# Calculating the frequency based on LC circuit

1. Mar 23, 2013

### guns

Hi all,

I am currently building up a lightning detector based on this website: http://www.techlib.com/electronics/lightningnew.htm
as the diagrams shows, the resonance tank included a 10mH inductor and a 10pF capacitor.
As I want to calculate the resonant frequency (using the formula f = 1/2pi*sqrt(LC)), the answer was 5Hz, which is far from the desired range (should be around 200kHz)
is there any mistake in my calculation or concept? Thanks for your help.

Guns

2. Mar 23, 2013

### Staff: Mentor

How did you get your answer? Did you consider the SI prefixes (milli, pico)?
I think there are brackets missing in your equation.

3. Mar 23, 2013

### guns

mfb,
I did it this way:
1/2pi*sqrt(10^(-3)*10^(-12))
which approxmately equals to 5 Hz.

4. Mar 23, 2013

### milesyoung

Following up on mfb's post, you're missing brackets and are using the wrong values for the SI units:

10 mH = 10*10^(-3) H
10 pF = 10*10^(-12) F

1/(2*pi*sqrt(10*10^(-3)*10*10^(-12))) Hz = 503.3*10^3 Hz = 503.3 kHz

5. Mar 24, 2013

### guns

oh my, silly me.
Thanks mfb and milesyoung!