# Démodulation Fsk

**Topics:**Frequency-shift keying, Modulation, Frequency modulation

**Pages:**29 (4103 words)

**Published:**January 15, 2013

Tech-note

Author: Bob Watson

FSK: Signals and Demodulation

The most commonly used signal parameters

for describing an FSK signal are shown in

Figure 2. The minimum duration of a mark

or space condition is called the element

length. Typical values for element length are

between 5 and 22 milliseconds, but element

lengths of less than 1 microsecond and greater

than 1 second have been used. Bandwidth

constraints in telephone channels and signal

propagation considerations in HF channels

generally require the element length to be

greater than 0.5 millisecond. An alternate way

of specifying element length is in terms of the

keying speed. The keying speed in “bauds” is

equal to the inverse of the element length in

seconds. For example, an element length of

20 milliseconds (.02 seconds) is equivalent to

a 50-baud keying speed.

Frequency measurements of the FSK signal

are usually stated in terms of “shift” and cen-

DATA

Binary FSK (usually referred to simply as

FSK) is a modulation scheme typically used

to send digital information between digital

equipment such as teleprinters and computers. The data are transmitted by shifting the frequency of a continuous carrier in a binary

manner to one or the other of two discrete

frequencies. One frequency is designated as

the “mark” frequency and the other as the

“space” frequency. The mark and space correspond to binary one and zero, respectively. By convention, mark corresponds to the higher

radio frequency. Figure 1 shows the relationship between the data and the transmitted signal.

quency and the mark or space frequencies.

The deviation is also equal, numerically, to

one-half of the shift.

FSK can be transmitted coherently or noncoherently. Coherency implies that the phase of each mark or space tone has a fixed phase

relationship with respect to a reference. This is

similar to generating an FSK signal by switch-

1

time (a)

0

SIGNAL

AMPLITUDE

BINARY FSK

ter frequency. The shift is the frequency difference between the mark and space frequencies. Shifts are usually in the range of 50 to 1000 Hertz. The nominal center frequency is

halfway between the mark and space frequencies. Occasionally the FM term “deviation” is used. The deviation is equal to the absolute

value of the difference between the center fre-

t

(b)

t

MARK

(c)

SPACE

FREQUENCY

Frequency shift keying (FSK) is the most

common form of digital modulation in the

high-frequency radio spectrum, and has

important applications in telephone circuits.

This article provides a general tutorial on FSK

in its many forms. Both modulation and

demodulation schemes will be discussed

Figure 1. FSK modulation. Binary data (a) frequency modulates the carrier to produce the FSK signal (b) which has the frequency characteristic (c).

FREQUENCY

NOMINAL

CENTER

FREQUENCY

F2

SHIFT

DEVIATION

DEVIATION

F1

t

ELEMENT

LENGTH

SHIFT = | F2 - F1 |

CTR FREQ =

DEVIATION =

F2 + F1

2

SHIFT F2 + F1

=

2

2

KEYING SPEED IN BAUDS =

1

ELEMENT LENGTH

(IN SECONDS)

Figure 2. FSK parameters.

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The Communications Edge ™

Tech-note

Author: Bob Watson

ing between two fixed-frequency oscillators to

produce the mark and space frequencies.

While this method is sometimes used, the

constraint that transitions from mark to space

and vice versa must be phase continuous

(“glitch” free) requires that the shift and keying rate be interrelated. A synchronous FSK signal which has a shift in Hertz equal to an

exact integral multiple (n = 1, 2,...) of the

keying rate in bauds, is the most common

form of coherent FSK. Coherent FSK is capable of superior error performance but noncoherent FSK is simpler to generate and is used for the majority...

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