Before discussing radio receivers, it is necessary to explain certain terms used to define their characteristics.
'Sensitivity' is the ability of a receiver to receive weak signals. The sensitivity of a receiver is normally quoted as being that level of signal which produces a standard ratio of signal-to-background noise, eg 1uV input to give not less than 10dB signal-to-noise ratio. This means that a 1uV input signal will give rise to level of received signal that is about three times the level of the noise present with the antenna removed and the receiver correctly terminated. The sensitivity of a communication receiver can usually be varied by means of an RF gain control.
'Selectivity' is the ability of a receiver to receive one signal and disregard others on adjacent frequencies. On crowded amateur bands it is necessary for stations to operate on frequencies very close to each other. In order to be able to receive the desired signal with the minimum amount of adjacent-channel interference, a receiver with a high selectivity must be used. Selectivity may be quoted, for example, as follows: 2.5kHz at 6dB down: 4.1kHz at 60dB down.
'Bandwidth'. A receiver with high selectivity is said to have a narrow bandwidth. In the above example, the receiver has a bandwidth of 2.5kHz at 6dB down.
'Frequency stability' is the ability of a receiver to remain tuned to the desired signal. If a receiver is not stable it is said to 'drift'. Stability is determined by the design and construction of the oscillator stage in a receiver.
'Dynamic range' is the range of input signal over which a receiver will function satisfactorily, ie it is the difference in level between the maximum signal the receiver will accept and the minimum signal which will give a usable output. The dynamic range is quoted in decibels (dB); in a good receiver it would be of the order of 90-105dB.
'Automatic gain control' (AGC) is the automatic control of the sensitivity of a receiver by the strength of the signal to which the receiver is tuned. For weak signals, the sensitivity needs to be high, but for strong signals a low sensitivity suffices. AGC is useful when there is fading on a signal; the sensitivity is varied in accordance with the signal to produce an almost constant audio output level.