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The Wikipedia article is very short and doesn't explain the concept very well, and there aren't any other sites that I can find that give a simple explanation. What does it mean by 100% modulation? I understand the basic concepts behind amplitude modulation, frequency modulation, and pulse width modulation, but I have never really understood what is meant by the "amount" of modulation or modulation depth.
Can somebody please shed some light on the subject?
Thanks!
In the image below a amplitude modulated sine wave:
A still overview of the most important modulation depths (0, 50, 100 and 200%):
The animation was created using gnuplot using the following script:
unset xtics
set yrange [-3:3]
set samples 10000
do for [d=0:200] { plot sin(2*pi*3*x)*(1+(sin(2*pi*x/10))*d/100) title sprintf("%3i%%",d); }
AM over modulation causes the carrier wave to invert it's phase when the modulating signal has an amplitude that is above a certain level. Broadcast AM typically never does this because the complexity of an accurate demodulator is too great for the thousands and millions of receivers.
AM is just the mathematical multiplication of two signals and regular broadcast AM remains as a 2 quadrant multiplier whereas full modulation uses all four quadrants.
100% modulation is where the modulating signal drives the carrier to zero and is theoretically the maximum modulation that can be successfully demodulator by a regular AM envelope detector.
Over modulation isn't really of any significance to FM systems (unlike AM). If the modulation signal amplitude is too great, any decent frequency modulator will limit the signal so that it can't push the bandwidth of the modulated signal too wide in the frequency spectrum. In effect, the modulating signal becomes clipped.
With amplitude modulation, the carrier signal is alternately driven above and below its resting (unmodulated) value. At 100% modulation, the peaks will be driven to twice the resting power, and the "valleys" will reach zero. If the modulation level is increased beyond this value, the valleys will attempt to go below zero. Since the power level can't go below zero, the signal will "clip" instead, producing distortion. The peaks may clip as well, but that will be dependent on the capability of the power stage.
For frequency modulation, going past the designed deviation value will produce a signal whose bandwidth exceeds that for which the receiver was designed. The receiver will experience distortion because part of the signal has been filtered out, and the excessive bandwidth of the transmitted signal may interfere with stations on adjacent frequencies.
