Eli5: what is the radio looking for when it auto scans and decides to stop on a radio station?

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Sometimes it will skip over stations that you can barely hear and decides to stop on static. What tells it to stop or keep going?

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Audio has a range of around 20Hz-20kHz. This is what we’d call the baseband (it’s the frequency of the signal that you are trying to send). Radio waves at 20Hz-20kHz do not travel that well and need ENORMOUS antennae (the length of an antennae is proportional to the wavelength of the signal and so inversely proportional to the frequency (the lower the frequency the longer the antenna..

Radio modulates this baseband signal with a much higher frequency (called the carrier frequency) that is easier to transmit and can use realistically sized antennae. The carrier frequency is just a pure sinewave at the specified frequency. Modulation is just superimposing the baseband signal onto this carrier frequency. There are different ways to do this (AM, FM, PM, etc), but the resulting waveform looks a lot like a somewhat distorted sinewave at the carrier frequency.

When you select a frequency on your radio, what you are doing is adjusting a filter that eliminates frequencies outside of the radio station you selected. AM stations are listed by a number which reflects the station’s carrier frequency in KHz. For example AM 610 is a station that broadcasts with a carrier frequency of 610kHz. FM stations are in MHz, so 101.5 FM uses a carrier frequency of 101.5 MHz.

To determine if a radio station is in range you just looks at the amplitude of that station’s carrier frequency. If it sees a large amplitude it assumes the radio station is in range. If the amplitude is small then it assumes the radio station is out of range. Even if the radio station is broadcasting silence (record ended with DJ out of the room for example), the carrier frequency will still be present at full amplitude and is detectable.