Design and Construction of Fm Transmitter Report

8/19/2019 Design and Construction of Fm Transmitter Report

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BY

AKINWANDE JUBRIL AKINFOLARIN

NDA/PGS/FE/M/1808/14

SUBMITTED TO

DEPARTMENT OF ELECTRICAL / ELECTRONICS ENGINEERING

NIGERIAN DEFENCE ACADEMY

IN PARTIAL FULFILMENT OF THE REQUIRMENT FOR THE

AWARD OF MASTERS OF ENGINEERING (M.Eng) ELECTRONICSAND COMMUNICATIONS ENGINEERING

MARCH 2016


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ABSTRACT

The transmission of audio signals is commonly achieved through the use of frequency

modulation techniques. This report is a demonstration of the use of a varactor diode and a

differential oscillator to produce a frequency modulated signal with good fidelity at the

receiver.

The varactor diode modulator approach was adopted in the design of the Voltage Controlled

Oscillator (VCO), while a BJT differential oscillator design, which produces a negative

resistance was used to generate the carrier signal to be modulated. The circuit was able to

produce very good quality sound within a 30 meters radius.


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DECLARATION

This report is presented in partial fulfilment of the requirements for the award of the degree,Masters of Engineering (M. Eng) Electronics and Communication Engineering.

This report is an original work carried out by me under the supervision of Dr. Nyitamen. It

has not been presented to any other university or higher institution, or for any other academic

award in this university. Where use has been made of the work of other people it has been

fully acknowledged and referenced.

___________________________________ ______________________

AKINWANDE, JUBRIL AKINFOLARIN DATE


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1 CERTIFICATION

This is to certify that the project titled “Design and Construction of a FM TRANSMITTER”

carried out by AKINWANDE JUBRIL has been read and approved for meeting part of the

requirements for the award of masters of engineering (m.eng) electronics and communications

engineering, Nigerian Defense Academy, Kaduna. Nigeria.

…………………………………………. …………………………

Dr. D.S. Nyitamen Date

(Project Supervisor)

………………………………………….. …………………………

Dr. D. S. Nyitamen Date

(Head of Department)


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ACKNOWLEDGEMENTS

I am grateful for the guidance of my project supervisor Dr. D.S Nyitamen, whom really

rendered useful advice that helped with the successful completion of this work.

I sincerely appreciate the support of Mr. Robinson Edeh, whose experience with electronic

components really helped in the construction of this FM transmitter. I am also very grateful

for the support of my wife, Ruqayyah Akinwande without whom the successful completion

of this work may have been impossible.

And my utmost gratitude goes to Almighty Allah, for making the completion of this project a

success.


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TABLE OF CONTENTS

ABSTRACT ................................................................................................................................ i

DECLARATION ....................................................................................................................... ii

CERTIFICATION ................................................................................................................... iii

ACKNOWLEDGEMENTS ...................................................................................................... iv

LIST OF TABLES ................................................................................................................... vii

LIST OF FIGURES ................................................................................................................. vii

CHAPTER 1 .............................................................................................................................. 1

1.1 INTRODUCTION ....................................................................................................... 1

1.2 WHY FREQUENCY MODULATION ...................................................................... 1

1.3 FM TRANSMISSION SYSTEM ................................................................................ 2

a) Microphone: ................................................................................................................ 2

b) Audio Amplifier: ......................................................................................................... 3

c) RF Oscillator: .............................................................................................................. 3

d) Modulator: ................................................................................................................... 3

