FM-Modulation

[nathan_hale1]

Hi guys, this is my first post on this site.
I have a question about Frequency Modulation that i read about else where on this site. It says there "FM is the method of impressing data (intelligence) onto an alternating wave by varying the frequency of the wave......."

My question is...... if the frequency of the carrier wave is being varied, how does the receiving radio play the song ( for example) without me having to change the receiving frequency every second? How is someone able to just tune into 98.5 MHz ( for example ) and just keep listening. I mean if the frequency is changing in frequency modulation shouldn't the guy at the receiving radio continuously keep changing the frequency to a high or low freq. ? But this doesn't happen. People just tune into 98.5MHz and forget it and somehow the song is being played. How does it happen? Why is the end user not required to change the receiving freq. or change dials?

Thanks for your replies.

 

[cabletech]

Sounds like you are just getting into radio theroy. Several things, 1) go to your local liberay and lookup radio theroy books, there are several very good ones out there.

2) Do a google serch on radio theroy lots of good information.

The short answer. The 'frequency' of 98.5 is the actual radio frequency,(RF). The 'FM' part is the modulation part.

Think of the RF as a highway, and the modulation is the vehicles.

Now the modulation part is super imposed on the RF and the actual frequency does not change, just the modulation.

You can also think of this in a differant way, 112 volts at 60 cycles (HZ). 112v is the rf, 60hz is the modulation.

Good luck

 

[n5ims]

The 98.5 part is the center frequency and the FM frequency variation is the deviation from that center frequency. This (FM broadcasting - Wikipedia, the free encyclopedia) may help you understand this better than I could put it into a few brief sentences.

 

[nd5y]

The transmitter frequency doesn't change very much. When the transmitter is modulated it only goes +/- 75 kHz from the center carrier frequency which is way less than the bandwidth window of the receiver. An FM broadcast receiver receives at least 100 kHz higher and lower than the center frequency it is tuned to.

 

[mancow]

Visualize it as grooves in an old vinyl record. The grooves or "tracks" don't appear to move when you look at it. They are all nice and concentric. However, if you were to look with a high powered magnifier you would see small deviations that match up with the high and low points of the sound recorded on it.

So, to your radio station tuning question, think of it as a certain spot on the record. You could tune to that spot on the record repeatably by placing the needle in the same spot right? Well that same spot is similar to that same spot on the dial, the microscopic variations in the tracks are the "deviation" of the signal that are "discriminated" and converted to sound in proportion to audio.

The only difference in a practical sense is that instead of a physical vinyl disc which has a needle riding on it and vibrating back and forth to make sound the radio station is occupying a signal of a certain frequency range and small errors in the frequency are used in the similar manner. Of course, the radio station has the advantage of being able to change the information instantly whereas the record's information is permanently molded in the vinyl.


This is grossly oversimplified I know, but hopefully it helps to visualize it.

This is a link to a very old black and white video made by the Army years ago on youtube and it is one of the best tutorial's I've ever seen. It explains it all in simple easy to understand terms with visuals.

http://www.youtube.com/watch?v=3soSK5GjQiY