How can a radio tune to the frequency of an FM station if the frequency is changing?

Question

How can a radio tune to the frequency of an FM station if the frequency is changing?

Answer 1

Assuming that you guys in the US use RDS on radio like us here in the UK (you will be able to tell this by seeing a little RDS logo on your radio), the answer is simple.

The AF flag on RDS (the LCD display on your radio will say AF on it in real small letters) contains information about a station's other FM frequencies, so that the radio can switch to a better signal whilst driving.

An example of this is when I am driving from one city listening to a BBC national radio station to another city, my radio switches frequency without me realising.  This sounds great, but can be rather annoying to.  Where I live, I like to listen to a local radio station BBC London, however as I live on the borders of London and the next county over (Berkshire), my radio keeps thinking it should be playing BBC Radio Berkshire and everytime I tune it into BBC London and drive down the end of my street it keeps switching to BBC Radio Berkshire.

I know DAB radio can do this too, but I'm not quite sure how as I have been out of the broadcast engineering game for a while.  Besides, I dont think the US uses DAB I think it maybe DRM or some such.  I'm sure one of the US radio techs here would know better than me.

Answer 2

FM, or frequency modulation, has to do with the type of "carrier wave," not the frequency of the wave that is carried. So, 102.7 FM broadcasts on 102.7mghz, but is sent on a carrier wave that travels by changing the frequency of the wave that delivers it.

AM, or amplitude modulation, is carried by a wave that is constant as far as frequency is concerned, but changes in size, (height would be the easiest way to describe it.) This wave, if I remember correctly, travels slower than the FM wave, which explains the difference in sound quality between the two.

It's been a VERY long time since I studied this in electronics school as a kid. But that is a basic rundown of how it works. VERY basic.

Answer 3

FM is frequency modulation. There is two ways to explain this.

the first it that the base frequency is sent out..say 107.1MHz. The audio varies the time that this has to happen. and this creates a voltage that the demodulator creates back in to a audio frequency.

The second is that the Audio alters the frequency by a slight amount ,up and down, that the demodulator creates a varying voltage that is the audio result.. at any rate the tuner is set to the base frequency and the receiver decodes it into sound.

Answer 4

Radio stations broadcast electomagnetic waves in the range of 88 to 107 megahertz or so.  Humans can hear sounds from about 20 Hz to about 20 kHz.  At the station, they  superimpose the signal (the song or whatever) onto the carrier frequency (the radio station's frequency, such as 99.1 MHz).  The superposition changes the carrier frequency only very slightly because the highest pitched sound waves are a good hundred times slower than the carrier frequency.  

So, on a carrier frequency of 99.1 megahertz, the total signal will vary from maybe 99.12 to 99.08, all in MHz.  That's why there are no even numbers in radio stations, it allows the signals to spread out a little further for stereo sound and error correction.