Frequency modulation

Passband modulation
Analog modulation
AM · SSB · QAM · FM · PM · SM
Digital modulation
FSK · MFSK · ASK · OOK · PSK · QAM
MSK · CPM · PPM · TCM · SC-FDE
Spread spectrum
CSS · DSSS · FHSS · THSS
See also: Demodulation, modem,
line coding, PAM, PWM, PCM

In telecommunications and signal processing, frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant. In analog applications, the difference between the instantaneous and the base frequency of the carrier is directly proportional to the instantaneous value of the input-signal amplitude. Digital data can be sent by shifting the carrier's frequency among a range of settings, a technique known as frequency-shift keying. FSK (digital FM) is widely used in data and fax modems. Morse code transmission has been sent this way, and FASK was used in early telephone-line modems.[1] Radioteletype also uses FSK.[2] FM modulation is also used in telemetry, radar, seismic prospecting and newborn EEG seizure monitoring.[3] Frequency modulation is known as phase modulation when the carrier phase modulation is the time integral of the FM signal. FM is widely used for broadcasting music and speech, two-way radio systems, magnetic tape-recording systems and some video-transmission systems. In radio systems, frequency modulation with sufficient bandwidth provides an advantage in cancelling naturally-occurring noise.

Contents

Theory

If the baseband data signal (the message) to be transmitted is x_m(t) 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 Ac 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} %2B f_{\Delta} x_{m}(\tau) \right] d \tau \right)
= A_{c} \cos \left( 2 \pi f_{c} t %2B 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 infinitely, although they become negligible beyond a certain point.

Sinusoidal baseband signal

Simply stated, a baseband modulated signal may be approximated by a sinusoidal continuous wave signal with a frequency fm. The integral of such a signal is:

\int_{0}^{t}x_m(\tau)d \tau = \frac{A_m \cos (2 \pi f_m t)}{2 \pi f_m}\,

In this case, equation (1) above simplifies to:

y(t) = A_{c} \cos \left( 2 \pi f_{c} t %2B \frac{f_{\Delta}}{f_{m}} \cos \left( 2 \pi f_{m} t \right) \right)\,

where the amplitude A_{m}\, of the modulating sinusoid is represented by the peak deviation f_{\Delta}\, (see frequency deviation).

The harmonic distribution of a sine wave carrier modulated by such a sinusoidal signal can be represented with Bessel functions; this provides the basis for a mathematical understanding of frequency modulation in the frequency domain.

Modulation index

As in other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to variations in the carrier frequency:

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

where f_m\, is the highest frequency component present in the modulating signal xm(t), and \Delta{}f\, is the peak frequency-deviation—i.e. the maximum deviation of the instantaneous frequency from the carrier frequency. 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_\Delta\,. While wideband FM uses more bandwidth, it can improve the signal-to-noise ratio significantly; for example, doubling the value of \Delta{}f\,, while keeping f_m constant, results in an eight-fold improvement in the signal-to-noise ratio.[4] (Compare this with Chirp spread spectrum, which uses extremely wide frequency deviations to achieve processing gains comparable to traditional, better-known spread-spectrum modes).

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 remains 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.

Frequency modulation can be classified as narrowband if the change in the carrier frequency is about the same as the signal frequency, or as wideband if the change in the carrier frequency is much higher (modulation index >1) than the signal frequency. [5] For example, narrowband FM is used for two way radio systems such as Family Radio Service, in which the carrier is allowed to deviate only 2.5 kHz above and below the center frequency with speech signals of no more than 3.5 kHz bandwidth. Wideband FM is used for FM broadcasting, in which music and speech are transmitted with up to 75 kHz deviation from the center frequency and carry audio with up to a 20-kHz bandwidth.

Carson's rule

Main article: Carson bandwidth rule

A rule of thumb, Carson's rule states that nearly all (~98 percent) of the power of a frequency-modulated signal lies within a bandwidth B_T\, of:

\ B_T = 2(\Delta f %2Bf_m)\,

where \Delta f\,, as defined above, is the peak deviation of the instantaneous frequency f(t)\, from the center carrier frequency f_c\,.

Noise quieting

Noise power decreases as signal power increases; therefore, the SNR increases significantly.

Bessel functions

The carrier and sideband amplitudes are illustrated for different modulation indices of FM signals (based on Bessel functions):

Modulation
index		 Sideband
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  −0.05 0.50 0.45 0.22 0.07 0.02 0.01
3.0  −0.26 0.34 0.49 0.31 0.13 0.04 0.01
4.0  −0.40 −0.07 0.36 0.43 0.28 0.13 0.05 0.02
5.0  −0.18 −0.33 0.05 0.36 0.39 0.26 0.13 0.05 0.02
5.53 0    −0.34 −0.13 0.25 0.40 0.32 0.19 0.09 0.03 0.01
6.0  0.15 −0.28 −0.24 0.11 0.36 0.36 0.25 0.13 0.06 0.02
7.0  0.30 0.00 −0.30 −0.17 0.16 0.35 0.34 0.23 0.13 0.06 0.02
8.0  0.17 0.23 −0.11 −0.29 −0.10 0.19 0.34 0.32 0.22 0.13 0.06 0.03
8.65 0    0.27 0.06 −0.24 −0.23 0.03 0.26 0.34 0.28 0.18 0.10 0.05 0.02
9.0  −0.09 0.25 0.14 −0.18 −0.27 −0.06 0.20 0.33 0.31 0.21 0.12 0.06 0.03 0.01
10.0  −0.25 0.04 0.25 0.06 −0.22 −0.23 −0.01 0.22 0.32 0.29 0.21 0.12 0.06 0.03 0.01
12.0  0.05 −0.22 −0.08 0.20 0.18 −0.07 −0.24 −0.17 0.05 0.23 0.30 0.27 0.20 0.12 0.07 0.03 0.01

