Frequency-shift keying

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Passband modulation
Analog modulation
Digital modulation
Hierarchical modulation
Spread spectrum
See also
An example of binary FSK

Frequency-shift keying (FSK) is a

binary FSK (BFSK, which is also commonly referred to as 2FSK or 2-FSK), in which the carrier is shifted between two discrete frequencies to transmit binary (0s and 1s) information.[2]

Modulating and demodulating

Reference implementations of FSK modems exist and are documented in detail.[3] The demodulation of a binary FSK signal can be done using the Goertzel algorithm very efficiently, even on low-power microcontrollers.[4]

Variations

Multiple frequency-shift keying

Continuous-phase frequency-shift keying

Main article:
continuous-phase frequency-shift keying

In principle FSK can be implemented by using completely independent free-running oscillators, and switching between them at the beginning of each symbol period. In general, independent oscillators will not be at the same phase and therefore the same amplitude at the switch-over instant, causing sudden discontinuities in the transmitted signal.

In practice, many FSK transmitters use only a single oscillator, and the process of switching to a different frequency at the beginning of each symbol period preserves the phase. The elimination of discontinuities in the phase (and therefore elimination of sudden changes in amplitude) reduces sideband power, reducing interference with neighboring channels.

Gaussian frequency-shift keying

Rather than directly modulating the frequency with the digital data symbols, "instantaneously" changing the frequency at the beginning of each symbol period, Gaussian frequency-shift keying (GFSK) filters the data pulses with a

DECT, Bluetooth,[5] Cypress WirelessUSB, Nordic Semiconductor,[6] Texas Instruments,[7] IEEE 802.15.4, Z-Wave and Wavenis devices. For basic data rate Bluetooth
the minimum deviation is 115 kHz.

A GFSK modulator differs from a simple frequency-shift keying modulator in that before the baseband waveform (with levels −1 and +1) goes into the FSK modulator, it passed through a Gaussian filter to make the transitions smoother to limit spectral width. Gaussian filtering is a standard way to reduce spectral width; it is called pulse shaping in this application.

In ordinary non-filtered FSK, at a jump from −1 to +1 or +1 to −1, the modulated waveform changes rapidly, which introduces large out-of-band spectrum. If the pulse is changed going from −1 to +1 as −1, −0.98, −0.93, ..., +0.93, +0.98, +1, and this smoother pulse is used to determine the

carrier frequency, the out-of-band spectrum will be reduced.[8]

Minimum-shift keying

Main article: Minimum-shift keying

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particular spectrally efficient form of coherent FSK. In MSK, the difference between the higher and lower frequency is identical to half the bit rate. Consequently, the waveforms that represent a 0 and a 1 bit differ by exactly half a carrier period. The maximum

orthogonal
.

Gaussian minimum-shift keying

Main article:
Gaussian minimum-shift keying

A variant of MSK called Gaussian minimum-shift keying (

GMSK) is used in the GSM mobile phone
standard.

Audio frequency-shift keying

Audio frequency-shift keying (AFSK) is a

binary
one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in performing the modulation at

carrier (using a conventional technique, such as AM or FM
) for transmission.

AFSK is not always used for high-speed data communications, since it is far less efficient in both power and bandwidth than most other modulation modes.[

AC-coupled
links, including most equipment originally designed to carry music or speech.

AFSK is used in the U.S.-based Emergency Alert System to notify stations of the type of emergency, locations affected, and the time of issue without actually hearing the text of the alert.

Multilevel frequency-shift keying

Phase 1 radios in the Project 25 system use 4-level frequency-shift keying (4FSK).[9][10]

Applications

1200 baud AFSK signal
Listen to an example of a 1200 baud AFSK-modulated signal.
Problems playing this file? See media help.

In 1910, Reginald Fessenden invented a two-tone method of transmitting Morse code. Dots and dashes were replaced with different tones of equal length.[11] The intent was to minimize transmission time.