1.4 AIM AND OBJECTIVES OF THE PROJECT .......................................................... 3

1.5 SIGNIFICANCE OF THE PROJECT ........................................................................ 4

1.6 METHODOLOGY ...................................................................................................... 4

1.7 SCOPE OF THE PROJECT ........................................................................................ 5

2 CHAPTER 2 LITERATURE REVIEW ............................................................................. 6

2.1 ORIGIN OF FM TRANSMISSION ........................................................................... 6

2.2 FREQUENCY MODULATION (FM) TRANSMITTER .......................................... 6

2.3 DIRECT FM TRANSMITTER ................................................................................... 6

2.3.1 ADVANTAGES OF DIRECT FM ...................................................................... 7

2.3.2 DISADVANTAGES OF DIRECT FM ............................................................... 7

2.4 INDIRECT FM TRANSMITTER .............................................................................. 7

2.4.1 ADVANTAGE OF INDIRECT FM .................................................................... 9

2.4.2 DISADVANTAGE OF INDIRECT FM ............................................................. 9

2.5 REVIEW OF PROJECT WORKS ON FM TRANSMITTERS ................................. 9

2.5.1 MULTICHANNEL FM TRANSMITTER BY F. MC_SWIGGAN. [12] .......... 9

2.5.2 SINGLE TRANSISTOR FM TRANSMITTER BY D. MOHANKUMAR [13]

10

2.5.3 2 WATT FM TRANSMITTER BY SINNER[14]............................................. 11

3 CHAPTER 3 DESIGN AND IMPLEMENTATION ....................................................... 13

3.1 BASIC BUILDING BLOCKS OF AN FM TRANSMITTER ................................. 13


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3.2 SOUND SENSOR ..................................................................................................... 13

3.2.1 BIAS DESIGN FOR THE ELECTRET CONDENSER MICROPHONE ........ 14

3.3 AUDIO AMPLIFIER ................................................................................................ 15

3.3.1 AUDIO AMPLIFIER DESIGN ......................................................................... 15

3.4 VARACTOR-DIODE FREQUENCY MODULATOR............................................ 17

3.4.1 DESIGN OF VARACTOR-DIODE TANK CIRCUIT ..................................... 17

3.5 YAGI ANTENNA DESIGN ..................................................................................... 21

4 CHAPTER 4 TEST AND RESULTS .............................................................................. 24

4.1 INTRODUCTION ..................................................................................................... 24

4.2 TEST EQUIPMENT ................................................................................................. 24

4.3 CONSTRUCTION AND ASSEMBLY TOOLS ...................................................... 24

4.4 CONSTRUCTION AND ASSEMBLY .................................................................... 25

4.5 COMPONENT LIST................................................................................................. 25

4.6 TEST RESULT ......................................................................................................... 27

4.6.1 WAVEFORM MEASUREMENT ..................................................................... 27

4.6.2 VOLTAGE AND CURRENT MEASUREMENT ............................................ 28

4.6.3 TRANSMISSION RANGE MEASUREMENT ................................................ 29

5 CHAPTER 5 CONCLUSION AND RECOMMENDATION ......................................... 30

5.1 CONCLUSION ......................................................................................................... 30

5.2 LIMITATION ........................................................................................................... 30

5.3 RECOMMENDATION ............................................................................................ 30

REFERENCES ........................................................................................................................ 32

6 APPENDIX ...................................................................................................................... 33


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LIST OF TABLES Table 4.1 RESISTORS

Table 4.2 CAPACITORS

Table 4.3 INDUCTORS

Table 4.4 TRANSISTORS

Table 4.5 DIODES

Table 4.6 VOLTAGE AND CURRENT MEASUREMENT

LIST OF FIGURES

Figure 1.1 BASIC BLOCK OF A FM TRANSMITTER .......................................................... 2

Figure 2.1 BLOCK DIAGRAM OF DIRECT FM TRANSMITTER ....................................... 7

Figure 2.2 PORTABLE MULTICHANNEL FM TRANSMITTER BY F. Mc SWIGGAN .. 10

Figure 2.3 SINGLE TRANSISTOR FM TRANSMITTER BY D. MOHANKUMAR .......... 11

Figure 2.4 2 WATT FM TRANSMITTER BY SINNER ........................................................ 12

Figure 3.1 BUILDING BLOCK OF THE FM TRANSMITTER............................................ 13

Figure 3.2 ELECTRET MICROPHONE BIAS....................................................................... 14

Figure 3.3 AUDIO-AMPLIFIER CIRCUIT ............................................................................ 15

Figure 3.4 VARACTOR TANK CIRCUIT ............................................................................. 17

