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Calculating frequency

Discussion in 'General Electronics Discussion' started by Electro132, Mar 30, 2015.

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  1. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Hi,

    I'd like to know how do i calculate the frequency for a LC tank? I've seen so many different formulas i lost my way and managed to get to Physics some how where the formula is V = C / (inverted Y symbol). I am interested on learning the basic way first as seeing the entire thing seems like a blur.

    Cheers
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello. You want 1/2pi sqroot LC
    Adam
     
  3. pebe

    pebe

    83
    11
    Sep 3, 2013
    Long ago I was taught a catchy little jingle to remember the formula:-

    "One over two pie root L C
    Equals the resonant frequency.".
     
    Tha fios agaibh and Arouse1973 like this.
  4. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Thanks guys.

    So from what you told me, it's pretty much

    1/2 x pie (3.14) square root L (which is the coil length, diameter and thickness) x C = the resonant frequency

    Am i on the right path?
     
  5. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    Yep! Don't forget to multiply the root also. 1/(2*pi*root(L*C)) Example: 1/(2*3.142*root(1uH*10nF) = 1.59 MHz

    1 uH (Micro Henry) being 1*10^-6 and 10nF (Nano Farad) being 10*10^-9

    Thanks
    Adam
     
  6. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Ah i see. I am just a little confused with the 1 uH for the coil. How do you find out what the value is for it?

    Also i have been working with this formula to find out the inductance of the coil:

    L1 = D^2 n^2
    ---------------
    45D + 100 L2

    where: L1 = Coil inductance (uH)
    D = Coil Diameter (cm)
    L2 = Winding Length (cm)
    n = Number of windings


    I just wanted to make sure this is the right formula to find the inductance. Can anyone check please?

    Thanks
     
    Last edited: Apr 1, 2015
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,363
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    Jan 21, 2010
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    It was an example! You need to enter your values you have.
    Adam
     
  9. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Ok thanks for the link

    Cheers
     
  10. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Hi Adam,

    i have a question: why is 1uF equal to 1x10(-6) while 10nF = 10x10(-9)? Does it go something like this:

    1nf = 10 x 10(-1) 1uF = 1 x 10(-6) <------- This is the last one on the chart, so how can it be to the power of -6 (0.0000001)?
    1.2nf = 10 x 10(-2)
    1.5nf = 10 x 10(-3)
    1.8nf = 10 x 10(-4)
    etc..

    I just don't understand how 10nf is equivalent to 10 x 10(-9). Can you explain please.

    Thanks
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,363
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    Jan 21, 2010
    Pico (p) is 1e-12
    Nano (n) is 1e-9
    Micro (u) is 1e-6
    Milli (m) is 1e-3

    These are all multipliers so 10nF is 10 * 10e-9 F, or 1e-8 F or 10,000pF or 0.01uF.
     
  12. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    that's it. i've tried to understand how to calculate the formula you gave and its doing my head in. How do you calculate the capacitor when its in nF or pF? i know how to get the inductance for the coil (L) though.

    Thanks
     
  13. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Guys,

    Don't worry about it. I figured it out. It took a while but got the hang of it.

    Thanks heaps. :)
     
    Arouse1973 likes this.
  14. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Well done
    Adam
     
  15. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    hey can someone check this please:

    I'm using an LC tank which has an inductance of 0.00018 uF (6.5 coil turns, length = 6cm, diameter = 0.5mm) and a 1nf cap giving me 375.13 mHz as the frequency.

    Thanks
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Well, the frequency is 1/(2*pi*sqrt(L*C)).

    Inductances are measure in Henries, so I presume it's 180pH. So 1/(6.28* sqrt(180e-12 * 1e-9)) = 1/(6.28 * sqrt(180e-21)) = 1/(6.28 * 4.2e-10) = 1/2.7e-9 = 380MHz

    Note that 375mHz is 1,000,000,000 times too small :)
     
  17. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Sorry MHz is what it was supposed to be not mHz. Also the uF was supposed to be uH, sorry my mistake. had too many calculations :)

    With the coil's diameter, i wanted to ask is that to do with the coil itself (0.5mm, 1mm, 1.25mm) or the diameter of the coil ring that i make using a screwbit? i initially thought it was the coil itself but wanted to double check.
     
    Last edited: Apr 12, 2015
  18. duke37

    duke37

    5,344
    753
    Jan 9, 2011
    This seems unrealistic to me so I have not done any calculations.
    To get down to 0.375Hz, you will need a much bigger capacitor and thousands of turns.
    6.5 turns of 0.5mm diameter over 60mm length is a helix not a coil !
    If you are looking for 375MHz, then the capacitor will not work, it will be running above its resonant frequency.
     
  19. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Sorry guys, i made a mistake in explaining. Here's the right details:

    d = 5 mm
    n = 6 cm
    L2 = 6.5

    so i did the calculation again and the frequency was 38.33 MHz. Can you guys check please if you got the same?
     
  20. Electro132

    Electro132

    261
    3
    Feb 12, 2013
    Hey,

    I was trying out the formula to find out the frequency for this example but couldn't get the right answer as my calculations produced 1.1253954e+12 Hz. Its weird. Can anyone figure it out please? I converted my answer to khz and got 1 125 395 400 kilohertz instead.

    LC oscillator Example.jpg
     
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