Noise occurs naturally in any active device or circuit, and limits the minimum levels of useful signals. With a cell phone, for example, it can interfere with a weak signal, and interrupt a call. Therefore, it is important to design circuits to minimize the effects of noise. To do this, the noise must be quantified and measured in the form of noise parameters, comprised of Fmin, Gamma,opt (mag and phase), and Rn.
An impedance tuner is often used in conjunction with a noise figure analyzer (or alternate noise measurement instrument) to measure the noise figure, or noise power, as a function of source impedance presented to the DUT, from which the Noise Parameters are extracted.
Note, the term Noise Figure is a commonly referenced parameter when discussing LNAs, and most often refers to the 50ohm noise contribution of a device. The 50ohm Noise Figure of a device can be directly measured using the Noise Parameter system, or extrapolated from the Noise Figure contours. Direct measurement is achieved by using an impedance tuner to present exactly 50ohm to the DUT and measuring the associated noise figure (note, the tuner can correct for the non-50ohm system impedance normally presented without a tuner). Noise Figure extrapolation is a standard function within a noise parameter measurement system and uses mathematically determined contours to calculate the expected Noise Figure contribution at 50ohm.