Figure 3.5 COMPLETE FM TRANSMITTER CIRCUIT WITH DESIGN VALUES .......... 21

Figure 3.6 YAGI ANTENNA STRUCTURE [24] .................................................................. 23

Figure 4.1 PRE-AMPLIFIED VS AMPLIFIED AUDIO WAVEFORM ............................... 28


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CHAPTER 1

1.1 INTRODUCTION

In the last 30 years wireless communication has deeply changed the human lifestyle[1]; it has

enhanced the exchange of information across the globe quickly and efficiently. Transmission

of audio message wirelessly provides the exchange of information in real time. Wireless audio

transmission involves the transfer of audio (acoustic) energy over a distance through the

atmospheric medium, while maintaining or allowing minimal distortion to the characteristics

of the audio signal, such that the integrity of the information being conveyed is maintained.

An audio signal is a naturally occurring analogue signal with frequencies in the audio-

frequency range of roughly 20 to 20,000 Hz. Audio signals (Sound Waves) are mechanical

waves generated from vibrations within a medium. It travels at a relatively slow speed of about

350m/s and it is also affected by attenuation caused by the medium they travel in, hence

limiting the distance to which they can travel and remain intelligible.

Long range audio message transmission can be achieved with the use of frequency modulation

technique, which involves the process of imposing the audio signal (low frequency signal) onto

a higher frequency signal (carrier signal) by varying the frequency of the carrier wave in

accordance with the audio signal, in order to produce a modulated signal with the

characteristics of an electromagnetic wave, which is more suitable for long range transmission.

This method was pioneered by Edwin Howard Armstrong for FM broadcasting[2].

1.2 WHY FREQUENCY MODULATION

Audio signals are inherently low frequency signals; and when they are converted into an

electrical signal with the aid of a transducer (e.g. Microphone); they produce low frequency

electrical signals with low amplitudes. At low frequencies radiation is poor and the signals get

highly attenuated, also transmission of low frequency signal requires large antenna sizes[3].

However, at higher frequencies (> 20 kHz), radiation of electrical signal is efficient and

practical antenna sizes are smaller[4]; hence if the audio signal can be translated to a signal of

higher frequency, then transmission of the audio signal becomes practicable.


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Modulation provides the technique by which the audio message can be embedded within a high

frequency signal (i.e. carrier wave); thereby allowing us to take advantage of the benefits of

transmitting at high frequency. The process of modulating an audio signal onto a carrier signal

involves causing a variation in one of the 3 variables (i.e. amplitude, phase, frequency) of the

carrier signal in accordance with the modulating signal while keeping the other two variables

constant.

Modulation of audio signals, is commonly achieved using Amplitude Modulation (AM) and

Frequency Modulation (FM). Frequency modulation is achieved by varying the frequency of

the carrier wave with respect to amplitude changes in the audio signal (i.e. modulating signal);

while AM is the variation of the amplitude of the carrier wave with respect to the audio signal.

AM provides wider coverage than FM, but frequency modulation is more resilient to noise andsignal strength variation compared to AM, and this makes FM more suitable for mobile

applications.

