When you tune your radio (digital or analog) to receive say 100 MHz frequency and while in the environment there are hundreds of channels everywhere around the radio. How does it chooses to receive the frequency only at 100 MHz? How does the radio receiver works? Is it possible to explain this in simple concept?
Answer
This is a resonance in the circuit--- when you have a bunch of different frequencies driving a resonant system, the response is only strong for those frequencies which are close to the natural frequency of the resonant oscillator.
You can see the same phenomenon in mechanical systems. If you have a mechanical mass on a spring, and you apply a force which varies with time, the amplitude of oscillation is
$$ { F(\omega)\over \omega^2 - \omega_0^2 + i\Gamma} $$
Where $F(\omega)$ is the Fourier component of the force at the frequency $\omega$, $\omega_0$ is the natural frequency of the oscillation, and $\Gamma$ is a small damping parameter. In the limit of small $\Gamma$, you pick out only the Fourier component of F near the resonant frequency, those components which are different in frequency cancel because they push and pull at the wrong time given the natural vibration frequency of the oscillator.
This natural Fourier transform property of linear oscillators is the basis of human hearing, where the hairs in the ear are tuned to resonate only very close to one frequency. It is also the basis for radio tuning, or any other linear frequency sensitive response.
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