As was discussed in the introduction to this chapter, externally generated interference is the major source of noise in most measurement systems. If, however, external noise is sufficiently suppressed, the noise generated internally in all electronic equipment and component will predicate. Such internal noise therefore limits the ultimate measurement sensitivity that can be achieved. Most measurement systems are designed to maintain an internally generated noise level below the sensitivity at which the system's instrument are intended to operate. Therefore, the internal noise is not noticeable at the output. If operation is extended(NY amplification, for example) to lower signal levels, that is, down to the limit where the signal approaches the internally generated noise level, further sensitivity cannot be achieved without reducing the internal noise level