Démodulation Fsk

Topics: Frequency-shift keying, Modulation, Frequency modulation Pages: 29 (4103 words) Published: January 15, 2013
The Communications Edge ™

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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