Frequency-shift keying: Difference between revisions

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Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK). BFSK literally implies using a couple of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency. The time domain of an FSK modulated carrier is illustrated in the figures to the right.

Other forms of FSK

Minimum-shift keying

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly 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 used to represent a 0 and a 1 bit differ by exactly half a carrier period. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.

FSK is commonly used in Caller ID and remote metering applications: see FSK standards for use in Caller ID and remote metering for more details.

Audio FSK

Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone. Normally, the transmitted audio alternates between two tones: one, the "mark", represents a binary one; the other, the "space", represents a binary zero.

AFSK differs from regular frequency-shift keying in performing the modulation at baseband frequencies. In radio applications, the AFSK-modulated signal normally is being used to modulate an RF 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. In addition to its simplicity, however, AFSK has the advantage that encoded signals will pass through AC-coupled links, including most equipment originally designed to carry music or speech.

in FSK the baud=bitrate

transmitter

this uses a concept of modulation which we are familiar about. As the name indicates there is a combination of the input signala nd the carrier signal. The input signal and the carrier signal are combined in "BALANCE MODULATOR",i.e., there are 2transformers and

4diodes

working

  logic 1:
         the diodes d1 and d2 are in ON state 
         the diodes d3 and d4 are in OFF state
         the output is in phase with the input
 logic 0:
         the diodes d3 and d3 are in ON state 
         the diodes d1 and d2 are in OFF state
         the output is in phase with the input
===================================================

         d1 is connected between the positive of T1(transformer) and positive of T2
         d2 is connected between the negative of T1(transformer) and negative of T2
         d3 is connected between the positive of T1(transformer) and negative of T2
         d4 is connected between the negative of T1(transformer) and positive of T2

Applications

1200 baud AFSK signal

Listen to an example of a 1200 baud AFSK-modulated signal.

Problems playing this file? See media help.

Most early telephone-line modems used audio frequency-shift keying to send and receive data, up to rates of about 300 bits per second. The common Bell 103 modem used this technique, for example. Even today, North American caller ID uses 1200 baud AFSK in the form of the Bell 202 standard. Some early microcomputers used a specific form of AFSK modulation, the Kansas City standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows data transmission through unmodified voiceband equipment. Radio control gear uses FSK, but calls it FM and PPM instead.

AFSK is also used in the United States' Emergency Alert System to transmit warning information. It is used at higher bitrates for Weathercopy used on Weatheradio by NOAA in the U.S., and more extensively by Environment Canada.

The CHU shortwave radio station in Ottawa, Canada broadcasts an exclusive digital time signal encoded using AFSK modulation.

External links

dFSK: Distributed Frequency Shift Keying Modulation in Dense Sensor Networks

@@ Line 23: / Line 23: @@
 ===transmitter===
+ the carrier signal. The input signal and the carrier signal are combined in "BALANCE MODULATOR",i.e., there are 2[[transformers]] and
-=======================================================================================
- this uses a concept of [[modulation]] which we are familiar about. As the name indicates there is a combination of the input signal
- and the carrier signal. The input signal and the carrier signal are combined in "BALANCE MODULATOR",i.e., there are 2[[transformers]] and f
 [[diodes]]
- '''working'''
+'''working'''
    logic 1:
           the diodes d1 and d2 are in ON state