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Two main qualities of any radio receiver either Amateur Band, Broadcast Band, Marine Band, Citizens Band, Business Band or Shortwave, etc. are its Sensitivity and Selectivity. Although there are many important specification measurements for any radio receiver the most important is its Sensitivity it is the key specification for any radio receiver.

 

Sensitivity:

 

 The radio receiver noise floor will determine its Sensitivity measured in microvolts for a given SINAD (Signal to Noise Ratio and Distortion). The receivers front end noise measurement either measured in signal to noise ratio dB’s (decibel’s), noise figure or noise factor, a radio receiver can use any noise measurement. The use of low noise semiconductors in the receivers front end circuit is crucial to providing the radio with a low noise floor. Semiconductors like GaAs (Gallium Arsenide) FETs or low noise BJT’s (Bipolar Junction Transistors) or any low noise semiconductor circuits are used. It is very important for any radio receiver that its front end be as low noise as possible as the later stages will amplify this noise and this noise will be audible from the speaker. The noise can be either man made or internal. For a Superheterodyne receiver the use of an R. F. Amplifier before the R. F. Converter (mixer and oscillator) will improve the receivers Sensitivity.

 

Selectibility: 

 

A radio receiver’s Selectability is a complex topic with many mathematical formulas and Filters to describe its operation. This article will describe Selectivity in simple basic easy to understand language without the mathematical formulas. Selectivity of any radio receiver is described as the ability to only receive the radio stations that the radio is tuned into while rejecting all other frequencies which can be the adjacent channel station, man made other unwanted frequencies. Rejecting unwanted frequencies will give you a good indication of the radios quality usually specified as adjacent channel rejection ratio. The ratio of a nearby stations frequency to the frequency that the radio is tuned into is the receivers Selectability. High Q-LC filters or other types of filters are used in the later stages of the radio as this will reject the unwanted frequencies or for Superheterodyne radios extra I. F. (Intermediate Frequency) amplifier stages are used or high Q-LC Filters, Ceramic Filters, Crystal Filters, (Mechanical Filters which are often used in Collins Radios) or any filter that will provide a narrow bandwidth can be used. Linking Filters together in a chain to obtain more then one pole can be used sometimes extra poles are inside the Filters. It is common that internal filters which can be 2-8 poles are used. The extra pole filter circuits are usually used in the I.F. stages.

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