1.3 FM TRANSMISSION SYSTEM

A FM transmission system, primarily comprises 3 basic sub-sections:

a) Microphone

b)

Audio Amplifier

c)

Modulator

d) RF Oscillator

Figure 1.1 BASIC BLOCK OF A FM TRANSMITTER

a) Microphone: A microphone is a device which converts sound waves into electrical

signals. When sound wave is impinged on the microphone, the varying air pressure onthe microphone generates an electrical signal representation of the sound, which


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corresponds in frequency to the original signal. This is an essential block in audio

processing, because for the sound wave to be processed it is required to be transformed

into an electrical representation.

b) Audio Amplifier: The electrical signal produced by the microphone has low amplitude

and requires amplification[4]. The audio amplifier section receives the output from the

microphone and increases its amplitude to a desired level before being fed into the

modulator.

c) RF Oscillator: The function of the RF oscillator is to produce a high frequency signal

in the FM range (88 – 108MHz), called a carrier wave. The carrier wave is a sinusoidal

signal with constant amplitude and constant frequency. The frequency at which the FM

transmitter operates, is referred to as the carrier wave frequency.

d) Modulator: The modulator provides the means by which the electrical signal

representation of the sound wave is embedded within the carrier wave. In frequency

modulation (FM), this is achieved by varying the frequency of the carrier wave in

relation with amplitude changes in the modulating signal (i.e. audio signal). The

resultant is a modulated wave of high frequency that contains the audio signal. This is

a very important part of a FM transmission system, because it allows the advantages of

high frequency signal transmission to be exploited such as:

I. Practical antenna length: The Length of the antenna is directly related to the

wavelength of the wave; and the higher the frequency, the shorter the wavelength.

Hence the smaller the antenna required[5]

II.

Higher Energy Transmission: The energy carried by a wave depends upon its

frequency. The higher the frequency of the wave, the greater the energy possessed by

it. As the audio signal frequencies are small, they cannot be transmitted over large

distances if radiated directly into space .[4]

1.4 AIM AND OBJECTIVES OF THE PROJECT

The purpose of this project is to design and build a FM transmitter for the transmission of an

audio message wirelessly to a receiver up to 1000 meters apart; for the purpose of

communication or information conveyance.


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The objectives of this project are:

I. To generate an electrical signal representation of an audio message using a transducer.

II. To modulate the electrical signal (low frequency signal) generated onto a high

frequency carrier signal using frequency modulation.

III. Transmission of the carrier wave (electromagnetic wave) from the transmitter to the

receiver wirelessly and reproduction of the audio message at the receiver.

1.5 SIGNIFICANCE OF THE PROJECT

Transmission of audio message wirelessly provides the exchange of information in real time;

and also transfer of audio signal from one point to another without the use of wired electrical

connections. This has wide applications such as the following:

Transfer of audio sound to loud speakers situated at far corners in large halls, stadia, big open

events without the need to run long cables to them.

Communication between people within a building or offices.

1.6 METHODOLOGY

The FM transmitter will be based on direct frequency modulation technique using a varactor

frequency modulator. The varactor frequency modulator will comprise an active device

(transistor) and a varactor diode in parallel with a LC tank circuit. The varactor diode behaves

like a capacitor when reverse biased; the modulating signal will be applied to the reverse-biased

varactor diode and as the modulating signal voltage varies the fixed reverse bias voltage will

be increased or decreased (i.e varied) in proportion to the varying modulating signal voltage.

Variation in the reverse bias voltage across the varactor diode, will produce a varying varactor

diode-capacitance and consequently produce a varying deviation in the resonant frequency of

the LC tank circuit in proportion to the modulating signal.

This behaviour of the varactor diode will be exploited in generating a frequency modulated

wave. A differential LC oscillator will be designed to produce a carrier frequency within the

FM range (88 – 108 MHz), and a suitable varactor diode in parallel with the LC tank circuit

will be selected to produce the required varying frequency deviation within the ±75KHz

bandwidth allowed for FM transmission.

