KYAMBOGO
UNIVERSITY
FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRICAL AND ELECTRONICS
ENGINEERING
BACHELORS OF ENGINEERING IN TELECOMMUNICATIONS
ENGINEERRING
YEAR THREE SEMESTER ONE
TETE 3104 RADIO AND TV ENGINEERING PRACTICAL
ASSIGNMENT
PRACTICAL ONE: AMPLITUDE MODULATION
PRESENTED BY
NAME
REGISTRATION NO
ODOCH HERBERT
18/U/ETE/10168/PE
YEHANGANE PROMISE
19/U/ETE/20193/PE
SENDIJJA DANCAN
18/U/ETE/10153/PE
AINEMBABAZI MICHAEL
18/U/ETE/10166/PE
ODEKE JOSEPTH
18/U/ETE/10136/PE
ISINGOMA JOHN
18/U/ETE/10164/PE
MAYINJA DENIS
18/U/ETE/10162/PE
MUKIIBI ANDRAW
18/U/ETE/10145/PE
INTRODUCTION: AM MODULATION
Signals are transported between a transmitter and receiver over some form of transmission
medium. However, original signals are selected in a form that is suitable for transmission.
Therefore, they must be transformed into a form that is more suitable for transmission. The
process of impressing low-frequency information signals onto a high frequency carrier signal
is called modulation. In other words, modulation is the process of changing some
characteristics (phase, frequency, amplitude) of a carrier wave in accordance with the
instantaneous value of the modulating signal.
There are three types of modulation techniques and they are; Amplitude Modulation,
Frequency modulation and Phase Modulation
Amplitude Modulation
Amplitude modulation is defined as the modulation in which the amplitude of the carrier wave
is varied in accordance with the instantaneous amplitude of the modulating signal, keeping
its(carrier) frequency and phase constant.
In Amplitude Modulation, the amplitude of the carrier wave c (t) is varied linearly with the
instantaneous amplitude of the message signal m(t). The standard form of amplitude modulated
wave is defined by
Where Ka is a constant called the amplitude sensitivity of the modulator, Ac is the amplitude
of carrier signal, m(t) is the message signal, fc is the carrier frequency.
Methods of Generating AM waveforms
There are two methods to generate AM and they are;
Low level modulation: the message signal and carrier signal are modulated at low power levels
and then amplified.
High level modulation: the carrier and message signals are sufficiently amplified to the
transmitting levels and modulation is done at high power levels.
Different types of AM:
1. Double Sideband with carrier (we will call it AM): This is the most widely used type of AM
modulation. In fact, all radio channels in the AM band use this type of modulation.
2. Double Sideband Suppressed Carrier (DSBSC): This is the same as the AM modulation
above but without the carrier.
3. Single Sideband (SSB): In this modulation, only half of the signal of the DSBSC is used.
4. Vestigial Sideband (VSB): This is a modification of the SSB to ease the generation and
reception of the signal.
EXPERIMENT: DDESIGN OF AM MODULATOR
Aim:

This project aims to design AM modulator on software. The modulating signal that is
used has a frequency of 15kHz frequency and carrier of 125kHz frequency.
Objectives



