Superposition of EM waves and "magic" behind tuned receiver circuit

Question

I'm struggling to understand how the receiver circuit (eg. FM radio) is able to extract a particular frequency out of the sum of all received frequencies by its antenna?

I've checked dozen of websites, videos, and pictures, but still, that part is a "black box" to me.

This video is the closest I come, but still, it's really hard to understand how multiple different frequencies can coexist on the antenna at the same time without interference, and the receiver is able to "extract" only one of them?

There are dozen of radio stations around us, and they are using different frequencies to emit audio signals, but they are not synchronized by phase of the transmitted EM waves, and still, EM waves pass directly to our antennas without issues.

Does it have something with photons or not? What is the "magic" behind it?

Thanks

Answer 1

Classically, it was done with resonance. Early radio receivers had a collection of resonators tuned to the frequency of the station (the "tuned radio frequency" approach). The resonators would only respond to frequencies close to the desired frequency. Variable capacitors adjusted the resonators to "tune in" the station you wanted to listen to. The later "superheterodyne" approach used frequency conversion to enable most of those resonators to be at a fixed frequency, with most of the tuning done by adjusting the frequency converter.

Modern digital receivers basically simulate this with algorithms, often supported by physical resonators to do coarse signal selection.

Answer 2

The reason why information can be transmitted in a useful way by electromagnetic waves is linearity of the electromagnetic wave equations, or in other words, the linear superposition principle. To put it simply, if you add together a signal (in the sense of an electromagnetic wave solution) in the frequency band 90.0-90.1 Mhz and another signal in the frequency band 100.0-100.1 Mhz, the result will again be a solution to the electromagnetic wave solution. Since this is true for all imaginable frequency combinations (including the composition of said frequency bands themselves), the frequencies do not influence each other. That is, by adding the signals together, there will not appear any new frequencies that are not part of the individual signals. Hence, information from the individual radio stations is conserved due to this linear superposition principle.

Water surface waves are a good example of a medium where the linear superposition principle does not hold. Hence, it would not be a good idea to transmit speech through water surface waves. If two people spoke at the same time, you would hardly understand anything behind all the intermodulation distortion.

Your example of colors is misleading, since the brain is "engineered" to compose multiple colors into one (and, therfore, fool you about the true nature of electromagnetism). If you use a prism, however, and decompose the colors into their spectral components, you see that nature is different behind the veil of sensory delusion.

As to the technical details of the extraction of a specific radio channel from the mixture of all channels, valid answers have already been given. Let me just add some more details. It would certainly be possible, to design something like a prism for radio waves, to sort out different frequencies at different angles. But this would involve several conductors in certain arrangements, and then it is much easier to use just one conductor (an "antenna") that pre-selects a certain frequency range by its resonant properties. However, the main purpose of the antenna is not to narrow-select a certain radio station (to the contrary, you would like to be able to receice a bunch of stations without swapping antennas all the time). The main job is done by the receiver (which is the "prism" for radio stations). The most important principle today for selecting channels is: Multiplication.

By multiplying a 100.05 Mhz sinusoidal signal to the 100.0-100.1 Mhz radio station, two distinct signals are generated (note that what happens in the receiver is therefore now a nonlinear operation! we are not in free air anymore!). The first one is the sum frequency, which will be a band at about 200 Mhz. This signal changes way too fast to be useful, and is thrown away (by applying a lowpass filter). The second signal is the difference frequency, which is in the band -0.05 MHz to +0.05 Mhz. Surprise, we only need a lot slower circuits to decode this one. All the other radio stations will also result in sum and difference signals in the mixture, but the 90 MHz station for example will reappear at about -10 MHz (difference) and +190 Mhz (sum), both being so fast, we can throw them away again.

Now that we have a signal that is around DC (-0.05 MHz to +0.05 Mhz) we can just filter out the other nasty high frequency stuff, and do some more post-processing, depending on whether we have an amplitude modulated (AM) radio station, or a frequency-modulated (FM) one. In any case we have isolated the frequency range where the information of interest is located. That is how modern software defined radios (SDR) work, sometimes in the GHz range (mobiles, WiFi, etc.), where it is hard to find reliable electronic circuits operating that fast.

As to your final question: this does not have anything to do with photons. Radio transmission only works if there is a huge amount of photons in the radio wave. If you had a signal so weak that the number of photons played a role, you would be overwhelmed by shot noise. I think the auditory correspondence of this phenomenon could best be compared by the sound of a heavily broken CD, where the voices are severely hacked into sound fragments. But this just as an analogy, don't take it too serious.

Answer 3

We imagine a sea of electromagnetic waves of all frequencies, bathing the antenna. They induce AC currents in the antenna elements at all the frequencies present in that random bath, which consists of all the different radio signals being broadcast at that moment by all the different radio transmitters in operation at that moment.

The radio receiver's job is to take the AC signal from the antenna and 1) filter only ONE narrow band of frequencies out of that broad spectrum of frequencies, and 2) demodulate the content of that one narrow band, discard the carrier frequency (corresponding to the center frequency of that narrow band), and extract the signal which was used to modulated the carrier at the transmitter.

Job 1) is done by sending all that bathwater through a frequency-selective electronic filter which selects out of the chaotic bath only ONE narrow range of frequencies corresponding to the station you wish to hear. Spinning the tuning knob on your receiver allows you to tweak the filter circuit to the frequency you want, thereby excluding all the others you do not want.

At commonly-used radio frequencies, the photon model does not enter into the picture. The "magic" is in the filter circuit, which blocks out everything except the signal frequency you want to receive.

Answer 4

The 'magic' is integrated circuits(electronic) which cut off the envelope wave.In the early years of radio transmission we used AM and the "integrated circuit" was just a vacuum tube with a capacitor.For FM we have more complicated circuits.