Digital Modulation Techniques - Digital Communication: Principles and System Modelling - page 115

6.3 Frequency Shift Keying (FSK)

Frequency Shift Keying is analogous to Frequency Modulation (FM) in analog
domain, where the slope of the time varying angle of the signal, i.e., instantaneous
frequency of the carrier signal varies with respect to the baseband signal. In unipolar
or bipolar formatting of the digital symbol, there exist two amplitude levels defined
for two logic values one and zero. Therefore, two sinusoidal carrier waves of the
same amplitude A c but different frequencies f H and f L are used to represent the two
symbols. The binary FSK wave may be represented as shown in Fig. 6.10 .

Digital data: 10110

Fig. 6.10 FSK signal

High Frequency Carrier

2

0

-2

0 1 2 3 4 5

t-->

Low Frequency Carrier

2

0

-2

0 1 2 3 4 5

t-->

FSK

2

0

-2

0 1 2 3 4 5

t-->

6.3.1 Mathematical Model

According to the above discussion, the scheme of FSK modulation can therefore be
described in a tabular form as shown in Table 6.2 .

Table 6.2 FSK waveform
definition

Data to be transmitted [d(t)] FSK

Binary 1 A c cos ω H t

Binary 0 A c cos ω L t

The last column can be rewritten as,

ϕ FSK ( t ) = A c cos ω H tfd ( t ) = Logic 1 (6.11)