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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?

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4 Answers

up vote 2 down vote accepted

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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Does resonance here means the effect of amplifying that frequency so that it becomes larger, right? –  Ryan Jan 17 '12 at 13:20
    
@Ryan: yes--- the frequency dependent amplification factor is described by the formula in the answer. –  Ron Maimon Jan 17 '12 at 14:22
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Possibly simpler explanation.

Imagine a piano with people tapping different keys (notes) as morse code.

Although the result would be an almost random garble of noise. If you had a device that was tuned to listen to only a single note (or you had very good ears) it could detect when that note was being played or not in the midst of all the other notes, and so receive the signal.

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That's not an answer, just a restatement of the question. –  user2963 Jan 17 '12 at 14:00
    
Then the previous answer is wrong - there isn't necessarily a resonance in the circuit, DAB radios do the correlation entirely in software. –  Martin Beckett Jan 17 '12 at 15:56
    
I agree, I don't like the other answer either for that reason. I'm just saying, the question already acknowledges that the radio "tunes in" on a particular frequency, they want to know HOW that process happens. –  user2963 Jan 17 '12 at 16:04
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Everyday FM radio receivers employ several stages of techniques to output the desired signal.

Starting from the outside and working our way to the radio output, the antenna design and engineering is the first step. The dimensions and orientation of the antenna elements can be optimized for a particular frequency or frequency band, and or polarization of radio wave (usually horisontal or vertical). As an everyday example, the radio antenna from a car wouldn't work very well if used for a wifi router, due in part to this principle.
[Link] Antenna Length Calculator: www.kwarc.org/ant-calc.html
[Link] Antenna Polarisation: www.radio-electronics.com/info/antennas/basics/polarisation-polarization.php

After the radio waves have been received on the antenna and traveled down the antenna cable to the radio unit, it's common to have a filter as the first component. These filters further the selection work started by the antenna design, and create resistance for any radio wave that aren't in the range it's designed to allow. They can be analog or digital; and normally the more expensive the filter, the better job it will do of only allowing certain frequencies thru.
[Link] RF Filter Basics: www.radio-electronics.com/info/rf-technology-design/rf-filters/rf-filter-basics-tutorial.php

Once the signal has made it through the filter, common radio units do several steps to 'clean up' the signal, and perhaps convert it to an IF or Intermediate Frequency so that it travels better on the internal circuit boards in the radio unit. One common way to isolate the exact frequency you want is to use an oscillator to multiply the carrier frequency you're targeting. For example lets say you tune an old radio knob to 96.5MHz, which then sets the internal oscillator to produce a 96.5MHz signal. This reference signal is then used by the radio receiver to help it identify and strengthen any radio waves coming from the antenna at the 96.5Mhz frequency. It lets it know what to listen for, and like a child discerning their parent's voice in a noisy room, so then the radio will 'hear' that station because it knows what it's looking for.

Hopefully this gives a conceptual answer to your question. Some of the examples were generalized for simplicities' sake, and the best way to understand this might be to go buy a kit and build your own FM radio receiver:
http://www.somerset.net/arm/fm_only_one_transistor_radio.html

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When thousands of radio waves hit the antenna at the same time, a voltage is passed down the radio antenna, so does it know the frequency from the power of voltage? –  Ryan Jan 19 '12 at 5:04
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Conceptually, you tune into a particular signal by using Fourier analysis. (This may be implemented in various different ways, traditional radios use the method described in Ron's answer.)

Basically, the signal you want to receive is oscillating at (or near to) a known frequency. If you multiply the raw signal by a sine wave at the same frequency, and then average over time, all the other signals more or less cancel themselves out, leaving just the signal you want which you can then decode as appropriate (depending on whether it is AM, FM, digital radio, or whatever).

As always, reality is a bit more complicated than that, but I think that's the concept you wanted.

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