Frequency modulation

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"FM" redirects here. For other uses, see FM (disambiguation).
See also: Amplitude modulation
Modulation techniques
Analog modulation
AM · SSB · FM · PM · SM
Digital modulation
OOK · FSK · ASK · PSK · QAM
MSK · CPM · PPM · TCM · OFDM
Spread spectrum
v • d • e
FHSS · DSSS
See also: Demodulation

In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of 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 data can be sent by shifting the carrier's 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 narrow band 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.
An audio signal (top) may be carried by an AM or FM radio wave.

Contents

[edit] Applications in radio

An example of frequency modulation. This diagram shows the modulating, or message, signal, xm(t), superimposed on the carrier wave, xc(t)
An example of frequency modulation. This diagram shows the modulating, or message, signal, xm(t), superimposed on the carrier wave, xc(t)
The modulated signal, y(t), produced from frequency-modulating xc(t) with xm(t).
The modulated signal, y(t), produced from frequency-modulating xc(t) with xm(t).

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]

As the name implies, wideband FM (W-FM) requires a wider signal 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 called 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 effect, 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.

A high-efficiency radio-frequency switching amplifier can 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.

[edit] Theory

Suppose the baseband data signal (the message) 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_c \cos (2 \pi f_c t)\,

where fc is the carrier's base frequency and A_c is the carrier's amplitude. The modulator combines the carrier with the baseband data signal to get the transmitted signal,

y(t) = A_c \cos \left( 2 \pi \int_{0}^{t} f(\tau) d \tau \right)
= A_{c} \cos \left( 2 \pi \int_{0}^{t} \left[ f_{c} + f_{\Delta} x_{m}(\tau) \right] d \tau \right)
= A_{c} \cos \left( 2 \pi f_{c} t + 2 \pi f_{\Delta} \int_{0}^{t}x_{m}(\tau) d \tau \right)

In this equation, f(\tau)\, is the instantaneous frequency of the oscillator and f_{\Delta}\, is the frequency deviation, which represents the maximum shift away from fc in one direction, assuming xm(t) is limited to the range ±1.

Although it may seem that this limits the frequencies in use to fc ± fΔ, 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.

The harmonic distribution of a sine wave carrier 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.

[edit] Modulation index

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:

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

where fm is the highest modulating frequency of xm(t). If h \ll 1, the modulation is called narrowband FM, and its bandwidth is approximately 2fm. If h \gg 1, the modulation is called wideband FM and its bandwidth is approximately 2fΔ. 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; 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.

[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 BT of

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

where fΔ is the peak deviation of the instantaneous frequency f(t) from the center carrier frequency fc (assuming xm(t) is in the range ±1).

[edit] Bessel functions

The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals. Based on the Bessel functions.

Modulation index Carrier 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
0.00 1.00
0.25 0.98 0.12
0.5 0.94 0.24 0.03
1.0 0.77 0.44 0.11 0.02
1.5 0.51 0.56 0.23 0.06 0.01
2.0 0.22 0.58 0.35 0.13 0.03
2.41 0 0.52 0.43 0.20 0.06 0.02
2.5 −.05 0.50 0.45 0.22 0.07 0.02 0.01
3.0 −.26 0.34 0.49 0.31 0.13 0.04 0.01
4.0 −.40 −.07 0.36 0.43 0.28 0.13 0.05 0.02
5.0 −.18 −.33 0.05 0.36 0.39 0.26 0.13 0.05 0.02
5.53 0 −.34 −.13 0.25 0.40 0.32 0.19 0.09 0.03 0.01
6.0 0.15 −.28 −.24 0.11 0.36 0.36 0.25 0.13 0.06 0.02
7.0 0.30 0.00 −.30 −.17 0.16 0.35 0.34 0.23 0.13 0.06 0.02
8.0 0.17 0.23 −.11 −.29 −.10 0.19 0.34 0.32 0.22 0.13 0.06 0.03
8.65 0 0.27 0.06 −.24 −.23 0.03 0.26 0.34 0.28 0.18 0.10 0.05 0.02
9.0 −.09 0.25 0.14 −.18 −.27 −.06 0.20 0.33 0.31 0.21 0.12 0.06 0.03 0.01
10.0 −.25 0.04 0.25 0.06 −.22 −.23 −.01 0.22 0.32 0.29 0.21 0.12 0.06 0.03 0.01
12.0 0.05 −.22 −.08 0.20 0.18 −.07 −.24 −.17 0.05 0.23 0.30 0.27 0.20 0.12 0.07 0.03 0.01

[edit] Implementation

FM signals can be generated using either direct or indirect frequency modulation.
Direct FM can be achieved by directly feeding the message into the input of a VCO.
For indirect FM, 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 multiplier to give an FM signal. [2]

A common method for recovering the information signal is through a Foster-Seeley discriminator.

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

[edit] External links

[edit] References

  1. ^ 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. IRE. doi:10.1109/JRPROC.1936.227383. 
  2. ^ "Communication Systems" 4th Ed, Simon Haykin, 2001
  3. ^ "frequency-shift keying".
  • A. Bruce Carlson: "Communication systems, 2nd edition", McGraw-Hill, Inc, 1981, ISBN 0-07-085082-2
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