- Frequency modulation
telecommunications, frequency modulation (FM) conveys informationover a carrier waveby varying its frequency(contrast this with amplitude modulation, in which the amplitudeof the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital datacan be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying.
Suppose the baseband data signal (the message) to be transmitted is
and is restricted in amplitude to be
sinusoidal carrier is
where "fc" is the carrier's base frequency and "Ac" is the carrier's amplitude.The modulator combines the carrier with the baseband data signal to get the transmitted signal,
In this equation, is the "instantaneous frequency" of the oscillator and is the "
frequency deviation", which represents the maximum shift away from "fc" in one direction, assuming "x""m"("t") is limited to the range ±1.
Although it may seem that this limits the frequencies in use to "fc" ± "f"Δ, this neglects the distinction between "instantaneous frequency" and "spectral frequency". The
frequency spectrumof an actual FM signal has components extending out to infinite frequency, although they become negligibly small beyond a point.
harmonicdistribution of a sine wavecarrier modulated by a sine wave signal can be represented with Bessel functions - this provides a basis for a mathematical understanding of frequency modulation in the frequency domain.
As with other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the frequency of the carrier signal:
where is the highest modulating frequency of "x""m"("t"). If , the modulation is called "narrowband FM", and its bandwidth is approximately . If , the modulation is called "wideband FM" and its bandwidth is approximately . While wideband FM uses more bandwidth, it can improve
With a tone-modulated FM wave, if the modulation frequency is held constant and the modulation index is increased, the (non-negligible) bandwidth of the FM signal increases, but the spacing between spectra stays the same; some spectral components decrease in strength as others increase. If the frequency deviation is held constant and the modulation frequency increased, the spacing between spectra increases.
where is the peak deviation of the instantaneous frequency from the center carrier frequency (assuming is in the range ).
The noise power decreases as the signal power increases, therefore the SNR goes up significantly.
Wideband noise reduction
Since increases with the deviation ratio and the SNR is proportional to the deviation ratio, we can reduce signal power and keep the SNR constant as long as we increase the bandwidth.
The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals. Based on the
FM signals can be generated using either direct or indirect frequency modulation.
* Direct FM modulation can be achieved by directly feeding the message into the input of a VCO.
* For indirect FM modulation, the message signal is integrated to generate a phase modulated signal. This is used to modulate a crystal controlled oscillator, and the result is passed through a
frequency multiplierto give an FM signal ["Communication Systems" 4th Ed, Simon Haykin, 2001 ] .
A common method for recovering the information signal is through a
Applications in broadcasting
FM is commonly used at
VHF radio frequenciesfor high-fidelitybroadcasts of musicand speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radiosettings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.
Applications in hardware
FM is also used at intermediate frequencies by most analog VCR systems, including
VHS, to record the luminance (black and white) portion of the video signal. FM is the only feasible method of recording video to and retrieving video from magnetic tape without extreme distortion, as video signals have a very large range of frequency components — from a few hertzto several megahertz, too wide for equalizers to work with due to electronic noise below -60 dB. FM also keeps the tape at saturation level, and therefore acts as a form of noise reduction, and a simple limiter can mask variations in the playback output, and the FM captureeffect removes print-throughand pre-echo. A continuous pilot-tone, if added to the signal — as was done on V2000and many Hi-band formats — can keep mechanical jitter under control and assist timebase correction.
Applications in sound
FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis, was popularized by early digital
synthesizers and became a standard feature for several generations of personal computer sound cards.
Applications in radio
FM broadcasting" Edwin Armstrongpresented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New Yorksection of the Institute of Radio Engineerson November 6, 1935. The paper was published in 1936. [cite journal
first = E. H.
last = Armstrong
title = A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation
journal = Proceedings of the IRE
volume = 24
issue = 5
pages = 689–740
publisher = IRE
date= May 1936
doi = 10.1109/JRPROC.1936.227383]
As the name implies, wideband FM (W-FM) requires a wider
signal bandwidththan amplitude modulationby an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radiotransmission: hence the term " FM radio" (although for many years the BBCcalled it "VHF radio", because commercial FM broadcasting uses a well-known part of the VHFband; in certain countries, expressions referencing the more familiar wavelength notion are still used in place of the more abstract modulation technique name).
FM receivers employ a special detector for FM signals and exhibit a
phenomenoncalled capture effect, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically however, frequency driftor lack of selectivity may cause one station or signal to be suddenly overtaken by another on an adjacent channel. Frequency drift typically constituted a problem on very old or inexpensive receivers, while inadequate selectivity may plague any tuner.
An FM signal can also be used to carry a stereo signal: see
FM stereo. However, this is done by using multiplexingand demultiplexing before and after the FM process, and is not part of FM proper. The rest of this article ignores the stereo multiplexing and demultiplexing process used in "stereo FM", and concentrates on the FM modulation and demodulation process, which is identical in stereo and mono processes.
A high-efficiency radio-frequency
switching amplifiercan be used to transmit FM signals (and other constant-amplitude signals).For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a linear amplifier.This gives FM another advantage over other modulation schemes that require linear amplifiers, such as AM and QAM.
Frequency modulation can be regarded as a special case of
phase modulationwhere the carrier phase modulation is the time integral of the FM modulating signal.
Frequency-shift keying" is the frequency modulation using only a discrete number of frequencies. Morse codetransmission has been implemented this way, [Cite web
title=frequency-shift keying] as were most early telephone-line
modems and radioteletypeapplications.
By the phenomenon of "slope detection" whereby FM is converted to AM in a frequency-selective circuit tuned slightly away from the nominal signal frequency, AM receivers may detect some FM transmissions, though this does not provide an efficient method of detection for FM broadcasts.
FM modulation is also used in telemetry applications.
Carson bandwidth rule(Estimate of RF bandwidth required for an FM signal)
Frequency modulation synthesis(FM as an audio synthesis method)
FM-UWB(FM and Ultra Wideband)
Modulation, for a list of other modulation techniques
History of radio
FM broadcast band
* A. Bruce Carlson: "Communication systems, 2nd edition", McGraw-Hill, Inc, 1981, ISBN 0-07-085082-2
* [http://www.rfcafe.com/references/electrical/frequency_modulation.htm Frequency Modulation]
* [http://www.fas.org/man/dod-101/navy/docs/es310/FM.htm Frequency Modulation]
* [http://www.aerialsandtv.com/fmanddabradio.html FM receiving antennas]
* [http://www.r-type.org/static/add059.htm Ratio Detector with schematics]
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