calculating wavelength ?

Thread Starter

Alasttt

Joined May 13, 2015
68
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:

MikeML

Joined Oct 2, 2009
5,444
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).
 

OBW0549

Joined Mar 2, 2015
3,566
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.
You're off by a factor of 1000.

300000000 / 20000 = 15000, not 15.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
You're off by a factor of 1000.

300000000 / 20000 = 15000, not 15.
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.
 

OBW0549

Joined Mar 2, 2015
3,566
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

Joined Dec 13, 2013
2,458
the wavelength of the modulating signal isnt the wavelength of the carrier... if light is the carrier, figure the wavelength of that.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
the wavelength of the modulating signal isnt the wavelength of the carrier... if light is the carrier, figure the wavelength of that.
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).
 

OBW0549

Joined Mar 2, 2015
3,566
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).
You get that from the LED datasheet; it will specify the predominant wavelength of the emitted light.
 

Thread Starter

Alasttt

Joined May 13, 2015
68
You get that from the LED datasheet; it will specify the predominant wavelength of the emitted light.
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?
 
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