8 AMPLITUDE SHIFT KEYING (OR) DIGITAL AMPLITUDE MODULATION (OR) OOK SYSTEM
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ANALOG AND DIGITAL COMMUNICATION
2.8.1 Mathematical representation
ASK is the simplest type of digital CWmodulation. Here carrier is a sine
wave of frequency ‘fc’.
We
can
represent
the
carrier
signal
mathematically
as follows,
Vc(t) = Ac cos ωc t ...(1)
ASK can be mathematically expressed as,
A
VASK (t)= [1+ Vm(t)] c cos ωc t
2
...(2)
Where,
Vm(t) = Digital modulating signal (volts)
Ac
= Unmodulated carrier amplitude (volts)
ωc
=Analog carrier radian frequency
Case 1
For a bit 1 (logic 1) input, Vm(t) = +1 volt
Equation (2) becomes,
VASK(t) = [1+1]
Ac
2
cos ωct
VASK (t)= AC cos ωct
...(3)
Case 2
For a bit 0 (logic 0) input,Vm(t) = 1 volt
Equation (2) becomes,
VASK(t) =[11] Ac cos ωct
2
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V
ASK
(t) = 0 ...(4)
Thus, the VASK(t) is either Ac cos ωct (or) 0. Hence the carrier is ei
ther ‘ON or ‘OFF’. Therefore ASK is also called ONOFF keying.
2.8.2 Graphical representation
The Figure 2.5 shows the ASK Modulation in graphical manner.
Figure 2.5 ASK Output Waveform
2.8.3 ASKgenerator
The Figure 2.6 shows the Ask generation circuit
Figure 2.6 Block Diagram of ASKgenerator
• The
digital
signal
from
the
computer
is
a
unipolar NRZ (nonreturn to zero) signal which acts as a
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modulating signal, applied as a one input of product modulator.
• ASK – modulator is nothing but a multiplier followed by a
bandpass filter. The carrier signal is applied as a another input of
product modulator.
• Due to multiplication of the two signal, the ASK output will be
present only when a binary ‘1’ is to be transmitted.
• The ASK output corresponding to a binary ‘0’ is zero.
• We conclude that a carrier is transmitted when a binary ‘1’ is to
be sent and no carrier is transmitted. When binary ‘0’ is to be
sent is as shown in Figure 2.5
2.8.4 ASK –Detector
The Figure 2.7 below shows the ASKdemodulation circuit.
ASK
Signal
(or)
Received
signal
0
∫
Tb
Decision
making
device
Original
data
Threshold
(Carrier)
Ac cos wct
Figure 2.7 ASK  Demodulator Circuit
• The ASK –signal is applied as one input of multiplier and integrator.
• The locally generated coherent carrier is applied as another input of
multiplier
Case 1
Received signal is consider Ac cos wct then, the output of
multiplier is given by,
= Ac2 cos2 ωct
...(1)
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The output of multiplier given as a input to integrator, integrator
act as a LPF. Therefore LPF produce low frequency component only at
the output.
1 + cos 2ωc t
= Ac 2
2
=
Ac 2 Ac 2
+
cos 2 ωc t ...(2)
2
2
In equation (2), first term represents DCterm and second term
represents second order harmonic. Therefore LPF filtered out the second
term, First term only obtained at the output
Ac 2
=
...(3)
2
The output of integrator is given to Decision making device,
which is compared with threshold value and produce the output logic 1
(i.e binary ‘1’)
Case 2
Received signal consider as a zero, then the output of multiplier,
integrator and decision making device is equal to zero. Therefore the
output is logic ‘0’ (i.e) binary ‘0’.
Bit time (interval ) Tb
The bit interval is the time required to send one single bit. It is the
reciprocal of the bit rate.
Bit rate
Bit rate is the number of bits transmitted (or) sent in one second.
It is expressed in bits per second (bps).
Relation between bit rate & bit interval,
1
1
=
Bit rate =
= fb
Bit interval
Tb
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Baud rate
Baud rate
fb
= N
...(1)
For ASK, we use one bit (0 (or) 1) to represent one symbol.
Therefore , the rate of change of the ASK wave form (baud) is the same
as the rate of change of binary input (bps), thus bit rate equals the baud
rate.
fb
Baud =
N
fb
=
= fb
1
Where N= number of bits =1
2.8.5 Bandwidth of ASK
The Bandwidth of ASK in terms of bit rate is given by,
BW
= (fc + fa)  ( fc fa)...(1)
fb
Where, fa = fundamental frequency of binary input =
2
fb
fb
− fc −
BW
= fc +
2
2
= fc +
fb
f
− fc + b
2
2
BW
= fb
For ASK , Bandwidth is also equal to bit rate.
Advantages
(1)
Simple techniques
(2)
Easy to generate and detect.
Disadvantages
(1)
It is very sensitive to noise.
(2)
It is used at very low bit rates upto 100 bits/sec.
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2.9 FREQUENCY SHIFT KEYING
Definition
The frequency of a sinusoidal carrier is shifted between two
discrete values according to the binary symbol (0 (or) 1).
