Frequency modulation

Frequency Modulation is a Modulation Scheme where the incoming signal modulates the frequency of the Carrier Signal by a tiny fraction. Used e.g. in FM Radio.

A variation of Frequency Modulation is sometimes also used for digital Transmissions, this is then called Frequency Shift Keying or FSK-Modulation.

Visualisation

it would be nice to have a gnuplot picture here, is it possible to upload the gnuplot script for toying, too?

Mathematical Setting

The output voltage of a FM modulator with input voltage <math> u_{in} </math> is described by

<math> u_{out} = sin(2 \pi (f_{carrier} + u_{in} \cdot \Delta f)) </math>

where <math> \Delta f </math> is the frequency deviation from the center frequency at <math> u_{in} = 1V </math>.

Aquired Bandwidth

...can be determined using Carson's Bandwidth Rule: two times sum of the peak deviation <math> \Delta f </math> from the highest frequency occuring in the spectrum of the modulating signal <math> (f_m) </math>:

<math>bandwidth = 2 \ (\Delta f + f_{m}) </math>

Noise Immunity

is higher than the one of Amplitude Modulation Schemes since athmospheric disturbances and noise usually don't shift the frequency of a signal but add their contribution to the amplitude of the transmitted signal. Amplitude variations don't affect FM much.

Links

• Frequency Modulation on Wikipedia

• A nice introduction to the History of Radio on Wikipedia