Minimum detectable signal
Encyclopedia
Minimum detectable signal (MDS) in a radio receiver is the smallest signal power that can be received at its input, processed by its conversion chain and demodulated by the receiver, resulting in usable information at the demodulator output.
In this equation:
-174dBm
is the available noise power kT in a bandwidth B = 1Hz, expressed in dBm
. T is the system temperature in kelvin
s and K is Boltzmann's constant (1.38×10−23 joule
s per kelvin
= −228 dBW/(K·Hz)). If the system temperature and bandwidth is 290 K and 1 Hz, then the effective noise power available in 1 Hz bandwidth from a source is −174 dBm (174 dB below the one milliwatt level taken as reference). This is the system's noise floor at its input. Any signal of lower power may not be discerned from noise, except for specific situations.
1 Hz noise floor: calculating the noise power available in a one hertz bandwidth at a temperature of T = 290 K defines a figure from which all other values can be obtained (different bandwidths, temperatures). 1 Hz noise floor equates to a noise power of −174 dBm so a 1 kHz bandwidth would generate −174 + 10 log10(1 kHz) = −144dBm of noise power (the noise is thermal noise, Johnson noise).
(NF) is noise factor (F) expressed in decibels. F is the ratio of the input signal-to-noise-ratio (SNRi) to the output signal-to-noise-ratio (SNRo). F quantifies how much the signal degrades with respect to the noise because of the presence of a noisy network. A noiseless amplifier has a noise factor F=1, so the noise figure for that amplifier is NF=0 dB: a noiseless amplifier does not degrade the signal to noise ratio as both signal and noise propagate through the network.
If the bandwidth in which the information signal is measured turns out not to be 1 Hz wide, then the term 10 log10(bandwidth) allows for the additional noise power present in the wider detection bandwidth.
Equation
In this equation:
-174dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
is the available noise power kT in a bandwidth B = 1Hz, expressed in dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
. T is the system temperature in kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s and K is Boltzmann's constant (1.38×10−23 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s per kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
= −228 dBW/(K·Hz)). If the system temperature and bandwidth is 290 K and 1 Hz, then the effective noise power available in 1 Hz bandwidth from a source is −174 dBm (174 dB below the one milliwatt level taken as reference). This is the system's noise floor at its input. Any signal of lower power may not be discerned from noise, except for specific situations.
1 Hz noise floor: calculating the noise power available in a one hertz bandwidth at a temperature of T = 290 K defines a figure from which all other values can be obtained (different bandwidths, temperatures). 1 Hz noise floor equates to a noise power of −174 dBm so a 1 kHz bandwidth would generate −174 + 10 log10(1 kHz) = −144dBm of noise power (the noise is thermal noise, Johnson noise).
Noise figure and noise factor
Noise figureNoise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
(NF) is noise factor (F) expressed in decibels. F is the ratio of the input signal-to-noise-ratio (SNRi) to the output signal-to-noise-ratio (SNRo). F quantifies how much the signal degrades with respect to the noise because of the presence of a noisy network. A noiseless amplifier has a noise factor F=1, so the noise figure for that amplifier is NF=0 dB: a noiseless amplifier does not degrade the signal to noise ratio as both signal and noise propagate through the network.
If the bandwidth in which the information signal is measured turns out not to be 1 Hz wide, then the term 10 log10(bandwidth) allows for the additional noise power present in the wider detection bandwidth.