Frequency modulation

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“FM” redirects here. For other uses, see FM (disambiguation).

Topics in Modulation techniques

Analog modulation

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Digital modulation

ASK | PSK | FSK | QAM | OFDM | MSK

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Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. (Contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant.) In analog applications, the carrier frequency is varied in direct proportion to changes in the amplitude of an input signal. Digital data can be represented by shifting the carrier frequency among a set of discrete values, a technique known as frequency-shift keying.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrowband form is used for voice communications in commercial and amateur radio settings. 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.

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 hertz to several megahertz, too wide for equalisers 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 capture effect removes print-through and pre-echo. A continuous pilot-tone, if added to the signal — as was done on V2000 and many Hi-band formats — can keep mechanical jitter under control and assist timebase correction.

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.

An audio signal (top) may be carried by an AM or FM radio wave.

1 Applications in radio
2 Theory
- 2.1 Modulation index
- 2.2 Carson's rule
3 Miscellaneous
4 See also
5 External links
6 References

[edit] Applications in radio

An example of frequency modulation. The top diagram shows the modulating signal superimposed on the carrier wave. The bottom diagram shows the resulting frequency-modulated signal.

Edwin Armstrong presented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935. The paper was published in 1936. ^[1]

Wideband FM (W-FM) requires a wider bandwidth than amplitude modulation by 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 radio transmission: hence the term "FM radio" (although for many years the BBC insisted on calling it "VHF radio", because commercial FM broadcasting uses a well-known part of the VHF band; 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 phenomenon called capture, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically, however, frequency drift or 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 multiplexing and 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.

[edit] Theory

Suppose the signal to be transmitted is

$x_m(t)\,$

and is restricted in amplitude to be

$\left| x_m(t) \right| \le 1, \,$

and the sinusoidal carrier is

$x_c(t) = A \cos (2 \pi f_c t)\,$

where f_c is the carrier's base frequency and A is an arbitrary amplitude. Then the carrier will be modulated by the signal as in

$x_c(t) = A \cos \left( 2 \pi \int_{0}^{t} f(\tau)\, d \tau \right) = A \cos \left( 2 \pi \int_{0}^{t} \left[ f_c + f_\Delta x_m(\tau) \right] \, d \tau \right)$

where $f (t) = f c + f Δ x m (t).$

In this equation, f(t) is the instantaneous frequency of the oscillator and f_Δ is the frequency deviation, which represents the maximum shift away from f_c 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 f_c ± f_Δ, this neglects the distinction between instantaneous frequency and spectral frequency. The frequency spectrum of an actual FM signal has components extending out to infinite frequency, although they become negligibly small beyond a point.

For a simplified case, the harmonic distribution of a sine wave signal modulated by another sine wave signal can be represented with Bessel functions - this provides a basis for a mathematical understanding of frequency modulation in the frequency domain.

[edit] Modulation index

As with other modulation indices, in FM this quantity indicates by how much the modulated variable varies around its unmodulated level. For FM, it relates to the variations in the frequency of the carrier signal:

$h = \frac{\Delta{}f}{f_m} = \frac{f_\Delta |x_m(t)|}{f_m} \$

If $h \ll 1$ , the modulation is called narrowband FM, and its bandwidth is approximately $2 f m$ . If $h \gg 1$ , the modulation is called wideband FM and its bandwidth is approximately $2 f Δ$ . While wideband FM uses more bandwidth, it can improve signal-to-noise ratio significantly.

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. If the frequency deviation is held constant and the modulation index increased, the bandwidth stays roughly the same, but the spacing between spectra decreases.

[edit] Carson's rule

A rule of thumb, Carson's rule states that nearly all (~98%) of the power of a frequency-modulated signal lies within a bandwidth $B T$ of

$\ B_T = 2(f_\Delta +f_m)\,$

where f_Δ is the peak deviation of the instantaneous frequency f(t) from the center carrier frequency f_c (assuming x_m(t) is in the range ±1) and f_m is the highest modulating frequency of x_m(t).

[edit] Miscellaneous

Note that frequency modulation can be regarded as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal.

Frequency-shift keying is the simple case of frequency modulation by a simple signal with only discrete states, such as in Morse code or radioteletype applications.

Manchester encoding may be regarded as a simple version of frequency shift keying, where the high and low frequencies are respectively double and the same as the bit rate, and the bit transitions are synchronous with carrier transitions.

When used in supervisory signaling in telephony, the term frequency-change signaling has been used to describe frequency modulation.

The phrase frequency-modulated, an adjective, should have a hyphen when used attributively.

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.

[edit] See also

Amplitude modulation
Carson bandwidth rule (Estimate of RF bandwidth required for an FM signal)
Frequency modulation synthesis (FM as an audio synthesis method)
Modulation, for a list of other modulation techniques
History of radio
Phase modulation

[edit] External links

Frequency Modulation
Frequency Modulation
" Modulation Trainers" - Digital and Analog communication trainer kits PCM, DPCM, ASK, FSK, PSK, DPSK, QPSK ...

[edit] References

^ Armstrong, E. H. (May 1936). "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation". Proceedings of the IRE 24 (5): 689-740.