Editing Noise temperature (section)

==System noise temperature==
An RF receiver system is typically made up of an [[Antenna (radio)|antenna]] and a [[Receiver (radio)|receiver]], and the transmission line(s) that connect the two together.  Each of these is a source of [[Additive white Gaussian noise|additive noise]].  The additive noise in a receiving system can be of thermal origin ([[Johnson–Nyquist noise|thermal noise]]) or can be from other external or internal noise-generating processes.  The contributions of all noise sources are typically lumped together and regarded as a level of thermal noise.  The noise power spectral density generated by any source (<math> P / B</math>) can be described by assigning to the noise a temperature <math>T</math> as defined above:<ref>Skolnik, Merrill I., Radar Handbook (2nd Edition).  McGraw-Hill, 1990.  {{ISBN|978-0-07-057913-2}}</ref>

:<math>T = \frac{P}{B} \cdot \frac{1}{k_\text{B}}</math>

In an RF receiver, the overall system noise temperature <math>T_S</math> equals the sum of the effective noise temperature of the receiver and transmission lines and that of the antenna.<ref>{{cite book |last1=Skolnik |first1=Merrill I. |title=Introduction to Radar Systems |date=2001 |publisher=McGraw-Hill |location=Boston |isbn=0-07-290980-3 |pages=731–732 |edition=Third}}</ref>

:<math>T_{S} = T_{A} + T_{E}</math>

The [[antenna noise temperature]] <math>T_{A}</math> gives the noise power seen at the output of the antenna.  The composite noise temperature of the receiver and transmission line losses <math>T_{E}</math> represents the noise contribution of the rest of the receiver system.  It is calculated as the effective noise that would be present at the antenna input terminals if the receiver system were perfect and created no noise.  In other words, it is a cascaded system of amplifiers and losses where the internal noise temperatures are referred to the antenna input terminals.  Thus, the summation of these two noise temperatures represents the noise input to a "perfect" receiver system.