Because each receiver is tuned to a particular frequency.

And then e.g. you have things as

http://ham.stackexchange.com/questions/137/what-frequencies-am-i-not-allowed-to-use-north-of-line-a-in-the-usa

(I should add that in frequency space, your pic would be 3 functions that are very seperated from each other)

>>8059672
Could you elaborate on how it is tuned to that particular frequency? In the example (take sine of 1Hz, 2Hz, 4Hz together) if you sample at 1Hz you'll be incapable of seeing wether it went down or up right? (As the others add their energy to the wave?)

>>8059677
Seperated how? As in in/out of sync?

>>8059681
Do you know how a fourier analysis works?
Your function is a vector in a vector space and if you have the whole signal, multiplying it with particular oscillations tells you the amplitude for each frequency.

For more particular oscillators as receivers, I don't know the designs that achieve that, sorry

>>8059681
All antennas that we use are LC-Circuits. They have a resonance frequency of [eqn]f_0=\frac{1}{2\pi \sqrt{LC}}[/eqn].
This means that if e.g. you have a circuit with a frequency of 1kHz and send a 2kHz signal the circuit is hardly stimulated/you can measure almost no current. If you send a signal with a frequency of 1kHz the circuit is very stimulated and thus has a large current. You can change the frequency it amplifies by changing the coil (hard) and/or the capacitor (easy)

>>8059681
You can make very simple circuits using only an inductor and a resistor that allow certain frequencies through based on the values of the inductor and resistor

>>8059681
RC and LC Filter networks

>>8059749
>>8059840
the quality factor will never be anywhere near a tuned LC circuit
you would basically be throwing signal power away

Thank you for your responses. I have a better understanding now, thanks.

>>8059672
Is that all it is? I'd think with potentially 1000s of wireless devices in a small urban area you would need something more sophisticated.

>>8059647
Band pass filters