How does a radio receiver discriminate between different signals?


If there are lots of different radio signals with the same or very similar wavelengths, how is it possible for a radio to pick up just one signal?

In: Technology

Radio is based on oscillating waves. AKA differing frequencies that spike at different intervals on a graph. Some are really steep like a switchback while others are very shallow like a wandering path into a valley. The “steepness” or “frequency” of the wave pattern can be intercepted by an antenna tuned to the same pattern. These levels of steepness have been turned into numbers which then are indicated by your FM/AM radio dials and can fine tune into a single frequency being broadcast. Interference is a different story.

In general, it can’t.

If two signals over lap, both will be received and will, generally, garble the message. While frequency – the number of oscillations of the carrier signal – is important, bandwidth – the “width” of the information in hertz – is important, too.

For example, a radio station on 101.3 FM actually extends from 101.2 to 101.4. This spacing prevents signals from “clipping” into one another when adjacent channels are co-located. 101.1 doesn’t go past 101.2.

With two stations at the same frequency, they’re usually spaced far enough apart that the power of one broadcast is significantly weak within the area of reception of the other station, thus interference is avoided.

Propagation phenomena changes with different frequencies, though. AM broadcasts can travel farther at night, and some stations are required to reduce power at these times to reduce interference.

Additionally, it’s possible to encode transmissions to be dug out later. This is not uncommon in things like radars, where a code is embedded in the phase of the transmission and returns can be discriminated from interference despite being at the same frequency.

It isn’t. If two stations in the same area broadcast with too similar frequency they will interfere. This is why a radio station needs to apply to a state authority to get assigned a frequency, and the state agency has to make sure that there is always enough spacing between frequencies assigned to stations in the same area.

Having an assigned frequency is hence a big thing, and there is a lot of fighting and hassling for frequencies.

Things get more complicated with things like cell phones. Your phone will always stay in contact with the nearest cell tower, and that cell tower will communicate with many phones using the same frequency. That only works if the phones don’t transmit simultaneously. Hence, the tower tells each phone when it is allowed to send, switching from phone to phone many hundred times each second. The phones digitally compress the speech so that they can transmit, say, 10 millisecond of speech in their time slice even if that slice is only one millisecond long.

The wavelengths are just different enough that you can pick up individual signals. Technically speaking, you still might pick up sounds from other frequencies but they’re so high/low pitched compared to the wavelength you’re tuned for that you can’t really hear it.