Radio: How does a demodulator work?

  1. Can someone explain how a demodulator or detector for a radio works in layman's terms? It is the part of the radio that separates the sound from the carrier wave. For both am and fm. I would like to know how it actually does this.

    Also, can someone explain how a coil in a circuit affects its frequency?
    Last edited: May 11, 2013
  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    I'll try the last question, it's the easiest. :smile:

    A coil is what we use when we need an inductor. By itself, an inductor does not have a "frequency". A tuning circuit involves an inductor together with a capacitor, and one of these is usually made variable so that you can alter the circuit's resonant frequency. The frequency the combination tunes to is the frequency where the capacitor has an equal but opposite impedance to that of the inductor, so that if one of these is made variable, then the frequency of resonance is variable and you are able to tune your radio up and down the band listening for stations on different frequencies.
  4. sophiecentaur

    sophiecentaur 14,721
    Science Advisor
    Gold Member

    Wiki is usually a good source of first call for most information. Is there anything in this link that you need explaining more clearly? I don't think you can get more "layman's terms" than that.
  5. Thank you for the link. I'm still a little confused on how the capacitor demodulates for am. The fm can be filtered by removing a certain frequency and I understand how that can be done.
  6. davenn

    davenn 4,364
    Science Advisor
    Gold Member
    2014 Award

    The diode is the main device that does the demodulating ( detecting) of the AM audio from the RF signal. The capacitor in basic circuits can be done away with, but is often used as a low pass filter, allowing the lower frequency of the audio signal to pass through and stopping any of the higher RF frequencies from passing on to the ausio amplifier stages.

  7. berkeman

    Staff: Mentor

    It's not a capacitor, it's a diode that is used for the envelope detection of an AM signal.
  8. What does the diode do exactly? From what I understand, diodes only allow electricity to pass one way and not the other.

    How does the capacitor actually act as a low pass filter? Do capacitors only store charge at a certain frequency?

    Is the same kind of circuit used to filter fm? What does the filtering for fm?

    What is meant by phase shifting 90 degrees?
    Last edited: May 12, 2013

    Your questions indicate that you may need some more basic background. There are a lot of good beginner books that teach the basics of electronics. My first book was "Getting Started In Electronics". But that was back in the 60's. The modern version is by a different author, so I haven't read that one.
  10. berkeman

    Staff: Mentor

    Please do us all a favor. Please use wikipedia to read more about your questions, and then if you are confused by what you read, please post your follow-up questions here with links to what you have read.

    I know that you are trying to learn, but part of learning is learning how to learn. It is best to do lots of reading on your own first, before you ask others to help you understand. Learning how to learn is an important cornerstone of the PF. :smile:
  11. sophiecentaur

    sophiecentaur 14,721
    Science Advisor
    Gold Member

    The very fact that you think you understand FM demodulation but not AM, demonstrates that you have a long way to go. You cannot just parachute half way into a subject like this and expect not to be totally lost. Start with the basics or you'll never get anywhere.
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