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The noise figure is the difference in decibel (dB) between the noise output of the actual receiver to the noise output of an "ideal" receiver with the same overall gain and bandwidth when the receivers are connected to matched sources at the standard noise temperature T 0 (usually 290 K).
One definition of signal-to-noise ratio is the ratio of the power of a signal (meaningful input) to the power of background noise (meaningless or unwanted input): =, where P is average power.
Different types of noise are generated by different devices and different processes. Thermal noise is unavoidable at non-zero temperature (see fluctuation-dissipation theorem), while other types depend mostly on device type (such as shot noise, [1] [3] which needs a steep potential barrier) or manufacturing quality and semiconductor defects, such as conductance fluctuations, including 1/f noise.
For thermal noise, its spectral density is given by N 0 = kT, where k is the Boltzmann constant in joules per kelvin (J/K), and T is the receiver system noise temperature in kelvins. The noise amplitude spectral density is the square root of the noise power spectral density, and is given in units such as volts per square root of hertz, V / H z ...
Thus, receivers with a narrow bandwidth have a higher sensitivity than receivers with a large bandwidth and input noise can be decreased by cooling the receiver input stage. A noise diode is a device which has a defined excess noise ratio (ENR). When the diode is off (unpowered) the noise from it will be thermal noise defined by the above formula.
In telecommunications, the term noise power has the following meanings: The measured total noise in a given bandwidth at the input or output of a device when the signal is not present; the integral of noise spectral density over the bandwidth; The power generated by a random electromagnetic process.
Noise is an undesirable but inevitable product of the electronic devices and components; also, much noise results from intentional economies of manufacture and design time. The metric for noise performance of a circuit is noise figure or noise factor. Noise figure is a comparison between the output signal to noise ratio and the thermal noise of ...
A signal is applied to the input such that the output increases by 12 dB. The level of the signal needed to produce this is noted. In this case, it was found to be 0.25 microvolts. According to the radio designer, intelligible speech can be detected 12 dB above the receiver's noise floor (noise and distortion).