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Radio Electronics: Frequency Modulation (FM)

Signals that interfere with an intentional electronic broadcast are called static, and static is the main drawback of AM radio. To counteract static, a better method of superimposing information on a radio wave, called frequency modulation or FM, was developed in 1933.

In frequency modulation, the intensity of the carrier wave isn't varied. Instead, the exact frequency of the carrier wave is varied in sync with the audio signal. When the audio signal is higher, the frequency of the broadcast signal goes up a little. When the audio signal is lower, the frequency slows down a bit.

You can see the carrier wave that clocks the specific frequency of the broadcast station. In the middle, you can see the audio signal that is to be superimposed on the carrier wave. And at the bottom, you can see the resulting modulated signal. As you can see, the frequency decreases when the input signal gets lower and increases when the input signal is higher.

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Note that the frequency variations in a frequency-modulated signal are all within a small proportion of the carrier-wave frequency. Typically, the frequency stays within 100 kHz of the base frequency.

FM radio stations broadcast at frequencies in the range of 88 to 108 MHz, but the base frequency for each station always ends in 0.1, 0.3, 0.5, 0.7, or 0.9. That’s why FM radio stations have frequencies such as 89.3 or 107.5, but never 92.0 or 98.6.

Assigning base frequencies in increments of 0.2 MHz gives each station 100 kHz of room on either side of the center frequency for its frequency modulation. Thus, a station broadcasting at 103.1 actually sends signals whose frequencies range from 103.0 to 103.2. Most stations limit the variation from the base frequency to 75 kHz to leave some margin for error. This helps prevent adjacent stations from interfering with one another.

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FM modulators usually use a type of electronic component called a varactor, which is a type of diode that has an unusual characteristic: It has capacitance like a capacitor, and its capacitance increases when voltage is applied across the diode. In essence, a varactor is a voltage-controlled variable capacitor. The schematic symbol for a varactor, shown in the margin, looks like a cross between a diode and a capacitor.

Varactors can be used in oscillator circuits to create an oscillator that vibrates faster when voltage increases. This ability makes it ideal for an FM radio modulator. As the voltage of the audio input increases, the capacitance of the varactor increases and thus the frequency of the oscillator increases. When the voltage decreases, the capacitance of the varactor decreases and so does the oscillator’s frequency.

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