General
The noise figure is the difference in decibels (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 T0 (usually 290 K). The noise power from a simple load is equal to k T B, where k is Boltzmann's constant, T is the absolute temperature of the load (for example a resistor), and B is the measurement bandwidth.
This makes the noise figure a useful figure of merit for terrestrial systems where the antenna effective temperature is usually near the standard 290 K. In this case, one receiver with a noise figure say 2 dB better than another, will have an output signal to noise ratio that is about 2 dB better than the other. However, in the case of satellite communications systems, where the receiver antenna is pointed out into cold space, the antenna effective temperature is often colder than 290 K. In these cases a 2 dB improvement in receiver noise figure will result in more than a 2 dB improvement in the output signal to noise ratio. For this reason, the related figure of effective noise temperature is therefore often used instead of the noise figure for characterizing satellite-communication receivers and low noise amplifiers.
In heterodyne systems, output noise power includes spurious contributions from image-frequency transformation, but the portion attributable to thermal noise in the input termination at standard noise temperature includes only that which appears in the output via the principal frequency transformation of the system and excludes that which appears via the image frequency transformation.
Read more about this topic: Noise Figure
Famous quotes containing the word general:
“They make a great ado nowadays about hard times; but I think that ... this general failure, both private and public, is rather occasion for rejoicing, as reminding us whom we have at the helm,—that justice is always done. If our merchants did not most of them fail, and the banks too, my faith in the old laws of the world would be staggered.”
—Henry David Thoreau (1817–1862)
“We do not need to minimize the poverty of the ghetto or the suffering inflicted by whites on blacks in order to see that the increasingly dangerous and unpredictable conditions of middle- class life have given rise to similar strategies for survival. Indeed the attraction of black culture for disaffected whites suggests that black culture now speaks to a general condition.”
—Christopher Lasch (b. 1932)
“Why not draft executive and management brains to prepare and produce the equipment the $21-a-month draftee must use and forget this dollar-a-year tommyrot? Would we send an army into the field under a dollar-a-year General who had to be home Mondays, Wednesdays and Fridays?”
—Lyndon Baines Johnson (1908–1973)