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 an other, 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.