Agilent Fundamentals of RF and Microwave

Modern receiving systems must often process very
weak signals, but the noise added by the system
components tends to obscure those very weak signals.
Sensitivity, bit error ratio (BER) and noise figure are
system parameters that characterize the ability to
process low-level signals. Of these parameters, noise
figure is unique in that it is suitable not only for
characterizing the entire system but also the system
components such as the pre-amplifier, mixer, and IF
amplifier that make up the system. By controlling the
noise figure and gain of system components, the designer
directly controls the noise figure of the overall system.
Once the noise figure is known, system sensitivity
can be easily estimated from system bandwidth. Noise
figure is often the key parameter that differentiates
one system from another, one amplifier from another,
and one transistor from another. Such widespread
application of noise figure specifications implies that
highly repeatable and accurate measurements between
suppliers and their customers are very important.