Frequency modulation

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Frequency modulation (FM) is a form of modulation in which the frequency of a carrier is altered to convey information. There are variations of FM in electronic and electromagnetic (i.e., wireless) transmission systems. The information conveyed is analog, including such things as voice, music, and video. A form of frequency modulation can also be used to convey digital information, but it is usually called frequency shift keying (FSK) and will not be discussed here.

Analog Baseband Signal

FM in More Detail

An FM transmission is one in which a carrier at a particular frequency is established, and then modulated by adjusting the instantaneous frequency of the carrier sine wave proportionate to a given baseband signal. In FM, the baseband signal is an analog signal. In FSK, the baseband signal is digital.

Carrier Signal

FM is more noise immune than amplitude modulation AM because it is not sensitive to changes in signal power (i.e., amplitude) due to external sources, and can maintain the same signal output even in the face of a weakened signal. Rather than fading in and out, a FM signal is more likely to exhibit the cliff effect, remaining strong until it disappears completely. Another aspect of FM transmission is that an FM receiver is typically capable of locking onto the stronger of two competing FM signals, a phenomenon known as the capture effect. In an AM radio systems, these two signals would be blended and played simultaneously.

In general, FM requires more bandwidth than AM when modulated by the same baseband signal. This makes FM somewhat less spectrally inefficient but more robust and noise immune.

Examples of FM Systems

Frequency Modulated Signal

FM is found in radio transmission systems. In North America it occupies the VHF radio frequency bands and is used for high-fidelity transmission. Because it is capable of high-fidelity transmission, most music stations now use FM transmission technology. The audio component of an analog television signal is also transmitted using FM. This is found both in broadcast television and on the analog channels in an HFC network deployed by a cable TV company.

FM is also used by most analog VCR systems, most notably VHS. It captures and conveys the luminance (e.g., light intensity of the pixels) component of the video.

See Also

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2008q2/frequency_modulation.mp3%7Cdownload</mp3> | Frequency modulation (FM)