CHAPTER 3
Digital Modulation Techniques
What is digital modulation?
The techniques used to modulate digital information so that it can be transmitted
via microwave, satellite or down a cable pair are different to that of Analogue
transmission .
Types of Digital modulation techniques
1. Binary Signaling
2. QAM
3 .M ary signalling
1. Coherent technique
2. Non coherent technique
Fundamental digital modulation methods:
Binary Amplitude shift keying (on-off keying)
advantage:
simplicity
•disadvantage: ASK is very
susceptible to noise interference–
noise usually (only)affects the
amplitude, therefore ASK is the
modulation technique most
affected by noise
•application: ASK is
used to transmit digital data
over optical fiber
Equation for ask
carrier = Cos ωt
VASK =d x Cos ωt
d =data bits
VASK =Cos ωt for d=1
VASK =0 for d=0
Coherent ASK detection
Noncoherent ASK detection
Baud rate (Nb)
• For ASK we use 1 bit (0 or 1) to represent one
symbol. So the rate of symbol transmission
i.e. the baud rate will be same as the bit rate
Baud rate (Nb) =Bit rate (fb)
Bandwidth of ASK
BW= fc+ fb- fc+ fb = 2fb
Bandwidth of ASK
fmax=fc+ fb/2 , fmin=fc- fb/2
Frequency Shift Keying
Frequency Shift Keying (FSK) is a frequency modulation scheme (conveys
information over a carrier wave by varying its instantaneous frequency)
in which digital information is transmitted through discrete frequency
changes of a carrier wave .
FSK is the most common form of digital modulation in the high frequency radio
spectrum, and has important applications in telephone cct.
A modem converts the binary data from a computer to FSK for transmission
over telephone lines, cables, or wireless media.
Shifts are usually in the range of 50 to 1000Hz.
demodulation:
demodulator must be able to determine which of two possible frequencies is
present at a given time
advantage:
FSK is less susceptible to errors than ASK – receiver looks for specific frequency
changes over a number of intervals, so voltage (noise) spikes can be ignored
disadvantage:
FSK spectrum is 2 x ASK spectrum
application:
over voice lines, in high-freq. radio transmission, etc.
BFSK ( Binary Frequency Shift Keying )
General form of BFSK is :
Acos(2 f1t), binary 1
s(t)
Acos(2 f 2 t), binary 0
Here , in BFSK, the “1” is called the mark frequency and the “0” is called the space
frequency.
The optical field of FSK form is :
The total B.W of FSK is given by :
2Δf + 2B, where B is the bit rate.
FSK Trasmitter
Asynchronous FSK
Synchronous FSK
Synchronous FSK
Synchronous FSK
Synchronous FSK
BANDWIDTH OF FSK
Bandwidth of FSK
fHmax = fH+ fb
fHmin = fH- fb
fLmax = fL+ fb
fLmin = fL- fb
BW = fHmax - fLmin
= fH+ fb –{fL- fb}
= fH+ fb –fL+fb
= fH –fL +2fb
= 2fb +2fb
Advantages of FSK
1.Simple process to construct the circuit
2 .Zero amplitude variations
3.Supports a high data rate.
4.Low probability of error.
5.High SNR (signal to noise ratio).
6.More noise immunity than the ASK
7.Error-free reception can be possible with FSK
8.Useful in high-frequency radio transmissions
9.Preferable in high-frequency communications
10.Low-speed digital applications
Disadvantages of FSK
1.It requires more bandwidth than the ASK and
PSK(phase shift keying)
2.Due to the requirement of large bandwidth, this
FSK has limitations to use only in low-speed modems
which the bit rate is 1200bits/sec.
BPSK waveform
Generation of PSK
Generation of PSK with waveform
Coherent PSK detector
Coherent PSK detector
Coherent PSK detector
Bandwidth of BPSK
BW= fc+ fb – {fc - fb }
BW = fc+ fb - fc+ fb =2fb
Constellation diagram of PSK
Advantages of PSK
Disadvantages of PSK
Comparision of ASK,FSK,PSK
DPSK Generation
DPSK Generation
DPSK Generation
Table for DPSK(assume previous bit as 1)
Table for DPSK( assume previous bit as 0)
Differentially encoded sequences with phase
Waveform of DPSK
DPSK receiver
DPSK receiver
DPSK receiver
Bandwidth of DPSK
It is clear that the phase shift of DPSK signal is
dependent on the existing bit and the previous
bit .Thus we can say that in DPSK two successive
bits form a symbol.
Symbol duration = 2Tb
BW of DPSK signal is given by
BW = 2/ Ts
BW = 2/ 2Tb
BW = fb
Advantages and Disadvantages
QPSK
1.We can keep the baud rate same and increase the
bit rate by using multilevel modulation techniques.
2.The data group of bits are divided into groups of
two more bits and each group of bits is
represented by specific value of amplitude,
frequency or phase of carrier.
3. Symbol duration = 2Tb
QPSK
QPSK modulator
Constellation Diagram
Waveform of QPSK
Bandwidth of QPSK
It is clear that the phase shift of QPSK signal is
dependent on the existing bit and the previous
bit .Thus we can say that in QPSK two successive
bits form a symbol.
Symbol duration = 2Tb
BW of QPSK signal is given by
BW = 2/ Ts
BW = 2/ 2Tb
BW = fb
QPSK Receiver
Advantages of QPSK
QAM
Generation of QAM
QAM Receiver
QAM Equation
Bandwidth of QAM
BW of QAM
BW of QAM signal is given by
BW = 2fs
BW = 2/ Ts
Symbol duration = NTb
BW = 2/ NTb
BW = 2fb/ N
Constellation diagram
Waveform of QAM
Application of QAM
THANK YOU
Questions