Some early Continuous Wave (CW) transmitters employed an

keyed. Instead of turning the arc on and off, the key slightly changed the transmitter frequency in a technique known as the compensation-wave method.[12] The compensation-wave was not used at the receiver. Spark transmitters used for this method consumed a lot of bandwidth and caused interference, so it was discouraged by 1921.[13]

Most early telephone-line

audio cassettes.[15] AFSK is still widely used in amateur radio
, as it allows data transmission through unmodified voiceband equipment.

AFSK is also used in the United States'

NOAA
in the U.S.

The

CHU shortwave radio station in Ottawa, Ontario, Canada broadcasts an exclusive digital time signal encoded using AFSK modulation.[citation needed
]

Caller ID and remote metering standards

Frequency-shift keying (FSK) is commonly used over telephone lines for

remote metering
applications. There are several variations of this technology.

European Telecommunications Standards Institute

In some countries of

Dual-tone multi-frequency
(DTMF) system and a no-ring mode for meter-reading and the like. It's more of a recognition that the different types exist than an attempt to define a single "standard".

Telcordia Technologies

The

Bell 202
tone modulation. The data may be sent in SDMF – which includes the date, time and number – or in MDMF, which adds a NAME field.

British Telecom

British Telecom (BT) in the United Kingdom developed their own standard, which wakes up the display with a line reversal, then sends the data as CCITT v.23 modem tones in a format similar to MDMF. It is used by BT, wireless networks like the late Ionica, and some cable companies. Details are to be found in BT Supplier Information Notes (SINs) 227 Archived 2014-07-26 at the Wayback Machine(link broken 28/7/21) and 242 Archived 2014-07-26 at the Wayback Machine(link broken 28/7/21); another useful document is Designing Caller Identification Delivery Using XR-2211 for BT Archived 2016-03-06 at the Wayback Machine from the EXAR
website.

Cable Communications Association

The

V.23 tones. They developed a new standard rather than change some "street boxes" (multiplexors) which couldn't cope with the BT standard. The UK cable industry use a variety of switches: most are Nortel DMS-100; some are System X; System Y; and Nokia DX220. Note that some of these use the BT standard instead of the CCA one. The data format is similar to the BT one, but the transport layer is more like Telcordia Technologies, so North American
or European equipment is more likely to detect it.

See also

References

  1. ISBN 978-0-07-112672-4
    ., p 509
  2. ^ FSK: Signals and Demodulation (B. Watson) http://www.xn--sten-cpa.se/share/text/tektext/digital-modulation/FSK_signals_demod.pdf Archived 2012-09-07 at the Wayback Machine
  3. ^ Teaching DSP through the Practical Case Study of an FSK Modem (TI) http://www.ti.com/lit/an/spra347/spra347.pdf
  4. ^ FSK Modulation and Demodulation With the MSP430 Microcontroller (TI) http://www.ti.com/lit/an/slaa037/slaa037.pdf Archived 2012-04-06 at the Wayback Machine
  5. ^ ieeexplore.ieee.org, Sweeney, D.; "An introduction to bluetooth a standard for short range wireless networking" Proceedings. 15th Annual IEEE International ASIC/SOC Conference, Rochester, NY, US, 25-28 Sept. 2002, pp. 474–475. 2002.
  6. ^ Nordic Semiconductor. nRF24LU1+ Preliminary Product Specification v1.2Archived 2011-02-20 at the Wayback Machine
  7. ^ LPRF Archived 2017-01-04 at the Wayback Machine
  8. ^ Bhagwat, Pravin (10 May 2005). "Bluetooth: 1.Applications, Technology and Performance". p. 21. Retrieved 27 May 2015.
  9. ^ Essam Atalla et al. "A Practical Step Forward Toward Software-Defined Radio Transmitters". p. 1.
  10. ^ Steve Ford. "ARRL's VHF Digital Handbook". 2008. p. 6-2.
  11. ^ Morse 1925, p. 44; Morse cites British patent 2,617/11.
  12. ^ Bureau of Standards 1922, pp. 415–416
  13. ^ Little 1921, p. 125
  14. ^ Kennedy & Davis 1992, pp. 549–550
  15. ^ Peschke, Manfred; Pesche, Virgina (2016-12-24). "Kansas City Standard". swtpc.com. Archived from the original on 2016-12-24. Retrieved 2023-01-09.

External links

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