Receiver Sensitivity

a radio receiver’s capability of receiving weak signals or a quantitative criterion of that capability. In many cases, the criterion is defined as the minimum level, at a receiving antenna, of a radio signal at which the useful information contained in the signal may be reproduced with the required quality, for example, with sufficient loudness or adequate picture contrast. The signal level may be either the electromotive force (usually expressed in millivolts or in microvolts) induced by the signal in the antenna or the field strength (expressed in millivolts per m) near the antenna.

In the simplest radio receivers, sensitivity depends mainly on the extent to which signals are amplified. As the gain is increased, normal reproduction of information may be achieved for weaker radio signals; that is, the receiver sensitivity is raised. However, in complicated radio receivers, such as communications receivers, the method of raising the receiver sensitivity by increasing the gain is of no value, since the strength of the useful radio signals may turn out to be comparable to the intensity of external radio interference. Such interference acts on the antenna at the same time as the useful signals and distorts the information received. In this case, the ultimate sensitivity of a receiver is said to be interference-limited; such sensitivity not only is a receiver parameter but also depends on external factors.

Under the most favorable conditions—mainly in the reception of metric waves or waves of shorter wavelength and especially in space communications—external interference is weak and the intrinsic random noise (seeFLUCTUATIONS, ELECTRICAL) of a receiver itself becomes the main factor that limits receiver sensitivity. Under normal operating conditions, the intrinsic random noise of a receiver has a constant level. Therefore, noise-limited sensitivity is a completely specified parameter. In this case, the random-noise level, which is characterized by a noise factor and a noise temperature, is often used directly as a measure of receiver sensitivity. (See alsoTHRESHOLD SIGNAL.)