hi
i am interested in RF, i understand how AM circuits work, by changing the magnitude of the imput but i cant seem to find any info on how FM circuits work (if you google it you get lots of useless junk)
i mean yes i know that it varys the frequency of the wave but how?
in Am the signal is increased by adding more voltage so how in FM is the signal frequency changed?

 

The simplest way is by using a varicap diode, this is used as part of the oscillator tuning circuit. Feeding audio to it changes it's capacitance, thus altering the frequency. The way that used to be commonly used in amateur radio gear was called 'phase shift', this only produces small frequency changes, but as it took place at fairly low frequencies the shift was multiplied along with the frequency - also, it only needed narrow deviation.

 

if you are willing to buy a book then i suggest "Modern Digital And Analog Communicaiton Systems" by B.P.Lathi Oxford 3rd edition.

you'll find the answer in there.

__________________
Electrical Engineering student @ Ryerson University

 

There are different ways to change frequency of the oscillator.
Making frequency changes proportional to input signal is called FM.
One common way to do it is by altering capacitance of the circuit
using varicap diode. Many simple circuits do not use actual varicap
diode but they still do function pretty much the same way.

Take a look at http://www.mariucci.freeserve.co.uk/fm2.GIF

It's a nice little circuit with only one transistor.
Transistor works as an oscillator with tuned LC circuit
(L1 and CV) in the collector. The base is "grounded" by C2 and C3
(well, at least for the high frequency signals). Portion of the
output signal (from the collector) is brought back to emitter
via C4 so circuit can oscillate.

Great so you have an oscillator and you can change frequency using
CV but I doubt your hand can turn the CV back and forth fast enough
to call it modulation.

But if you bring audio signal to the base of the transistor through C1
you do get FM.

How? Well the trick is that this is LC oscillator. L is L1 of course
but C portion of our "LC" is not the CV (at least not CV alone).
There is another "capacitor" that's in parallel with the CV.
This "capacitor" is combination of C2 and parasitic capacitance
of the base-collector section of the transistor (let's call it Cbc).
C2 and Cbc are in series but C2 is MUCH bigger than the
parasitic capacitance of the base-collector section (0.1uF vs. few pF)
so C2 doesn't play any role in this. But changing Cbc (which acts like
regular varicap) you change capacitive component of the LC circuit.
By changing amplitude of the input signal, you change modulation
depth so you might want to put mic preamp like in
http://www.mariucci.freeserve.co.uk/fm1.GIF

Note, simple circuits sure can work but more complex designs
have better stability etc. If you build something like this make sure
to keep all connections short and choose capacitors with low
temperature coeficient (usually black tip).