Другое: Radio receiver sensitivity

Radio receiver sensitivity
Receiver sensitivity is one of the key specifications of any radio. The two main requirements of any radio receiver are that it should be able to separate one station from another, i.e. selectivity, and signals should be amplified so that they can be brought to a sufficient level to be heard. As a result receiver designers battle with many elements to make sure that these requirements are fulfilled
A number of methods of measuring and specifying the sensitivity performance of radio receivers are used. Figures including signal to noise ratio, SINAD, noise factor and noise figure are used. These all use the fact that the limiting factor of the sensitivity of a radio receiver is not the level of amplification available, but the levels of noise that are present, whether they are generated within the radio receiver or outside it.
Noise
Today technology is such that there is little problem in being able to achieve very large levels of amplification within a radio receiver. This is not the limiting factor. In any receiving station the limiting factor is noise - weak signals are not limited by the actual signal level, but by the noise masks them out. This noise can come from a variety of sources. It can be picked up by the antenna or it can be generated within the radio receiver.
It is found that the level of noise that is picked up externally by a receiver from the antenna falls as the frequency increases. At HF and frequencies below this the combination of galactic, atmospheric and man-made noise is relatively high and this means that there is little point in making a receiver particularly sensitive. Normally radio receivers are designed such that the internally generated noise is much lower than any received noise, even for the quietest locations.
At frequencies above 30 MHz the levels of noise start to reach a point where the noise generated within the radio receiver becomes far more important. By improving the noise performance of the radio receiver, it becomes possible to detect much weaker signals.

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