Selectivity of a Radio Receiver

the ability of a radio receiver to differentiate between the desired signal and spurious electromagnetic oscillations of various origins that interfere with reception and to reject these undesired signals. The term “selectivity” is also applied to the radio-receiver parameter that quantitatively describes this ability.

The desired signal is selected on the basis of some characteristic it possesses. Depending on the characteristic used, such types as frequency selectivity, amplitude selectivity, phase selectivity, and time selectivity are distinguished. Frequency selectivity is the type most widely encountered, since artificial sources of radio signals are designed to operate at certain different frequencies (usually called carrier frequencies) and in certain ranges of wavelengths (in accordance with the Radio Regulations).

The selectivity of a receiver is evaluated by that relative intensity of a signal received from an external source, such as a radio station, for which the signal has a marked interfering effect on the reception of a chosen weak signal. Adjacent-channel selectivity and secondary-channel selectivity are usually considered. Adjacent-channel selectivity is the selectivity for interference occurring in the frequency channel that is closest to the carrier frequency. The secondary channels of interest in superheterodyne receivers are the image channel and the channel whose frequency is equal to the intermediate frequency. These selectivities are often too low, and as a result the desired signal is noticeably distorted by interference.