AM vs FM Modulation

FM Modulation

Frequency modulation is achieved by changing the frequency of the carrier wave in response to the amplitude of the modulated signal. A higher-frequency carrier represents larger amplitude, and a lower frequency represents a lower amplitude. This is opposed to AM which changes the amplitude of the carrier wave in response to the modulated signal.

(image source: wikipedia)

The advantage of FM is a much higher signal integrity over AM because the frequency of a signal doesn't change with distance from source like it does with AM. This means that FM will have the same signal quality right up until its maximum range where the carrier wave is finally overwhelmed by noise and cannot be detected. The disadvantages are a larger bandwidth requirement for the signals since the frequency needs to be able to change within a set range for the modulation to work and the requirement for a more complex receiver since it is much more difficult to demodulate than AM.

There are several methods used to demodulate a FM signal. The most common is a Foster-Seeley discriminator, which uses a tuned RF transformer to convert the frequency changes into amplitude changes. This is done by feeding the FM signal into the center tap of the transformer. If the frequency of the signal equals the tuned frequency of the transformer, the two halves will be in balance and produce 0 voltage. Otherwise, a voltage imbalance is created that is proportional to the frequency.

A lot of newer radios will use a digital signal processor to demodulate FM as this eliminates the need for complex hardware in the receiver (assuming you already have a computer anyways).