how the heck does FM work,?

Comments

• user224@lemmy.sdf.org ⁨2⁩ ⁨months⁩ ago

  I expect it is finding a way to measure multiple frequencies around the tuned station and decodes the data from it’s deviation from the tuned frequency?

  Yep, and that’s bandwidth. Up to 200kHz for WFM I think.

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  • Max_P@lemmy.max-p.me ⁨2⁩ ⁨months⁩ ago

    Of course, don’t trust me on anything. I likely said something wrong.

    Looks pretty solid to me!

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• drspod@lemmy.ml ⁨2⁩ ⁨months⁩ ago

  The FM tuner is a narrow band pass filter centered at the carrier frequency - it doesn’t try to track the modulation. The FM demodulator then uses a VCO (voltage controlled oscillator) with a phase detector in a feedback loop, so that the output of the VCO tracks the modulated input signal. The input to the VCO then represents the baseband (demodulated) signal.

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• l_b_i@yiffit.net ⁨2⁩ ⁨months⁩ ago

  RF only has 2 components, Phase (frequency) and its amplitude. For Analog FM radio, you have a center frequency you tune to. The variance from the center frequency (phase) is the amplitude of the carried signal. For digital signals, you will have specified offsets from the center that represent specific binary codes.

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• litchralee@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

  This isn’t quite an ELI5, but ARRL has a 2004 article on FM fundamentals; it’s four pages intended for a beginner ham radio operator, but applicable to all FM nevertheless.

  But to answer your question directly:

  The frequency of the FM signal at any instant in time is called the instantaneous frequency. The variations back and forth around the carrier frequency are known as deviation

  FM can also be detected by a PLL. As shown in Figure 6, the PLL’s natural function of tracking a changing input frequency can be employed to generate a voltage that varies as the input frequency change

  In a nutshell, FM only ever has one instantaneous frequency at a time, which dances around the nominal center frequency (aka carrier). So the receiver has to detect the instantaneous frequency, relative to the carrier.

  To actually recover the original signal, the receiver must also account for the modulation index used by the transmitter, which describes how much the output will deviate for a given input frequency. The modulation index is usually standardized for the application, such as FM broadcasting, amateur radio FM, walkie talkie FM, etc.

  Because a larger modulation index means the same input signal will result in wider deviations, more RF bandwidth is used, spreading the signal wider and generally improving noise immunity.

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