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In FM, the carrier amplitude is kept at a constant; the frequency of the wave is altered  by a modulating signal. Let us look at the wave form again, and above it the modulating signal that causes it.

fig 2: FM wave and modulating signal causing it.  This drawing, and the previous one, are modified versions of ones produced by the US Navy, in its Navy Electricity and Electronics  Training Series (NEETS).



FM Wave



From this diagram  we can see that the frequency shifts (or more formally deviates) in accordance with the modulating signal; therefore we can glean two important rules for FM:


1. The ammount of frequency shift is proportional to the amplitude of the modulating signal.

2 The rate of frequency shift is proportional to the frequency of the modulating signal.


From this it follows that we can say the modulating audio changes the instantaneous frequency and not the amplitude of the carrier


This is something of a difficult concept and is best explained in practical terms. Consider a modulating signal of loudness x and having a frequency of 1000 Hz. Let the first cycle in fig 2 represent this graphically. The carrier wave will deviate from the nominal frequency by plus (first alternation) and minus (second alternation) x Hertz. It will do this 1000 times per second.  Let the second cycle of fig 2 represent the same loudness x, but half the frequency (500 Hz).  The same shift in frequency will be generated but  only at a rate of 500 times per second. Were we to reduce the loudness (i.e the amplitude) of the modulating signal, but keep the frequency of 500 Hz, then the rate of change would be 500 times per second as before, but the shift in the frequency of the carrier wave would be plus and minus x/2 Hz. Of course this is a simplified form, and a real wave would have a much more complex form, but from this we can glean an important insight:  the total power transmitted does not vary, but the carrier wave power does. This is different from AM entirely and as we shall see has a marked effect on the sidebands generated.


General Theoretical Concerns in FM II



Radio Principles