Analog Transmition

5.1
Chapter 5
Analog Transmission
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5.2
5-1 DIGITAL-TO-ANALOG CONVERSION5-1 DIGITAL-TO-ANALOG CONVERSION
Digital-to-analogDigital-to-analog conversion is the process ofconversion is the process of
changing one of the characteristics of an analogchanging one of the characteristics of an analog
signal based on the information in digital data.signal based on the information in digital data.
 Aspects of Digital-to-Analog Conversion
 Amplitude Shift Keying
 Frequency Shift Keying
 Phase Shift Keying
 Quadrature Amplitude Modulation
Topics discussed in this section:Topics discussed in this section:

5.3
Digital to Analog Conversion
 Digital data needs to be carried on an
analog signal.
 A carrier signal (frequency fc) performs
the function of transporting the digital
data in an analog waveform.
 The analog carrier signal is manipulated
to uniquely identify the digital data being
carried.

5.4
Figure 5.1 Digital-to-analog conversion

5.5
Figure 5.2 Types of digital-to-analog conversion

5.6
Amplitude Shift Keying (ASK)
 ASK is implemented by changing the
amplitude of a carrier signal to reflect
amplitude levels in the digital signal.
 For example: a digital “1” could not affect the
signal, whereas a digital “0” would, by making
it zero.

5.7
Figure 5.3 Binary amplitude shift keying

5.8
Figure 5.4 Implementation of binary ASK

5.9
Frequency Shift Keying
 The digital data stream changes the
frequency of the carrier signal, fc.
 For example, a “1” could be
represented by f1=fc +∆f, and a “0” could
be represented by f2=fc-∆f.

5.10
Figure 5.6 Binary frequency shift keying

5.11
Coherent and Non Coherent
 In a non-coherent FSK scheme, when
we change from one frequency to the
other, we do not adhere to the current
phase of the signal.
 In coherent FSK, the switch from one
frequency signal to the other only
occurs at the same phase in the signal.

5.12
Multi level FSK
 Similarly to ASK, FSK can use multiple
bits per signal element.
 That means we need to provision for
multiple frequencies, each one to
represent a group of data bits.

5.13
Phase Shift Keyeing
 We vary the phase shift of the carrier
signal to represent digital data.
 The bandwidth requirement, B is:
B = (1+d)xS
 PSK is much more robust than ASK as
it is not that vulnerable to noise, which
changes amplitude of the signal.

5.14
Figure 5.9 Binary phase shift keying

5.15
Figure 5.10 Implementation of BASK

5.16
Quadrature PSK
 To increase the bit rate, we can code 2 or
more bits onto one signal element.
 In QPSK, we parallelize the bit stream so that
every two incoming bits are split up and PSK
a carrier frequency. One carrier frequency is
phase shifted 90o
from the other - in
quadrature.
 The two PSKed signals are then added to
produce one of 4 signal elements. L = 4 here.

5.17
Figure 5.11 QPSK and its implementation

5.18
Constellation Diagrams
 A constellation diagram helps us to
define the amplitude and phase of a
signal when we are using two carriers,
one in quadrature of the other.
 The X-axis represents the in-phase
carrier and the Y-axis represents
quadrature carrier.

5.19
Figure 5.12 Concept of a constellation diagram

5.20
Figure 5.13 Three constellation diagrams

5.21
Quadrature amplitude modulation is a
combination of ASK and PSK.
Note

5.22
Figure 5.14 Constellation diagrams for some QAMs

Analog Transmition

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