2. 2
Modulation of Digital Data
Carrier Signal - High-frequency signal used for
digital-to-analog or analog-to-digital modulation
May change any aspect of an analog signal to
represent digital data: amplitude, frequency, and
phase
Modulation - process of changing one of the
characteristics of an analog signals based on
information in digital signal
4. Types of methods
Amplitude shift keying (ASK): some audio modem, optical
fiber, printer
Frequency shift keying (FSK): Computer Modem, HF and
VHF radio
Amplitude shift keying (PSK): Mobile, Satellite
(microwave) etc
Digital to Analog
Modulation
FSK
QAM
PSK
ASK
Bit rate is the number of bits transmitted per second. Baud rate is the
number of signal elements transmitted per second. Baud rate is less than or
equal to the bit rate
5. 5
Amplitude Shift Keying (ASK)
Strength of carrier signal is varied to represent a
1 or 0
Frequency and phase remain constant while
amplitude is changed
Amplitude remains constant during bit duration
Highly susceptible to noise interference since
noise usually affects amplitude
Minimum bandwidth required is equal to baud
rate
8. 8
Example 3
Find the minimum bandwidth for an ASK signal
transmitting at 2000 bps. The transmission mode is half-
duplex.
Solution
In ASK the baud rate and bit rate are the same. The baud
rate is therefore 2000. An ASK signal requires a
minimum bandwidth equal to its baud rate. Therefore,
the minimum bandwidth is 2000 Hz.
9. 9
Frequency Shift Keying (FSK)
Frequency is varied to represent a 1 or 0
Frequency during bit duration is constant
Amplitude and phase remain constant
Avoids most of noise problems of ASK; can
ignore voltage spikes
Limited by physical capabilities of medium
Bandwidth required is equal to baud rate plus
frequency shift
12. 12
Example 6
Find the minimum bandwidth for an FSK signal
transmitting at 2000 bps. Transmission is in half-duplex
mode, and the carriers are separated by 3000 Hz.
Solution
For FSK
BW = baud rate + fc1 - fc0
BW = bit rate + fc1 - fc0 = 2000 + 3000 = 5000 Hz
13. 13
Phase Shift Keying (PSK)
Phase of carrier is varied to represent 1 or 0
Peak amplitude and frequency remain constant
Phase remains constant during each bit duration
Not as susceptible to noise degradation as ASK
or to bandwidth limitations of FSK
15. 15
PSK constellation (Phase state diagram)
Constellation is a graphical representation of the phase
and amplitude of different bit combinations in digital-to-
analog modulation.
16. 16
4-PSK; 8-PSK
4-PSK
May utilize four variations of phase shift by 90 degrees
Each phase shift represents 2 bits (dibit); technique is
referred to as 4-PSK
Allows data transmission two times as fast as 2-PSK
8-PSK
Vary signal by shifts of 45 degrees; each shift may
then represent three bits (tribit) and send data three
times as fast
21. 21
Example 8
Find the bandwidth for a 4-PSK signal transmitting at
2000 bps. Transmission is in half-duplex mode.
Solution
For 4-PSK the baud rate is one –half of the bit rate (1 baud :
2 bit). The baud rate is therefore 1000. A PSK signal requires
a bandwidth equal to its baud rate . Therefore, the bandwidth
is 1000 Hz
22. 22
Example 9
Given a bandwidth of 5000 Hz for an 8-PSK signal, what
are the baud rate and bit rate?
Solution
For PSK the baud rate is the same as the bandwidth,
which means the baud rate is 5000. But in 8-PSK the bit
rate is 3 times the baud rate, so the bit rate is 15,000 bps.
23. 23
Quadrature Amplitude Modulation (QAM)
Combination of ASK and PSK so that a maximum
contrast between each signal unit (bit, dibit, tribit, and
so on) is achieved
x number of variations in phase and y variations in
amplitude
Number of phase shifts is always larger than number
of amplitude shifts due to amplitude susceptibility to
noise
QAM is therefore less susceptible to noise than ASK
Same bandwidth is required for ASK and PSK
24. 24
Credits
All figures obtained from
Data Communications and Networking, 3rd edition by
Behrouz A. Forouzan. McGraw Hill Publishing, 2004