Communication system 1 chapter 3 - part 1

Contents of Analogue Communication
1. Amplitude Modulation
1.Analysis of linear modulations such as AM, DSB,DSB-SC,SSB,
VSB, Linear modulation
2.Demodulation techniques
3.AM transmitter
4.AM receivers
2. Angle modulation
1.PM and FM modulation and demodulation techniques
2.Narrow band and Wide band Angle modulation techniques
3.FM transmitters
4.FM receivers

3.1 Amplitude Modulation
In modulation, a message signal, which contains the
information is used to control the parameters of a carrier
signal, so as to impress the information onto the carrier

Figure Amplitude modulation

AM Signal Waveform

AM – Percentage Modulation

Therefore The full AM signal may be written as
Draw the Frequency Spectrum of the above AM signal and calculate the
Bandwidth

Draw Frequency Spectrum for a complex input signal with AM
The frequency spectrum of AM waveform contains three parts:
1. A component at the carrier frequency fc
2. An upper side band (USB), whose highest frequency component
is at fc+fm
3. A lower side band (LSB), whose highest frequency component is
at fc- fm
The bandwidth of the modulated waveform is twice the information
signal bandwidth.

The diagram below shows the spectrum and
corresponding waveform of the output signal, given by
 Because of the two side bands in the frequency spectrum its often
called Double Sideband with Large Carrier.(DSB-LC)
• The information in the base band (information) signal is duplicated
in the LSB and USB and the carrier conveys no information.

Example:
A 2500 kHz carrier is modulated by audio signal with frequency
span of 50 −15000 Hz. What are the frequencies of lower and
upper sidebands ? What bandwidth of RF amplifier is required
to handle the output ?
50Hz 1500 Hz
AM
MODULATION

Modulation Index (m)
 m is merely defined as a parameter, which determines
the amount of modulation.
 What is the degree of modulation required to establish a
desirable AM communication link?
Answer is to maintain m<1.0 (m<100%).
This is important for successful retrieval of the original
transmitted information at the receiver end.
AM – Normalized Average Power
The normalized average power of the AM signal is

Example:
The maximum peak-to-peak voltage of an AM wave is 16
mV and the minimum peak-to-peak voltage is 4 mV.
Calculate the modulation factor
Solution .

Example:
A carrier wave of frequency 10 MHz and V DC peak value 10V is
modulated by a 5- kHz sine wave of amplitude 3V. Determine (i)
modulation factor (ii) sideband frequencies and (iii) amplitude of
sideband components. Draw the frequency spectrum.
 Carrier amplitude, VDC = 10V
Signal (message) amplitude, Vm = 6V
Carrier frequency, fc = 10 MHz
Signal frequency, fs = 5 kHz = 0.005 MHz
i) modulation depth, m=Vm / VDC =0.6
ii) Sideband frequencies are :
fc – fm ; fc + fm
10 – 0.005 ; 10 + 0.005
9.995 MHz ; 10.005 MHz
iii) Amplitude of the side band signal= Vm /2=6/2=3V

What are the different Forms of Amplitude Modulation ?
1. Conventional Amplitude Modulation (DSB-LC)
(Alternatively known as Full AM or Double
Sideband with Large carrier (DSB-LC)
modulation
2. Double Side Band Suppressed Carrier (DSB-SC)
modulation
3. Single Sideband (SSB) modulation
4. Vestigial Sideband (VSB) modulation

Double Side Band Suppressed Carrier (DSB-SC)Modulation
• The carrier component in full AM or DSB-LC does not convey
any information. Hence it may be removed or suppressed
during the modulation process to attain higher power efficiency.
• The trade off of achieving a higher power efficiency using
DSB- SC is at the expense of requiring a complex and
expensive receiver due to the absence of carrier in order to
maintain transmitter/receiver synchronization.

Derive the Frequency Spectrum for Double Sideband
Suppressed Carrier Modulation (DSB-SC)

All the transmitted power is contained in the two
sidebands (no carrier present).
The bandwidth is twice the modulating signal
bandwidth.
USB displays the positive components of sm(t) and
LSB displays the negative components of sm(t).
Generation and Detection of DSB-SC
The simplest method of generating a DSB-SC signal
is merely to filter out the carrier portion of a full AM
(or DSB-LC) waveform.
Given carrier reference, modulation and
demodulation (detection) can be implemented using
product devices or balanced modulators.

Single Side Band Modulation (SSB)
How to generate SSB signal?
• Generate DSB-SC signal
• Band-pass filter to pass only one of the
sideband and suppress the other.
For the generation of an SSB modulated signal
to be possible, the message spectrum must
have an energy gap centered at the origin.

Examples

Solution

Example

3. AM receivers
1.AM receivers

3. 2 AM receivers
Two basic types of receiver
 Coherent receiver – the frequencies generated in the
receiver and used for demodulation are synchronized to
oscillator frequencies generated in the transmitter.
 Noncoherent receiver – frequencies that are generated
in the receiver or the frequencies that are used for
demodulation are completely independent from the
transmitter’s carrier frequency
 For AM DSBFC scheme, the noncoherent receivers are
typically used
 Tuned Radio Frequency receiver (TRF)
 Superheterodyne Receiver
Types of Receivers

3. 2 AM receivers
Tuned Radio Frequency receivers(TRF)
 Block diagram of 3-stages TRF receiver that includes an
RF stage, a detector stage and an audio stage :
 Two or three RF amplifiers are required to filter and
amplify the received signal to a level sufficient to drive
the detector stage.
The detector converts RF signals directly to
information.
 An audio stage amplifies the information signals to a
usable level
 Simple and have a relatively high sensitivity

3. 2 AM receivers

3. 2 AM receivers
Three distinct disadvantages :
1. The bandwidth is inconsistent and varies with the center
frequency when tuned over a wide range of input
frequencies.
 As frequency increases, the bandwidth = f/Q increases.
Thus, the selectivity of the input filter changes over any
appreciable range of input frequencies.
2. Instability due to large number of RF amplifiers all tuned to
the same center frequency
 High frequency, multi stage amplifiers are susceptible to
breaking into oscillation.
3. The gains are not uniform over a very wide frequency
range.
 The non uniform L/C ratios of the transformer-coupled
tank circuits in the RF amplifiers.

Communication system 1 chapter 3 - part 1

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