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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?
    ANS:__________

    (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?
    ANS:___________pF

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

    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
    Science Advisor
    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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