JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission
Section: Analog Modulation

Frequency Modulation

For narrowband phase modulation or narrowband FM, we can approximate the RF signal as a linearly modulated signal. Both NBPM and NBFM look like AM with one sideband shifted 180 degree in phase. (See a discussion and an animation for details)


Figure: Phasor Diagram for Narrowband PM with tonal modulation.

In the animation below, we demonstrate the phasor motion for FM with tonal modulation. The three blue phasor represent

Note also that this almost gives exactly a constant envelope for the resulting signal: The phasor black in black has constant length, only its rotation relative to the x-axis changes. This rotation contains the modulated tone signal. In fact we would need to draw more sidebands to ensure an exactly constant envelope. The need for more sidebands implies that the RF bandwidth is larger than twice the bandwidth of the modulating signal.


Java Applet: Phasor diagram of a carrier with FM tone modulation. Blue: Carrier, and four side components. Black: Composite Signal.

Exercise

Carefully look at the above applet and try to find out the relative phases of each component. Note that the resulting signal no longer has constant envelope if you modify one of the phases.

That is, if the FM signal suffers phase distortion due to filtering, the signal will exhibit amplitude modulation effects.



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