Implementation

Modulation

FM signals can be generated using either direct or indirect frequency modulation:

Demodulation

See also: Detectors

Many FM detector circuits exist. A common method for recovering the information signal is through a Foster-Seeley discriminator. A phase-locked loop can be used as an FM demodulator. Slope detection demodulates an FM signal by using a tuned circuit which has its resonant frequency slightly offset from the carrier. As the frequency rises and falls the tuned circuit provides a changing amplitude of response, converting FM to AM. AM receivers may detect some FM transmissions by this means, although it does not provide an efficient means of detection for FM broadcasts.

Applications

Magnetic tape storage

FM is also used at intermediate frequencies by analog VCR systems (including VHS) to record both the luminance (black and white) portions of the video signal. Commonly, the chrome component is recorded as a conventional AM signal, using the higher-frequency FM signal as bias. FM is the only feasible method of recording the luminance ("black and white") component of video to (and retrieving video from) magnetic tape without distortion; video signals have a large range of frequency components – from a few hertz to several megahertz, too wide for equalizers to work with due to electronic noise below −60 dB. FM also keeps the tape at saturation level, acting as a form of noise reduction; a limiter can mask variations in 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.

These FM systems are unusual, in that they have a ratio of carrier to maximum modulation frequency of less than two; contrast this with FM audio broadcasting, where the ratio is around 10,000. Consider, for example, a 6-MHz carrier modulated at a 3.5-MHz rate; by Bessel analysis, the first sidebands are on 9.5 and 2.5 MHz and the second sidebands are on 13 MHz and −1 MHz. The result is a reversed-phase sideband on +1 MHz; on demodulation, this results in unwanted output at 6−1 = 5 MHz. The system must be designed so that this unwanted output is reduced to an acceptable level.[7]

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 in several generations of personal computer sound cards.

Radio

Main article: FM broadcasting

Edwin Howard Armstrong (1890–1954) was an American electrical engineer who invented wideband frequency modulation (FM) radio.[8] He patented the regenerative circuit in 1914, the superheterodyne receiver in 1918 and the super-regenerative circuit in 1922.[9] 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.[10]

As the name implies, wideband FM (WFM) requires a wider signal bandwidth than amplitude modulation by an equivalent modulating signal; this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against 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 part of the VHF band—the FM broadcast band). FM receivers employ a special detector for FM signals and exhibit a phenomenon known as the capture effect, in which the tuner "captures" the stronger of two stations on the same frequency while rejecting the other (compare this with a similar situation on an AM receiver, where both stations can be heard simultaneously). However, frequency drift or a lack of selectivity may cause one station to be overtaken by another on an adjacent channel. Frequency drift was a problem in early (or inexpensive) receivers; inadequate selectivity may affect any tuner.

An FM signal can also be used to carry a stereo signal; this is done with multiplexing and demultiplexing before and after the FM process. The FM modulation and demodulation process is identical in stereo and monaural 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 methods requiring linear amplifiers, such as AM and QAM.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM. Narrowband FM is used for voice communications in commercial and amateur radio settings. In broadcast services, where audio fidelity is important, wideband FM is generally used. In two-way radio, narrowband FM (NBFM) is used to conserve bandwidth for land mobile, marine mobile and other radio services.

See also

References

  1. ^ Stan Gibilisco (2002). Teach yourself electricity and electronics. McGraw-Hill Professional. p. 477. ISBN 9780071377300. http://books.google.com/?id=-Q6SBAKsmXkC&pg=PA477&dq=morse-code+frequency-shift-keying+sent-using-fsk. 
  2. ^ David B. Rutledge (1999). The Electronics of Radio. Cambridge University Press. p. 310. ISBN 9780521646451. http://books.google.com/?id=ZvJYLhk4N64C&pg=RA2-PA310&dq=radio-teletype+fsk. 
  3. ^ B. Boashash, editor, “Time-Frequency Signal Analysis and Processing – A Comprehensive Reference”, Elsevier Science, Oxford, 2003; ISBN 0080443354
  4. ^ Der, Lawrence, Ph.D., Frequency Modulation (FM) Tutorial, http://www.scribd.com/Silicon-Labs-Fm-Tutorial/d/6217907, Silicon Laboratories, Inc., accessed 2011 April 29, p. 5
  5. ^ B. P. Lathi, Communication Systems, John Wiley and Sons, 1968 ISBN 0 471 51832 8, p, 214–217
  6. ^ "Communication Systems" 4th Ed, Simon Haykin, 2001
  7. ^ : "FM Systems Of Exceptional Bandwidth" Proc. IEEE vol 112, no. 9, p. 1664, September 1965
  8. ^ A. Michael Noll (2001). Principles of modern communications technology. Artech House. p. 104. ISBN 9781580532846. http://books.google.com/books?id=6tDEZlwiMK0C&pg=PA104. 
  9. ^ US 1342885 
  10. ^ Armstrong, E. H. (May 1936). "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation". Proceedings of the IRE (IRE) 24 (5): 689–740. doi:10.1109/JRPROC.1936.227383. 

Further reading

External links