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Homework Help: LC Circuits with variable frequency

  1. Mar 31, 2009 #1
    1. The problem statement, all variables and given/known data

    A variable capacitor with a range from 10 pF to 420 pF is used with a coil to form a variable-frequency LC circuit to tune the input to a radio.

    (a) What ratio of maximum frequency to minimum frequency that can be obtained with such a capacitor?

    (b) If this circuit is to obtain frequencies from 0.54 MHz to 1.60 MHz, the ratio computed in (a) is too large. By adding a capacitor in parallel to the variable capacitor, this range can be adjusted. What capacitance should be added to obtain the desired frequency range?

    (c) What inductance should the coil have to obtain the desired frequency range?

    2. Relevant equations

    [tex]\omega[/tex]= [tex]\frac{1}{\sqrt{LC}}[/tex]

    3. The attempt at a solution
    no idea pls help
  2. jcsd
  3. Apr 1, 2009 #2

    George Jones

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    Staff Emeritus
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    Gold Member

    For (a), assume that the value of [itex]L[/itex] is unknown, but fixed (constant).

    What value for the variable capacitance gives you the biggest frequency?

    What value for the variable capacitance gives you the smallest frequency?
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