Selectivity

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Selectivity (also adjacent channel selectivity ) refers to the ability of a receiving apparatus , a desired station by a in the frequency band to separate closely spaced channels.

The most important variable is the bandwidth between the upper and lower limit frequency . Other influencing factors are:

Preselector

Preselectors (from Latin pre , vor 'and seligere , select', often also 'preselector' as an incorrect translation of English preselector ) are intended to isolate the useful signal from neighboring interfering signals.

In modern receivers, the selectivity is determined by filters of various types in the intermediate frequency "far ahead", i.e. as directly behind the mixer stage as possible , in order not to burden the following stages with signals that are outside the desired frequency segment.

Some broadband receiver concepts, u. a. RACAL, however, work with mixer stages directly as an input stage and without further preselection, whereby the mixer may be exposed to very high field strengths outside the desired range. This only works satisfactorily with strong useful signals, an antenna that is as selective as possible and extremely resilient, thus highly linear, complex mixers. If the reception of weak useful signals is desired despite other, strong incoming transmitters, a receiver with sufficient large-signal strength (see also receiver dynamics , intermodulation , cross modulation ) is required in order to reduce the effects of interference.

Amplifying pre-stages that work without selection exacerbate these problems and often form the basis for mixer stages that are overloaded by total signal power.

This can be remedied by a preliminary stage or selective preselectors that provide little or no amplification or suppress the undesired signals as best as possible with the help of selective filters such as band pass filters , low or high pass filters , suction circuits or other suitable screening means. They can be integrated into the receiver or connected as independent devices between antenna and receiver.

AFC

With a high degree of selectivity, the stability of the frequency-determining components is particularly important. Since temperature and aging are unavoidable, the receivers are now equipped with an AFC circuit .

literature

  • Ralf Rudersdorfer, with the collaboration of Ulrich Graf and Hans Zahnd: Radio receiver compendium - understanding how it works, areas of application and international assignments, determining and interpreting parameters, optimizing receiving systems . 1st edition. Elektor International Media BV, Aachen 2010, ISBN 978-3-89576-224-6 .
  • Martin Meyer: communication technology. Modern messaging concepts. 2nd Edition. Friedrich Vieweg & Sohn Verlagsgesellschaft, Wiesbaden 2002, ISBN 3-528-13865-3 .
  • EH Dennhardt: Guide to radio interference suppression . Published by Julius Springer, Berlin 1935.
  • H. Meinke, FW Gundlach: Pocket book of high frequency technology. Volume 3: Systems . 5th edition. Springer Verlag, Berlin / Heidelberg 1992, ISBN 978-3-540-54716-7 .

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