# Trying to understand FM

Thread Starter

#### DamageDealingMeatShield

Joined Jul 18, 2012
5
Hey all,

I've spent much of my time learning and building signal processing circuits for musicians (eg. distortion, wah, phasers, etc.) And started to learn about AM and FM radio. I've found AM to be real easy to understand and to implement. But with FM, it's more of a challenge to understand it. For instance, when looking at the following link, I see a tank circuit triggered via a transistor. Couldn't one replace the tank circuit with a sine wave oscillator and turn that on and off via a transistor or even a switch? Would that still be frequency modulation?

Thanks.

http://www.circuitstoday.com/medium-power-fm-transmitter-circuit

#### Papabravo

Joined Feb 24, 2006
12,554
No that would be on-off keying of a continuous wave. FM means that the modulating signal changes the frequency of the carrier. For example if the carrier is 144.96 MHz and the human voice produces a modulated band limited signal that is 5 kHz. wide and that signal is applied to the carrier in a unidirectional manner the signal would vary between 146.960 and 146.965
It is actually more complicated, but that is the executive summary.

http://en.wikipedia.org/wiki/Frequency_modulation

In the receiver those variations would be reconstructed into the original voice signal.

#### crutschow

Joined Mar 14, 2008
23,533
No that would be on-off keying of a continuous wave. FM means that the modulating signal changes the frequency of the carrier. For example if the carrier is 144.96 MHz and the human voice produces a modulated band limited signal that is 5 kHz. wide and that signal is applied to the carrier in a unidirectional manner the signal would vary between 146.960 and 146.965
It is actually more complicated, but that is the executive summary.

http://en.wikipedia.org/wiki/Frequency_modulation

In the receiver those variations would be reconstructed into the original voice signal.
The deviation frequency of the carrier is actually determined by the amplitude of the modulation signal and the modulation index, not its frequency. The rate the carrier frequency changes is determined by the modulation frequency. Thus if the carrier frequency is 144.96MHz and the maximum deviation frequency is 0.02MHz then a maximum amplitude 5KHz sinewave modulation signal will cause the carrier to deviate from 144.94MHz to 144.98MHz at a 5KHz rate. A half-maximum amplitude 5KHz modulation frequency will cause the carrier to deviate from 144.95MHz to 144.97MHz but still at the 5KHz rate.