# Calculating frequency

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

1. ### 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

5,164
1,080
Dec 18, 2013
Hello. You want 1/2pi sqroot LC

3. ### 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

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?

5,164
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

6. ### 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*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,363
2,758
Jan 21, 2010

5,164
1,080
Dec 18, 2013
It was an example! You need to enter your values you have.

261
3
Feb 12, 2013

Cheers

10. ### Electro132

261
3
Feb 12, 2013

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*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,363
2,758
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

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

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.

5,164
1,080
Dec 18, 2013
Well done

15. ### 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*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,363
2,758
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

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

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

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

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.