Page 93 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes 6.4.2 Frequency Shift Keying

FSK describes the modulation of a carrier (or two carriers) by using a different frequency for a
1 or 0. In this technique the frequency of the carrier signal is changed according to the data while
keeping the amplitude and phase constant. The transmitter sends different frequencies for a 1
than for a 0 as shown in Figure 6.9. The resultant modulated signal may be regarded as the sum
of two amplitude modulated signals of different carrier frequency.
Mathematically, the modulated wave y(t) can be shown as y(t) = f1(t) sin(2pfc1t + j) + f2(t)
sin(2pfc2t + j) where fc1 and fc2 are different carrier frequencies of two different signals. FSK is
classified as wide band if the separation between the two carrier frequencies is larger than the
bandwidth of the spectrums. Narrow-band FSK is the term used to describe an FSK signal whose
carrier frequencies are separated by less than the width of the spectrum than ASK for the same
modulation.

Figure 6.9: Frequency Shift Keying

FSK modulation

Carrier wave 1

Carrier wave 0

Signal

The advantage of FSK is that it provides better immunity from noise because the receiving
device looks for specific frequency changes over given number of periods and frequency is
almost unaffected from noise. The disadvantages of this technique are that again as it was with
amplitude modulation. The rate of frequency changes is limited by the bandwidth of the line,
and that distortion caused by the lines makes the detection even harder than amplitude
modulation. Today this technique is used in low rate asynchronous modems up to 1200 baud
only.
The bandwidth for FSK signal is the sum of the baud rate of the signal and the frequency shift.
The frequency shift is the difference between the two carrier frequencies.
6.4.3 Phase Shift Keying (PSK)

In this modulation method a sine wave is transmitted and the phase of the sine wave carries the
digital data or the phase of sine wave is varied to represent binary 1 or 0 and both the amplitude
and frequency of the analog waveform are kept constant. For a 0, a 0 degrees phase sine wave is
transmitted. For a 1, a 180 degrees sine wave is transmitted. As this method involves two states
of phase changes, it is called binary PSK or 2-PSK. This technique, in order to detect the phase of
each symbol, requires phase synchronization between the receiver’s and transmitter’s phase.

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