Ask A Scientist© Engineering Archive

Author:      david
Subject:     FM modulation

I am having difficulty understanding how a signal is an FM signal 
modulated onto a carrier wave.  My text book does not explain it to my 
satisfaction.  AM modulation is fairly easy to understand, so maybe we can 
relate the two somehow.
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AM modulates the AMPLITUDE of a fixed frequency carrier; FM 
modulates the FREQUENCY of a fixed amplitude carrier.  Actually, it does not 
have to be fixed amplitude:  one of the main reasons for developing FM 
technology was to become immune to amplitude changes; like lightening produces 
static on AM but not on FM.

Jade
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First, let's look at AM.  You can find diagrams of what an AM wave
schematically looks like on the internet.  The wave envelope gets larger or
smaller according to the size of the modulating signal signal.   A circuit
in the receiving radio containing a diode (in simplest form) can follow the
amplitude.  The ³zero² point is the average amplitude of the signal.  When
the amplitude of the signal is a little larger than the average, then the
audio output from the detector is positive.  When the amplitude of the
signal is a little smaller than average, then the audio output from the
detector is negative.

[Side note:  so what's all this frequency sideband stuff?  Who needs
sidebands and extra frequencies for AM?  It looks like you are only changing
the amplitude of the wave, not its frequency.   Well, it turns out that if
you modulate a carrier wave, physics says that this is really as if there is
another frequency present.  Think about what happens when we combine two
sine waves of slightly different frequency.  When the signals are in phase,
the signals add to each other and the amplitude gets large.  When the
signals are out of phase, the signals subtract from each other and the
amplitude gets small.  For example, suppose you combine a 1,000,000 Hz wave
with a 1,000,100 Hz wave.  The resulting amplitude rises and falls at 100
Hz.  It is actually not perfect.  To get a perfectly in-phase wave that
rises and falls at 100 Hz you need 1,000,100 Hz and 999,900 Hz.  If you have
a large carrier wave at 1,000,000 Hz, and add small amounts of the previous
waves, you get small and perfect 100 Hz modulation.]

There are also nice diagrams of an FM modulated signal on the internet.  In
FM, the frequency shifts a little higher or lower depending on the signal
audio signal.  The amount the frequency deviates from the carrier represents
the amplitude of the modulating.  When the waves get scrunched closer
together (higher frequency) then that signifies a positive audio signal.
When the waves expand a bit (lower frequency) then that signifies a negative
audio signal.

The detection circuit for an AM radio "follows" the envelope of the radio
wave and demodulated audio signals come out.  That's easy to see.

The detection circuit for an FM radio is more complicated and has a circuit
that locks onto the average frequency of the FM signal, and another circuit
that looks at how far the signal frequency deviates from the average
frequency.  Higher in frequency ­ positive.  Lower in frequency ­ negative
(or vice versa)  Again, demodulated audio signals come out.   I hope that
helps.

Bob Erck
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