Noise in RF Systems
Noise ultimately limits the sensitivity of any RF receiver. Understanding noise sources, how they combine in a cascaded system, and how to minimise their contribution is fundamental to receiver design.
Thermal (Johnson-Nyquist) Noise
Any resistor at temperature T generates noise power due to thermal agitation of electrons. The available noise power in bandwidth B is:
At room temperature (290 K): \(kT = -174\) dBm/Hz. This is the noise floor of any passive system at room temperature.
Noise Figure
Noise Figure (NF) is the degradation of SNR through a component, referenced to a 290 K source:
A perfect noiseless amplifier has NF = 0 dB. A 50 Ω resistive attenuator of X dB has NF = X dB.
Friis Formula for Cascaded Systems
The first stage dominates. This is why an LNA with low NF and high gain at the front of a receive chain is critical. Lossy elements (cables, switches) before the LNA directly add to system NF.
System Noise Temperature
Equivalent noise temperature \(T_e = (F-1) \times 290\) K. Useful for low-noise systems such as radio telescopes and satellite receivers where temperatures of a few kelvin are achieved.