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