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Am I doing this correctly?

Discussion in 'Announcements & Open Forum' started by LeapFrog, Jan 8, 2018.

  1. LeapFrog

    LeapFrog Wielding Hanlon's Razor

    Feb 15, 2016
    Likes Received:
    source: https://br.answers.yahoo.com/question/index?qid=20090525124333AAHrJmB
    LC circuit question for crystal receiver.
    My attempt for a
    20 Meters example.

    Solve for L
    F = 14MHz
    C = 10pF

    L = 1 / (4π² (14e6)² (10e-12) )

    4π² = 39.4784176044
    14e6 = 14,000,000.0 squared is = 1.96e+14
    1.96e+14 x 10e-12 = 1960
    L= 1 / (39.4784176044 x 1960.00)
    39.4784176044 x 1960.00 = 77377.6985046
    L = 1 / 77377.6985046
    L = 0.00001292362 or 12.92 µH

    Do the numbers add up?
    I know I need to revisit Scientific notation.
    Thank You
    #1 LeapFrog, Jan 8, 2018
    Last edited: Jan 8, 2018

  2. nomadradio

    nomadradio Analog Retentive

    Apr 3, 2005
    Likes Received:
    Looks pretty darn close to me.

    Since what we do tends to get 'tweaked' to tune, I just use the reactance chart from the ARRL handbook. Resonance is the frequency where the reactance of the capacitor equals the reactance of the inductor. The chart has capacitance value along diagonal lines running down from left to right. Inductance value is along diagonal lines going up from right to left. Finding 13 uH and 10pf, the frequency where the lines cross for those two values lands right around 14 MHz.

    Should work, so long as your circuit has a variable element in either the inductor or capacitor to "tweak" it on frequency.

    #2 nomadradio, Jan 8, 2018
    Last edited: Jan 9, 2018
    Shadetree Mechanic and LeapFrog like this.
  3. LeapFrog

    LeapFrog Wielding Hanlon's Razor

    Feb 15, 2016
    Likes Received:

    Thank You for looking this over; I really appreciate the mention (and checking against) of the ARRL handbook, yet another reason why I need to get a copy!

    I will most definitely be using a variable element to tweak the values in-circuit; as I couldn't be bothered trimming the L millimeters at a time or adding minuscule amounts of C to finally get to peak resonance. :)

    I (just) found a reactance chart online (didn't know they made those), here it is just in case anyone needs a copy.

    I'm going to go study and read about reactance now, thank you once again Nomad.
    Also, I appreciate your explanation of resonance!

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