Threshold Signal

a signal at the input of a receiving device; its magnitude is the minimum that provides the required probability of registering a signal for a specified probability level of false registration, or operation caused by noise.

The concept of a “threshold signal” is used in analyzing the quality of systems of communication, sonar, radar, radiogoni-ometry, and optical location. The magnitude of a threshold signal can be evaluated in terms of energy, power or field intensity at the aperture of an antenna in radio-receiving devices, quantity of photons in optical receivers, current or voltage, or pressure or temperature at the input of relay systems. When applied to radio receivers and solid-state photodetectors, the threshold signal is often assumed to be equal to the optical density referred to the input of a receiver.

For a given noise level the power of the threshold signal can be as small as desired— for instance, in cases where it is possible to store the signal coherently and for an infinite length of time. In the optical range the magnitude of the threshold signal for a given observation period is sometimes determined not only by the external background noise and by the noise orginating in the receiver itself but also by the quantum fluctuations of the signal that is being registered. Current state of engineering (as of 1975) makes it possible to obtain threshold signals in the optical range that correspond to several dozens or even several units of quanta.