# calculating wavelength ?

Discussion in 'Wireless & RF Design' started by Alasttt, May 14, 2015.

1. ### Alasttt Thread Starter Member

May 13, 2015
68
0
Hi All,

Ive got some data, the time period is 50us. F = 1/t so F = 1/0.05ms which gives a frequency of 20Khz.

Wavelength = c/f. That means 300 000 000 m/s / 20k gives a wavelength of 15m.

This is very large am I doing something wrong ?, Looking at the spectrum 15m is so big it doesnt even appear.

Last edited: May 14, 2015
2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
Radio signals are in MegaHz. 20kHz is within a young person's (not mine) hearing range, so is considered audio...

Radio communications with submerged submarines happens on ~15kHz Very Long Wave Radio Waves (not acoustic).

3. ### OBW0549 Well-Known Member

Mar 2, 2015
1,241
844
You're off by a factor of 1000.

300000000 / 20000 = 15000, not 15.

4. ### Alasttt Thread Starter Member

May 13, 2015
68
0
Right thanks. That still is 15km that seems really large to me most wavelengths are nm. It is a light signal from a fibre that I want to shine onto a photo transistor, but I doubt the photo transistor will work at a wavelength of 15km.

5. ### OBW0549 Well-Known Member

Mar 2, 2015
1,241
844
The wavelength of the light being transmitted through the fiber will indeed be a matter of nanometers; but the wavelength of your data signal, which you say is 20 KHz, will be 15km. Two VERY different things.

alfacliff likes this.
6. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
428
the wavelength of the modulating signal isnt the wavelength of the carrier... if light is the carrier, figure the wavelength of that.

7. ### Alasttt Thread Starter Member

May 13, 2015
68
0
Right thanks a lot, yes I think thats where I am going wrong. The wavelength of my data (modulating signal) is 20khz. When i connect an LED and transmit it through a fibre light is the carrier. How do I find the wavelength of the carrier ? (light in this instance).

8. ### OBW0549 Well-Known Member

Mar 2, 2015
1,241
844
You get that from the LED datasheet; it will specify the predominant wavelength of the emitted light.

9. ### Alasttt Thread Starter Member

May 13, 2015
68
0
Brilliant thanks. I assume that means the carrier frequency will also change. So the freq of the signal through the fibre will not be 20khz?