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EKT 313 ELECTRONIC COMMUNICATIONS
FREQUENCY MODULATOR/ DEMODULATOR PROJECT
PROJECT 1: FREQUENCY MODULATOR CIRCUIT
1.0
PROJECT OBJECTIVES
At the end of the project:
•
Students are able to design a frequency modulator circuit and understand the
function of the modulator in communications system
•
Students are able to construct and test their own designed FM modulator
•
Students become familiar with the use of design and simulation tools in design
process
2.0
INTRODUCTION
In this project, students are required to design a complete circuit of modulator for
Frequency Modulation (FM). The modulator is needed to be design, construct and test.
The use of design and simulation tools such as ORCAD is recommended at early stage of
design process.
Frequency modulation is a form of angle modulation where the message signal is used to
vary the carrier frequency. In this project, you will choose to design an FM modulator
using phase-locked loop (PLL) OR voltage-controlled oscillator (VCO) only. Both
are using a different integrated circuit (IC).
2.1 Phase-locked loop:
A phase-locked loop (PLL) is basically an electronic feedback system that
provides frequency selective tuning and filtering without the need for coils or
inductors. It consists of three basic functional blocks:
•
Phase comparator
•
Low-pass filter
•
Voltage-controlled oscillator
Figure 1 shows the block diagram of PLL.
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EKT 313 ELECTRONIC COMMUNICATIONS
FREQUENCY MODULATOR/ DEMODULATOR PROJECT
Figure 1: Block diagram of the PLL components
A PLL circuit can be used to create an FM signal. The input to the phase
comparator is the carrier signal, and assuming that the carrier frequency is within
the capture range of the PLL, the VCO output frequency will be equal to the
carrier frequency. Now if the VCO control voltage is the sum of the message
waveform and the loop error voltage, then the VCO output frequency will vary
about the carrier frequency according to the message. Thus, the VCO output is the
desired FM signal. In this project, you will be using CD4046 IC for PLL circuit.
2.2 Voltage-controlled oscillator
Voltage-controlled oscillator (VCO) is an oscillator that can be tuned over a wide
range of frequencies by applying a voltage (tuning voltage) to it. In FM
modulator, carrier frequency is directly varied by the message through VCO.
When dc voltage increases, the frequency of the output signal decreases. In other
words, a dc voltage controls the oscillator frequency. Typically, the frequency
decreases linearly with an increase in dc voltage. In this project, you will be using
LM566C IC for VCO circuit and LM386 IC as an amplifier for the VCO circuit.
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EKT 313 ELECTRONIC COMMUNICATIONS
3.0
FREQUENCY MODULATOR/ DEMODULATOR PROJECT
PROJECT METHODOLOGY AND INSTRUCTIONS
3.1 Components lists:
Integrated Circuit CD4046
Integrated Circuit LM566C
Integrated Circuit LM386
Resistor
Variable resistor
Capacitor
3.2 Circuit diagram & project specification:
1) Define your project specification by setting the frequency of your carrier
frequency and modulation signal.
2) Choose your design by referring to the circuit diagram below as an example.
Figure 2 shows the FM modulator using CD 4046 as phase-locked loop and
Figure 3 shows the FM modulator using LM566C as voltage-controlled oscillator
and LM386 as an amplifier. Set the value of the missing parameters according to
your frequency.
* Before beginning your design, you are advised to review details of CD4046,
LM566C and LM386 by studying the datasheet.
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EKT 313 ELECTRONIC COMMUNICATIONS
FREQUENCY MODULATOR/ DEMODULATOR PROJECT
Figure 2: FM modulator circuit using CD4046
Figure 3: FM modulator using LM566C and LM386
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EKT 313 ELECTRONIC COMMUNICATIONS
FREQUENCY MODULATOR/ DEMODULATOR PROJECT
3.2 Design & simulations:
By utilizing any design and simulation tools such as ORCAD etc, design your
own circuit of FM modulator according to your specifications. Simulate your
design and observe the result.
3.3 Circuit construction and testing:
Construct your circuit of FM modulator according to your circuit design. With the
available devices/ equipment, test your circuit for its operation and functionality.
Compare your results with the simulation results.
4.0
RESULTS AND PROJECT REPORT
At the end of Project 1 timeline, you will be evaluating on the workability and results
data of your designed circuit. A formal report of the project needs to be submitted upon
evaluation. The report should contain explanations about your project and design, results,
observations, discussion and conclusion.
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