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?
In: Technology
You know how, when you have 2 tuning forks, and you strike one and bring it close to the other, the other one starts to sound as well, especiallly if they are “tuned” to the same frequency?
The effect is called resonance, and it happens because the two metals are shaped just right and when it starts to vibrate, a slight feedback loop in the metal itself happens, and it amplifies the signal.
Well, that’s basically how radios work, but instead of physical resonance, it uses electrical resonance.
The same way that a sound wave can have many frequecies, but you voice won’t necessarily cause a tuning fork to vibrate, a radio will have a circuit that is “tuned” to a specific frequency.
Electrical resonance uses the antenna length that should be related to the wavelength of the frequency you want to capture, and a circuit that helps filter out unwanted frequencies, like a coil. Think of a coil as an electrical damper spring that absorbs certain frequencies but lets others through.
**So how do radios work?**
Lets talk about the first method of transmitting signals, Amplitude Modulation.
Audio frequencies 20Hz to 20kHz are low frequency, and as electrical signals, are easily absorbed. High frequency signals in the 100kHz above are much better for transmitting. So how do we transcribe an audio signal to a radio signal?
We start with the “carrier wave”, or a high frequency signal. Think of a constant sine wave at a constant frequency. Now we alter (or modulate) the amplitude or the height of the wave using the audio signal, and we get.. Amplitude Modulation. You encode the audio signal changes as changes in the carrier wave *amplitude*,
[https://en.wikipedia.org/wiki/Amplitude_modulation](https://en.wikipedia.org/wiki/Amplitude_modulation)
Now we have a signal that contains information (the audio, encoded in the height of the wave) but with a high frequency, that can be transmitted a long distance.
The simplest AM reciever uses a wire, a coil (to tune the circuit to the desired frequency) and a diode to extract the “height” of the original audio wave from the carrier wave. The signal still contains the carrier wave so use a capacitor that resists changes by storing some charge an releasing, effectively filtering out the carrier wave and restoring the audio signal:
[https://en.wikipedia.org/wiki/Crystal_detector](https://en.wikipedia.org/wiki/Crystal_detector)
**So how to stations work?**
Just like tuning forks, each station is transmitting at a different frequency. The signals mix all together in the “air”, but the antennas and tuned circuits resonate at the desired frequency, letting you tune into one radio station.
Things are never cut-and-dried of course and station frequencies must have separation so that their signals don’t overlap. This is because the act of modulation generates “side bands”, additional resonant frequencies, weaker than the main carrier frequency, but strong enough to cause interference.
Amplitude modulation has a flaw, in that by modulating the amplitude, you also modulate the power, which leads to signal degredataion. Also lower carrier frequencies can only encode so much information in the amplitude, so that something like high quality music can’t be transmitted effectively.
To fix this, Frequency Modulation was developed. Instead of changing the amplitude, they varied the frequency, based on the input signal. Imagine you have a transmitter operating at a certain carrier frequency, like 88.3MHz. You then have a knob that you twist based on the audio signal, and this changes the fequency of the carrier wave by a few kHz, like 88-89MHz. This is frequency modulation. You encode the audio signal changes as changes in the carrier wave *frequency*,
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