2.9.1 Mathematical representation
FSK is sometimes called binary FSK (BFSK).The general
expression for FSK is,
V (t)
FSK
=VC cos {2(fc + Vm(t)∆f)t}
...(1)
Where,
Vc
=Peak analog carrier amplitude.
fc
=Analog carrier centre frequency.
Vm(t) =binary input (i.e logic 1 (or) logic 0)
∆f
=Peak shift in the analog carrier frequency.
From equation (1) it can be seen that the peak shift in carrier
frequency (fc) is proportional to the amplitude of binary input signal Vm(t).
Case 1
For a logic ‘1’ input Vm(t)=+1V. the equation (1) becomes,
VFSK(t)= Vc cos{2(fc + 1 ∆f)t}
V
FSK
(t) = Vc cos{2(fc+∆f)t}
...(2)
Case 2
For logic ‘0’ input, Vm (t) = 1 V, the equation (1) becomes,
VFSK(t)= Vc cos { 2p(fc  1. Df)t}
VFSK(t)=Vc cos {2(fc ∆f )t} ...(3)
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In BFSK , the centre frequency fc is shifted up and down in the
frequency domain by the binary input signal, is as shown in Figure 2.8.
∆f
+∆f
fc
fs
Logic 1
fm
Logic 0
Figure 2.8 FSK in frequency domain
When the binary input signal changes from a logic 0 to a logic 1
and vice versa, the output frequency shifts between two frequencies,
(1)
Space (or) logic 0 frequency (fs)
(2)
Mark (or) logic 1 frequency (fm)
2.9.2 Frequency Deviation (∆f)
It is half the difference between mark and space frequencies,
fmfs
∆f
=
... (4)
2
Where,
∆f
= frequency deviation (HZ)
fm fs = absolute difference between the mark and
frequencies.
space
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2.9.3 Graphical representation
The Figure 2.9 shows the Graphical representation of FSKModulation.
binary input
1
0
1
0
1
t
Carrier signal
FSK Output Free
t
fm
fs
fm
fs
fm
Figure 2.9 FSKOutput Waveform
Where,
fm mark frequency
fs Space frequency
2.9.4 FSK generation
The Figure 2.10 below shows the FSK  generation circuit.
NRZBinary
input
FSKModulator
(VCO)
FSK
Output
Figure 2.10 Block diagram of FSK generator
The VCO act as a FSK – generator , the input
as control input of VCO.
binary data is given
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If binary input is not applied (i.e) there is no input signal the VCO
generates the centre frequency equal to carrier frequency.
For logic 1 input , the VCO output frequency shifted to mark
frequency fm (i.e)( fc +∆ f).
For logic 0 input , the VCO output frequency shifted to space
frequency fs (i.e) (fc ∆f ).
We conclude that the VCO output frequency changes back and forth
between space and mark frequencies.
2.9.5 FSKdetection
There are three methods to demodulate the FSKSignal.
(1)
Non – coherent FSK demodulator
(2)
Coherent FSK demodulation
(3)
PLLFSK demodulator
2.9.5.1 Non Coherent FSKdemodulator
The Figure 2.11 below shows the FSK  demodulator circuit
BPF
‘fS’
FSK
Output
~
Envelope
detector
dc
comparator

Power
splitter
+
dc
‘fm’
BPF
~
Envelope
detector
Rectified signal
Output
Data
or
(Original
data)
Figure 2.11 Block Diagram of FSK demodulator
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FSK  demodulation is quite simple with a circuit such as the one
shown in Figure 2.11.
FSK  input signal is simultaneously applied to the inputs of both
band pass filters (BPF) through a power splitter.
The respective filter passes only the mark (or) only the space
frequency on to its respective envelope detector.
The envelope detectors, in turn indicate the total power in each pass
band, and the comparator responds to the largest of the two powers.
If Noninverting is greater when compare to inverting than it is taken
as logic 1 and vice versa for logic 0.
This type of FSK –detection is referred to as noncoherent detection.
There is no frequency involved in the demodulation process that is
synchronized either in phase .frequency (or) both with the incoming
FSKsignal.
2.9.5.2 Coherent FSKreceiver
The Figure 2.12 shows the block diagram for a coherent
FSK receiver.
Multiplier
LPF
Carrier
FSK
Input

Power
Splitter
+
Multiplier
LPF
Output
Data
or
Original
Data
Carrier
Figure 2.12 Block Diagram of a Coherent FSK receiver
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The FSK –input signal is simultaneously applied to the inputs of both
multipliers through power splitter.
Locally generated frequencies are applied as another input of
multiplier .The two frequencies are not same as transmitter
reference frequency, it is
impractical to reproduce a local
reference that is coherent with both of them. So coherent FSK –
detection is seldom used.
The multiplier outputs are passed through low pass filters and the
filter outputs are applied to a comparator.
Comparator responds to the largest of the two powers.
If noninverting is greater than the inverting input than the output is
logic 1 and vice versa for logic 0.
2.9.5.3 PLLFSK demodulator
Figure 2.13 PLL FSK Demodulator
The most common circuit used for modulation of BFSK is the phase
Locked loop (PLL) which is shown in Figure 2.13.
Generally, the natural frequency of PLL is made equal to the
center frequency of FSK –modulator (i.e) carrier frequency (fc) before