr is used to define signal strength at RF and AF frequencies. The symbol is an abbreviation for "decibels relative to reference level."

The dBr increment is based on the decibel , a logarithm ic measure of relative signal strength. Suppose a reference signal has a power of P watts, and the signal being measured has a power of P watts. Then the relative signal strength in dBr, symbolized S dBr , is:

S dBr = 10 log 10 P / P

Signal strength in dBr can also be calculated in terms of the effective signal voltage if the impedance remains constant. Suppose a reference signal has a root-mean-square ( rms ) voltage of V volts and the signal being measured has an rms voltage of V volts at the same impedance. Then the relative signal strength in dBr is:

S dBr = 20 log 10 V / V

If the dBr figure is positive, then the measured signal is stronger than the reference signal. If the dBr figure is negative, then the measured signal is weaker than the reference signal.

A dipole antenna has a gain of 0 dBd. An isotropic radiator has a gain of -2.15 dB

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