Like it picks up a signal and processes it and from that I can hear music. But is it just the variation in frequency? And if so, then how do stations work?
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Imagine the music you listen to is a dish (music waveform). The radio station takes the dish and puts a very distinctive spice (carrier signal) in it and mixes it up according to a certain recipe (modulation). It then sends the spiced food (modulated waveform) out via delivery (broadcast).
Your receiver at home takes the food from the delivery guy and has a nose (demodulator) that can sense which spice is contained in the dish and uses an anti-spice (demodulation) to rid the food of the spice flavour so you can only taste the original dish. By telling your receiver which spice to smell for (setting the frequency), you can tune into different radio stations that use distinctive spices.
Sometimes, different restaurants (radio stations) use the same spices, which can confuse your receiver and make you hear two stations on the same frequency. In that case, the receiver is trained to pick the food that has a stronger aroma (signal strength) and stay on it.
As others have said, the frequency wobbles, but how it wobbles is the magic part.
The wobbles basically look very much like the waves in a smooth pond of water when a stone is tossed into it.
Sound is waves of air pressure that our ears can detect. Most humans can hear the sound vibrations from as low as about 20 hz up to as high as about 20 thousand hz (20 kilohertz).
Radios can, currently, transmit and receive from as low as a few hundred thousand hz (kilohertz) up to several billion hz (gigahertz).
The FM broadcast band in most countries is around 100 MHz, in your example a broadcast station may be at 101.1 MHz.
There’s a really cool thing that happens when two different frequencies are combined. We call this combining process “mixing”. When we mix two frequencies together we get back four frequencies. We get the two original frequencies, plus we also get the sum and the difference of the two frequencies.
Suppose we want to mix 101 MHz with 11 KHz. First let’s get the two frequencies in the same unit size to make the math easier. 101 MHz is the same as 101,000 KHz.
When we mix the two frequencies, 11 KHz and 101,000 KHz, we get back four frequencies. The frequencies we get back are 11 KHz, 101,000 KHz, 101,011 KHz, and 100,989 KHz.
If the 11 KHz signal is the audio, the music, and the 101,000 KHz is the radio base frequency, then the other two frequencies are the base frequency with the audio superimposed on it. The superimposed signal is called “modulated”.
We can then transmit that combined signal to a radio receiver. If the radio receiver is set to the same base frequency, 101,000 KHz, then the receiver can simply mix the same base frequency with the modulated signal, 101,011 KHz. Mixing these two signals gives us four frequencies, the same two plus the sum and the difference. So we get back 101,000 KHz, 101,011 KHz, 202,011 KHz, and 11 KHz.
If we know that the audio, the music, is never going to be more than 30 KHz then we can filter out all of the signals above that frequency. This leaves us with the original audio frequency of 11 KHz, the audio signal, the music.
But music of just a simple 11 KHz signal would be really boring because that would just be a single tone, and a rather annoying tone at that. But if that signal is wobbling around it would be a more interesting and pleasant signal. The wobbling signal mixed with the steady 101,000 KHz base signal will produce the wobbling signal, the base signal, plus the wobbling sum and difference signals.
When the receiver mixes the wobbling high frequency signal with the steady high frequency signal we get the four signals back, including the original low frequency wobbling signal.
Then we just filter out the high frequency signals, amplify the low frequency signal remaining, and send the signal to the speakers so that they can vibrate the air so that we can hear it, or feel it if the sound is powerful enough and at a low enough frequency, like at a rock concert.
The basic principles are pretty straightforward.
The key idea is _superposition_ – basically, you can add two waves together, and they will create a new wave. An important observation is that you can also subtract a wave you know is there, and recover the other wave that was added.
From here it’s a pretty short step. The “radio frequency”, for example 101.1 MHz, is the frequency of what’s known as the _carrier wave_, the shared known base waveform between the sender and the receiver. The sender takes the sounds they want to transmit (which is a wave, of course), and then adds that wave to the carrier wave, which makes a new wave with tiny little changes to a certain aspect of the carrier wave. The receiver also knows the carrier wave, so they can subtract it out and recover the original wave.
The “tiny little changes” are done to one of the wave dimensions, either amplitude or frequency, hence the names AM (amplitude modulation) and FM (frequency modulation).
This of course hand-waves away an immense amount of engineering and technical detail required to actually “add” or “subtract” a sound wave and an electromagnetic wave, broadcast it at power, and output a coherent signal.
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