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In an FM, the frequency is modulated. So whatever reaches the receiver is not a constant frequency. This being the case, at Receiving an AM/FM Signal: "In an FM radio, the detector is different, but everything else is the same. In FM, the detector turns the changes in frequency into sound, but the antenna, tuner and amplifier are largely the same as for AM" is confusing me.

How can a tuner tune to single frequency (as in AM) in FM while fundamentals itself say frequency is modulated?

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  • \$\begingroup\$ Keep in mind that normal FM radio is still a narrow bandwidth signal. Look at the ratio of the frequency change to the base frequency, and you will see the frequency only changes a small amount. \$\endgroup\$ Feb 22, 2013 at 17:00

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You never, not in AM and not in FM, listen at only one frequency. That would be a perfect filter. No, we always listen at a little piece of a band, this is called the bandwidth. So for example: you say you listen at 7MHz, but actually you're listening at 6.999 - 7.001 MHz.

Since the frequency doesn't change in AM, we can have a very narrow bandwidth: usually less than 250Hz, when we transmit CW, since that only has one tone. When we transmit speech over AM, the bandwidth has to be more, conventionally 9kHz in Europe.

In FM, we do change the frequency. So while the base wave is, let's say, 100MHz, this may vary from e.g. 99.05MHz to 100.05MHz. The bandwidth of phone is approximately 3kHz, but to allow higher frequencies as well the common bandwidth of FM is 15 - 20kHz. So when you say you listen at 100MHz, this would be from 99.99 to 100.01 MHz, with a bandwidth of 20kHz.

So for AM, we have a rather narrow band filter, while we have a rather broad filter for FM. This is, as you said, because FM changes the frequency, while AM changes the amplitudo.

More information: http://www.cybercollege.com/frtv/frtv017.htm (from "Basic Differences Between AM and FM" to the end)

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  • \$\begingroup\$ Glad to be of help! \$\endgroup\$
    – user17592
    Feb 22, 2013 at 10:24
  • \$\begingroup\$ If the AM receiver is tuned with a narrow bandwidth such as 250Hz, you will only pick up the low frequency components of the modulating signal; up to 125 Hz if correctly tuned. You can improve the bandwidth by deliberately mistuning by 125Hz, and receiving 250Hz of one sideband. Either way, speech would be unintelligible until you increase the receiver bandwidth to several kHz (conventionally 9 kHz in Europe). This - not tuning difficulty - is what directs AM receiver bandwidth. \$\endgroup\$ Feb 22, 2013 at 11:08
  • \$\begingroup\$ Thanks, didn't know that. I edited the answer accordingly. \$\endgroup\$
    – user17592
    Feb 22, 2013 at 11:11
  • \$\begingroup\$ The last sentence of your third paragraph isn't correct. The bandwidth of the modulating signal doesn't translate directly to the bandwidth occupied by the modulated carrier. Commercial VHF FM has a modulating bandwidth of 53kHz but a maximum deviation of +/-75kHz (and this in turn results in a real modulated bandwidth of 312kHz through the IF strip). This relates more to S/N ratio than it does to input bandwidth. You are modulating the frequency of the carrier by the amplitude of modulating signal. The resulting sidebands are a Bessel function of the input. \$\endgroup\$
    – user207421
    Mar 19, 2013 at 22:02
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https://en.wikipedia.org/wiki/Detector_%28radio%29

The bandwidth of the modulated signal is typically a very small fraction of the carrier frequency so the difference between AM, FM, narrowband, "wideband" (still narrow relative to the carrier frequency), analog, digital, et. al. is small since one may be talking about 1% or less of the carrier frequency being the bandwidth of the signal whatever frequency variation exists due to the information modulated on the signal and the modulation technique. Modulating any information on a carrier creates frequency components at a different frequency than the carrier signal's center or static frequency. AM or FM might have these "side bands" of modulation generated frequencies symmetrically up and down from the carrier center frequency, or some kind of single sideband or special modulation might eliminate or vary the upper sideband's content relative to the lower sideband. So whether the total signal maximum bandwidth (width between the upper and lower sidebands at the maximum modulation bandwidth condition) is 1kHz, 10kHz, 75kHz, 100kHz, and whether the modulation is AM or FM, 100kHz is a small fraction of 100MHz carrier frequency and 10kHz is a small fraction of 1MHz carrier frequency. So the antenna does not typically need to be broadband relative to a fraction of the carrier frequency. The tuner is just tuning the LO local oscillator to the difference between RF carrier frequency and the 1st IF frequency (or near 0 if direct conversion / low IF) in either case so the tuner is similar in any case mostly depending on what the carrier frequency band is (~ 1MHz for common AM broadcast radio in the USA and ~ 100MHz for common FM broadcast radio in the US).

The detector for AM might be something like an envelope detector (q.v. the wikipedia article). The detector for FM might be something like a PLL or discriminator of some sort (q.v. the wikipedia article). The detector works on the output of the IF (or baseband if IF is near or at 0 Hz) so whatever radio circuitry gets the RF signal filtered, tuned, mixed, amplified, downconverted to the last IF is possibly somewhat generic regardless of whether AM or FM is used.

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  • \$\begingroup\$ Thanks for the detailed explanation on the effect of miniscule deviation of frequency from carrier freq. \$\endgroup\$
    – Aadishri
    Feb 22, 2013 at 10:33
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They are designed to have a wide bandwidth that accommodates FM.

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  • \$\begingroup\$ I was referring to an FM receiver, not AM! \$\endgroup\$ Mar 20, 2013 at 12:36

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