Receiver  specifications

Any communications receiver, analog or digital, audio or video, should do certain basic things properly.

Sensitivity
The sensitivity of the receiver is the ability to recover weak signals and process them into the readable data. The most common method to express receiver sensitivity is to state number of alternating current signal microvolts at antenna, required to produce a given signal-to-noise  (S/N) ratio. At times, the signal-plus-noise-to-noise ratio, which is denoted by S+N/N, is given.

When you look at specifications table for the radio receiver, you might see, for instance, better than 0.3   V for 10 dB S/N. This means that the signal of 0.3   V or less, at antenna terminals, will result in the S/N ratio of 10 dB. This figure is given as an example and not as any sort of standard separating good from bad. Technological advancements are always improving sensitivity figures for communications receivers. Besides that, the poor figure for one application, or on one frequency band, can be good for some other intended use, or on some of the other frequency band.The front end, or 1st RF amplifier stage, of a receiver is most significant stage with regard to sensitivity. Sensitivity is related directly to the gain of this stage; but amount of noise it generates is more significant. A good front end should generate the best possible S/N or S+N/N ratio at its output. All the subsequent stages will amplify front-end noise output as well as front-end signal output.

Selectivity
Selectivity is ability of the receiver to respond to the desired signal, but not to the undesired ones. This means that a receiver should have a frequency window within which it is sensitive, but outside it is not at all sensitive. This window is established by a preselector in early RF amplification stages of receiver and is honed to precision by the narrowband filter in the later amplifier stage.
Assume that you want to receive an LSB signal the suppressed-carrier frequency of which is 3.885 MHz. The signal information is contained in the band from 3.882 to 3.885 MHz, a span of 3 kHz. The preselector makes receiver sensitive in the range of around plus or minus 10 % of the signal frequency; other frequencies can be attenuated. This reduces chance for a strong, out-of-band signal to impair performance of receiver. The narrowband filter responds to the frequencies 3.882 to 3.885 MHz, such that signals in adjacent channels are rejected. The better this filter works the better adjacent channel rejection.

Dynamic range
The signals at the receiver input vary over the several orders of magnitude. If you are listening to weak signal and a strong one comes on same frequency, you do not want your eardrums damaged! But, you want the receiver to work well if a strong signal appears at the frequency near (but not on top of) the weak one. Dynamic range is ability of the receiver to maintain a fairly constant output and to keep its sensitivity in presence of signals ranging from very weak to extremely strong. Dynamic range can be specified in decibels and is characteristically over 100 dB.

Noise figure
All the RF amplifier circuits generate wideband noise. The internal noise a receiver produces, better the S/N ratio. This becomes more important as the frequency increases. It is paramount at high frequencies and above (above 30 MHz).The noise figure (NF) of a receiver can be specified in various different ways. The lower the noise figure, better the sensitivity. You may see a specification such as NF: 6 dB or better.

Noise figure depends on type of active amplifier device used in front end of a receiver. The Gallium-arsenide field-effect transistors (GaAsFETs) are well known for low levels of noise they generate, even at high frequencies. Other types of FETs are used at the lower frequencies. Bipolar transistors tend to be noisy.