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Most of the current radio modems works in frequency modulation with synthetized oscillators. In this kind of oscillators, the frequency command is realized by a vericap diode on which is overlayed the modulation. A "zero" induces a shift of -Fm, and a "one" induces a shift of +Fm. During a long sequence of "zero" and "one" level, the locking loop tends to catch the shift due to the modulation. To avoid this undesirable phenomenon, one of the used techniques (FMO or Manchester) consists to cut these levels into a clear alternation of 0 and 1 so that the average signal value is null. In this way, what ever is the content of the modulation, the frequecy carrier doesn't vary.
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