eli5: On old radios, how does turning the dial tune into a different station? what is happening inside the radio to achieve this?
In: 70
On the realy old designs, you change the value of a variable capacitor. They typically look like [https://en.wikipedia.org/wiki/Variable_capacitor#/media/File:Variable_Capacitor.jpg](https://en.wikipedia.org/wiki/Variable_capacitor#/media/File:Variable_Capacitor.jpg)
The simple capacitor is two parallel metal plates that have an air gap in between. Their capacitance depends on the area and the distance between the plates. If you one sideways the area where they overlap is reduced and the capacitance is reduced. That is exactly what happens in the variable capacitor in the image above. There are multiple layers of plates not just one to make it smaller.
A capacitor and an inductor can form an LC-oscillator. L is used for the value if inductance, I was already taking for current, and C stand for the capacitance.
It is a bit simplified way to produce a signal at a specific frequency. The frequency depends on the inductance of the inductor and the capacitance of the capacitor. The inductor will be fixed and the capacitor variable so you can change the frequency to one of the radio stations.
You can compare it to a spring with a weight attached, If you stretch out the spring it will start to oscillate up and down and the frequency depends on how strong the spring is and how large the weight is. You can change the frequency by just changing the weight, which is equivalent to changing the capacitor in an LC-oscillator.
The signals are then used to filter out the radio station you are listed to. The low-frequency sound can then be extracted from the high-frequency radio waves. In a simple AM receiver, the capacitor and inductor are used to filter the signal directly.
Regardless of how the LC-oscillator is used, you change the station by adjusting the frequency it oscillates at.
In newer designs, you might change the value of a resistor by mechanically moving a contact. That change will change the frequency of another type of oscillator. Today it is likly input to a microchip that in some way will control an oscillator that is used to pick out the signal you are interested in.
The dial was a variable capacitor, with metal plates that would sweep to mesh across an insulator. A coil wound around a ferrite rod was connected in parallel. Together, these made a “tuned circuit” which resonates at the desired frequency and selects the station. This circuit is “tuned” the same way the space inside a violin emphasizes certain tones.
I deal with radio frequency at work and was trying to understand a similar topic so I shall have a go – in part to get clarity on my own knowledge.
Old radios that dealt in AM sent and received signals at a set frequency but adjusted the strength (amplitude) of the signal when sending data – look at Artic Monkeys AM album cover.
FM radios keep the strength the same but alter the frequency of the signal to send data.
Signals are sent within a band width of 20, 40, 80 and 160 MHz
When you tune your radio to 90.5 FM, you are turning it to the frequency the radio station is sending out its signals – assuming it’s on a 20 MHz bandwidth, your radio is listening to all signals from 80.5 – 100.5 FM
So if there’s another channel broadcasting at say 70.5, if you turn your radio to 80.5 you end up hearing half of one station and half of another
Tuning a station is mostly done by filtering all the signals down to just the frequency you want.
Basic filter circuits are made from electric components like resistors and capacitors.
Capacitors are devices that store electric charge usually on two thin metal plates. You can adjust what frequency they help to filter by adjusting the distance between the plates.
Turning the knob changed the distance between the plates.
To tune into a station, there has to be something which naturally vibrates at the right frequency. This way, when the right frequency of radio waves hit it, it will easily vibrate, but other frequencies will not cause much vibration at all.
This vibration could be a lot of different things, such as a small quartz ‘spring’ that ‘boings’ at the exact right frequency.
Traditionally, instead of a physical object vibrating, you’d have an electrical circuit that does the same thing. Two electrical components are put together, one which acts like an electrical spring which pushes on electrons and one which acts like an electrical weight to slow down the vibration of the ‘spring’.
By making the spring stronger or the weight lighter, you can make the frequency higher.
If you make your electrical ‘spring’ tighter by pulling it apart (this would need its own eli5) it raises the frequency, tuning your radio to a higher frequency signal.
You could also make your ‘weight’ heavier or lighter. The easiest way to do this is to make a large coil (the ‘weight’) but connect your wire to different parts of the coil. By connecting to different parts of the coil, you’re effectively changing its length, which is analogous to its ‘weight’.
Imagine you had a whole bunch of little TV screens all showing different videos. You’re having trouble concentrating on just one video because the others get distracting. So you cut a rectangle hole in a giant piece of cardboard and you use it to cover all the TVs but the one you want to watch. You’ve basically made a “filter” to filter out all the content except the one you care about. If you change your mind and want to watch a different video you just move the cardboard until the hole reveals just the video you care about.
An electric circuit in the radio does the same thing. The radio’s antenna picks up all the signals from all the radio stations but that circuit filters out all signals except the signal of the station you want to hear. Turning the dial changes the way the filter acts, just like moving the cardboard. The “hole” in the filter changes “position” to concentrate on a different signsl.
The radio technically receives all frequencies at all time, but the tunning system is essentially a very fine filter that only lets through the frequencies you want to listen to.
If you’d imagine frequencies like swinging on a swingset, if you pull just at the right time you’ll accelerate, but if you push or pull at the wrong time you’ll actually slow down.
When you tune the radio you’re changing when and where these capacitors push and pull the radio signal so you amplify the frequency you want to hear and filter out all others
So there is AM radio and FM radio. One is amplitude modulated (the signal strength goes up and down) and the other is frequency modulated (the frequency goes slightly up and down)
The radio’s electronic schematic is split into several blocks.
One is the amplifier and it amplifies the radio frequency of the station you want to receive. One variable capacitor and a coil are set at a resonant frequency matching the radio station frequency. Some transistors amplify the signal.
Other is the oscillator. Is oscillates at a frequency higher (or lower) with 10,7Mhz (for FM) or 455Khz (for AM). A second variable capacitor and second coil at resonance set the oscillator frequency.
One more block is mixing the 2 frequencies. The result is two more frequencies: the sum of the 2 original frequencies (radio station and oscillator) and also their difference.
The lowest frequency now has a fixed value: 10,7Mhz or 455Khz. This is now filtered, amplified and decoded. The audio signal is amplified further and fed into the speaker.
If the radio can receive multiple bands (AM, FM, SW, LW), for each band there is 2 different variable capacitors (AM and LW may share the same variable capacitors).
Some simpler AM radios had wide band radio amplifier and a fixed capacitor with one variable coil for the oscillator.
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