Can an FM station be broacast with an AM frequency??

Discussion in 'General Electronics Chat' started by mabsj2, May 4, 2008.

  1. mabsj2

    Thread Starter Member

    Apr 24, 2008
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    read some article and i have been thinking alot about this.

    Can an FM station be broadcast over an AM frequency like 600KHz and can an AM station be broadcast with an FM frequency like 99MHz??

    but there is one thing i discovered, in TV broadcasting, the Video part of the signal has its Amplitude modulated while the the Audio is Frequency modulated and yet the whole signal (audio and Video) is broadcast over an FM frequency.

    so does this mean that its possible to interchange the frequencies for any modulation or i am just mistaken!!!!!!
     
  2. Caveman

    Active Member

    Apr 15, 2008
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    Those frequencies are just what the FCC has basically stated that it is to be used for. You could theoretically transmit any type of modulation on any frequency. It may be illegal however.
     
  3. mabsj2

    Thread Starter Member

    Apr 24, 2008
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    Thanks men, so my suspicions are correct........
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Actually, the current FM radio broadcast band occupies part of the spectrum originally assigned to TV. The FM band is between TV channel 5 (low band VHF) & 6 (high band VHF).
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    In the US it is between channel 6 (82-88MHz) and channel 7 (174-180MHz.), at least until February 2009.

    http://www.ntia.doc.gov/osmhome/allochrt.pdf
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Oops. Until 2/09.
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Well yes and no.

    It's all to do with the frequency range of the signal you want to modulate onto the carrier.

    As a rough guide you require twice the max frequency in the modulating signal, for FM.

    Audio signals normally range from 20 hz to 20Khz, which would require a swing if 20KHz above and below the base frequency to transmit, for FM.

    Imagine the effect on a station operating at say 100kHZ carrier frequency (the bottom end of the long wave). The frequency modulated output would vary from 80Khz to 120Khz.

    Doing this would allow only 4/5 stations on the whole of the long wave, without overlap.

    Now look at the same thing with a carrier of 100Mhz.
    The swing is now from 99.98 Mhz to 100.02 Mhz, allowing plenty of room for stations on the normal FM band (88 - 108Mhz)


    In practise the modulating signal on AM bands is limited to 3Khz and on the FM bands to 15Khz for this reason.

    Hope this helps.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The modulating signal for most FM radio stations goes up to 53kHz for stereo and up to 72kHz for alternate audio like music for restaurants and stores or for data.
     
  9. mabsj2

    Thread Starter Member

    Apr 24, 2008
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    Thanks for that...
     
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