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
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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