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So if FM radio signals modulate the frequency of the EM wave

  1. Oct 27, 2012 #1
    then how can you tune into, say, 101.1 when its not at 101.1 MHz all the time given the frequency change?
  2. jcsd
  3. Oct 27, 2012 #2
    An FM radio receiver tuned to 101.1 Mhz is actually receiving a band of frequencies centered on 101.1 Mhz. I think the bandwidth for the FM broadcast band in the US is [strike]20.5[/strike] Mhz. So a receiver tuned to 101.1 Mhz would actually be receiving frequencies in the range of [strike]90.85[/strike] to [strike]111.35[/strike] Mhz.

    The broadcast bandwidth is +/- 90 Khz, not 20.5 Mhz. And the assigned bandwidth is 200 Khz per channel.
    Last edited: Oct 28, 2012
  4. Oct 27, 2012 #3


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    Why just take a guess (mistaken) when responding to questions like this? The Wikipedia page explains FM radio in the US fairly well. Here is an excerpt:

    “...in the US, FM channels are 200 kHz (0.2 MHz) apart.

    Bandwidth of 200 kHz is not needed to accommodate an audio signal — 20 kHz to 30 kHz is all that is necessary for a narrowband FM signal. The 200 kHz bandwidth allowed room for ±75 kHz signal deviation from the assigned frequency, plus guard bands to reduce or eliminate adjacent channel interference. The larger bandwidth allows for broadcasting a 15 kHz bandwidth audio signal plus a 38 kHz stereo "subcarrier"—a piggyback signal that rides on the main signal. Additional unused capacity is used by some broadcasters to transmit utility functions such as background music for public areas, GPS auxiliary signals, or financial market data.”
    Last edited: Oct 27, 2012
  5. Oct 27, 2012 #4
    Thanks for the correction. Yes, I answered that way to quick. 20.5 Mhz is actually the width of the entire FM broadcast band.
  6. Oct 28, 2012 #5


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    101.1 MHz is the center frequency of a band that spans a range from 100.025 to 101.175 MHz. The next band is 101.225 to 101.375, with center frequency 101.3 MHz, etc.
  7. Oct 28, 2012 #6


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    even 20 - 30 kHz is quite wide, most commercial and amateur radio NBFM modes use 5, 12 or 15kHz these days

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