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FM using the 555

Discussion in 'General Electronics Discussion' started by jesster, Dec 19, 2013.

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  1. jesster


    Dec 19, 2013
    Hi, I'm trying to build a FM circuit from discrete components using the LM555 as an oscillator . I get how to modulate the input wave with the 555, but how would you demodulate/ get a modulated sine wave from the square output? My first idea was a sharp low pass filter to remove all the odd harmonics, but the filter would depend on the frequency--which is changing, right?
    Does anyone have any ideas?

    Thank you very much! :)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    frequency modulation or pulse width modulation?

    I'll assume you are modulating the output of the 555 to produce a PWM equivalent of a sine wave. Presumably also, the 555's frequency is much higher than that of your sine wave.

    To get the sine wave back, you need to filter the output with a low pass filter having a it's 3db point above that of the sine wave, and below that of the 555.

    Depending on how smooth you want the output, and the relative frequencies, you may need a more complex filter (typically one with a sharper cut off)
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Another assumption is that the OP wants the FM signal as a frequency moduated sine wave. The 555 gives a frequency modulated square wave. I think the use of the word demodulate in the OPs post is not appropriate.

    The bandwidth of a frequency modulated signal is approx. 2*fm (fm being the highest frequency component of the modulating signal). Therefore if the frequencies of the modulating signal and the carrier signal are sufficiently far apart, a low pass filter with a corner frequency at or near fcarrier+fm can suppress the overtones of the square signal. The change in frequeny of the modulated carrier is then small compared to the absolute frequency of the unmodulated carrier, therefore the filter's influence on the carrier's fundamental (sine) is smal, too.
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