Understand the process of generating an AM Signal.
Design and simulate AM modulator by using Multisim based on given frequencies.
Verify/Test the circuit of the AM Modulator.
Equipment Required:
 Function Generator
 4-channel Oscilloscope
 Power Supply
 AC Voltage Sources
Components Required:
 BC107BP transistor
 Resistors
 Capacitors
Circuit Diagram and Analysis:
Simple BJT Amplitude Modulation Circuit
How it works
o A simple AM Modulator is essentially a CE (Common Emitter) Amplifier having a gain
of A. The carrier signal is the input to the amplifier. The modulating signal is applied
in the emitter resistance circuit.
o The carrier c(t) is applied at the input of the amplifier and the modulating signal is
applied to the emitter resistance circuit. The amplifier circuit amplifies the carrier by a
factor “A” so that the output is Ac(t).
o Since the modulating signal is part of the biasing circuit, it produces low frequency
variations in the emitter circuit. This in turn causes variations in “A”. The result is that
the amplitude of the carrier varies in accordance with the strength of the signal.
Consequently, amplitude modulated frequency is obtained at output.
o It may be noted that the carrier should not influence the voltage gain A; only the
modulating frequency should do this. To achieve this objective, carrier should have a
small magnitude and signal should have large magnitude.
Analysis of Common Emitter Amplifier
Using NPN Transistor BC107BP with DC Current gain β = 400, VCE =20V and IC = 28.6mA.
Then getting fixed voltage divider resistor values R1=27KΩ and R2=12KΩ.
We used three coupling capacitors with C1 set to 100pF, a low value to reduce on the distortion
on the positive scale after varying hence coupling carrier signal to the base, C2 set to 1000uF
sufficient to couple output to the load then C3 set to 14.2uF sufficient to couple the modulating
signal to the emitter resistance circuit. There is no bypass capacitor to the emitter.
We then needed the collector resistor R6 and emitter resistance R3 which we obtained as
follows
To get R6; From
𝑅6 =
𝑉𝐶𝐶−𝑉𝐶𝐸
𝐼𝐶
=
30−20
28.6𝑚
Hence, we get Collector resistance as; 𝑅6 = 350Ω
To get R3
𝑅2
12𝐾
First getting Base-ground voltage VB from 𝑉𝐵 = 𝑅1+𝑅2 × 𝑉𝐶𝐶 = 12𝐾+27𝐾 × 30
Therefore 𝑉𝐵 = 9.23𝑉
Then getting Emitter-ground voltage VE from 𝑉𝐸 = 𝑉𝐵 − 𝑉𝐵𝐸 = 9.23 − 𝑂. 7
Therefore 𝑉𝐸 = 8.53𝑉
To get R3 we used 𝑅𝐸 =
𝑅3 =
8.53
28.6𝑚
Hence 𝑅3 = 298.2Ω
𝑉𝐸
𝐼𝐸
but IC=IE hence
Procedures





Use multisim software to construct the Amplitude Modulation circuit as shown in the
circuit diagram.
Connect two ac voltage sources at the input so that one of them will generate the carrier
signal and other will generate the message signal.
Given the modulation index of 20%(0.2), choosing the peak(Vrms) of the carrier to be
8, to get the peak of the modulating signal, we used the relation
𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑆𝑖𝑔𝑛𝑎𝑙(𝐴𝑚)
𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝐼𝑛𝑑𝑒𝑥(𝑀) =
𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑐𝑎𝑟𝑟𝑖𝑒𝑟 𝑆𝑖𝑔𝑛𝑎𝑙(𝐴𝑐)
So for Ac =8Vrms, to get Am
𝐴𝑚 = 𝐴𝑐 × 𝑀 = 8 × 0.2 = 1.6𝑉𝑟𝑚𝑠 Hence 𝐴𝑚 = 1.6𝑉𝑟𝑚𝑠
Enter the values for Vrms and frequency in each of the ac voltage sources
Connect an oscilloscope to output (across the collector and ground) and obtain the
output waveform.
Observations
Circuit
Input Values
For the carrier signal generated by the ac voltage source V3
Frequency -125kHz
Amplitude – 8 Vrms
For the message signal generated by the ac voltage source V2
Frequency -15kHz
Amplitude – 1.6 Vrms
Oscilloscope Waveforms:
The top most waveform is for the carrier signal(c(t)), middle is for the
modulating(message)(m(t)) signal and the lower waveform is the output waveform which is
the amplitude modulated signal
Result
 When the circuit of Amplitude modulator was simulated, an output modulated
waveform was obtained which consisted of the message signal modulated with the
carrier signal. We faced a problem with distortions at first but it was all because of poor
Biasing of the transistor which was worked upon as illustrated in the circuit analysis
and calculations