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Homework Help: Simple explanation of a series rlc circuit in a radio

  1. Aug 7, 2009 #1
    hi, i wanted a simple explanation of a series rlc circuit in a radio
    as i know the rlc circuit consists of resistor,inductor,capacitor
    i know the function of each of them but what i dont understand is how by combinig them we can make a radio tuner!?
    i searched many websites but i cant find a simple explanation.
    i mean a resistor just oppose the electric current
    and inductor causes electromagnetic induction phenomena and can store energy in form of magnatic field
    and capacitor can store energy and discharge it (although very small charge)
    but what i dont understand is? how by changing the capacitance of the capacitor i can make rlc circuit receive for example a 99.8 hertz signal and by changing it's capacitance i can make it receive a radio signal of 110 hertz.
    thanks for reading
  2. jcsd
  3. Aug 8, 2009 #2


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    Staff: Mentor

    Welcome to the PF. A series or parallel LC circuit resonates at a frequency determined by the values of the L and C. At resonance, the circuit presents either a low or a high impedance, depending on whether the circuit is series or parallel.

    More info can be found at http://en.wikipedia.org/wiki/LC_circuit

  4. Aug 9, 2009 #3


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    Science Advisor

    As you can see from that Wikipedia reference, resonance is not a simple subject.
    From their functions, you would not expect such effects from capacitors and inductors.

    Perhaps you can just accept that such effects do occur and can be easily demonstrated.

    Capacitors have less impedance as frequency increases and inductors have more impedance as frequency increases. At some frequency, these impedances (actually called reactances) are equal and can cancel each other out because of phase relationships between them.

    If you have a series circuit of R... L... C and a variable AC signal source, the L and C will reach a state, as you vary the frequency, where their impedances cancel each other out and the effective result is just the R in circuit. This will mean that all the AC voltage will occur across R. So, you would see a peak in voltage across R and a dip in voltage across the L .. C part of the circuit.

    BUT that is not the only thing that happens. At the junction of the L and C, there will be a highly magnified version of the input signal. This is very surprising when you first see it, because you don't expect to see an amplified signal without an amplifier.

    You could take this larger signal and use it in a radio receiver to receive one signal better than another on a different frequency.

    These effects are not perfect and nearby frequencies are also passed to some extent. If the series resistor is small the resonance effect is more pronounced and the selectivity of the circuit is greater. So, it is better at rejecting nearby frequencies.

    As an example, you could reject 900 kHz while receiving 1000 kHz, but you probably couldn't reject 990 kHz while receiving 1000 kHz, with a simple tuned circuit.
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