Oscillation in the LC tank circuit is sustained by the negative resistance effect produced by the

cross-coupled BJT transistor. The inductor and capacitor in the LC tank are inherently lossy


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and diminish the energy stored in the inductor and capacitor as energy is being transferred

between the inductor and capacitor in the oscillation cycles, without compensating for this loss

the oscillation will decay. In order to sustain the oscillation indefinitely a negative resistance

can be introduced in parallel with the LC tank in order to counter the inherent loss present in

the inductor and capacitor. The negative resistance will be produced with a cross-coupled BJT

transistor design, which is known to give a negative resistance of -2/gm [6]. Where gm is the

transconductance of the transistor.

1.7 SCOPE OF THE PROJECT

This project report consists of five chapters. The chapter one contains Introduction of the

project, chapter two: Literature Review and theoretical background of the project, chapter

three: system design and calculation, chapter four: construction, testing and packaging, and

finally, chapter five: conclusion and recommendation.


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2 CHAPTER 2 LITERATURE REVIEW

2.1 ORIGIN OF FM TRANSMISSION

In 1933 Edwin Armstrong, invented a new circuit to improve AM (Amplitude Modulation)

radio. He came up with the first practical system for transmitting radio signals, using FM.[2]

Armstrong generated a frequency modulated signal using a phase modulator in order to

overcome the inherent challenges of frequency-instability in the direct frequency modulation

method. Since the invention of FM by Edwin Armstrong it has grown and become the preferred

method of audio transmission through radio signals.[7]

2.2 FREQUENCY MODULATION (FM) TRANSMITTER

FM signals can be produced by either directly varying the frequency of the carrier oscillator,

or by converting phase modulation to frequency modulation (indirect method). Depending on

the method employed, FM transmitters are classified into 2 types: Direct and Indirect frequency

modulation transmitter.

2.3 DIRECT FM TRANSMITTER

The frequency modulation is achieved by direct variation of the carrier signal by the

modulating signal. The Direct frequency modulation is commonly achieved using the transistor

reactance modulator or the varactor diode modulator approach[8] .The transistor reactance

modulator comprise an active device (transistor) and a RC network in parallel with a resonant

tank circuit. The RC network causes the transistor to present a capacitive or inductive effect at

its output which is a function of the transconductance (gm) of the transistor. The modulating

signal applied at the input of the transistor will cause varying changes in the transconductance

(gm) of the transistor; this variation produces a varying capacitance or inductance which is in

parallel with the tank circuit; consequently a variation in the oscillating frequency with respect

to the modulating signal is produced i.e a frequency modulated signal is produced. The varactor

diode modulator approach exploits the capacitive property of a reversed-biased varactor diode;

the modulating signal presents a varying reverse-biased on the varactor diode, and

consequently frequency deviation in accordance to the modulating signal is produced. FM


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signal can also be produced this way. Fig 2.1 shows the typical block diagram for a direct FM

transmitter.

Figure 2.1 BLOCK DIAGRAM OF DIRECT FM TRANSMITTER

2.3.1 ADVANTAGES OF DIRECT FM

It is easier to obtain high frequency deviation

It requires simpler circuitry. [9]

2.3.2 DISADVANTAGES OF DIRECT FM

Additional circuitry (i.e. Automatic Frequency Control loop) is required to achieve good

frequency stability.

Requires a Pre-emphasis stage to reduce hiss and high frequency noise.[10]

2.4 INDIRECT FM TRANSMITTER

Indirect FM transmitters produce the FM signal whose phase deviation is directly proportional

to the amplitude of the modulating signal. With this method the phase angle is varied while the

frequency and amplitude remain constant. i.e. phase modulation. In order to achieve frequency

modulation from phase modulation, the modulating signal must be of the same frequency as


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the carrier frequency.[11] This is commonly achieved by first amplitude modulating the

modulating signal in order to produce a constant frequency signal with varying amplitude. The

AM signal is then phase shifted by 900 and then added to the carrier signal, which is usually

generated by a crystal oscillator. Since both the produced AM signal and the carrier signal have

the same frequency the generated output is a FM signal. The concept is best illustrated

mathematically as shown:

If the modulating signal em and carrier signal ec is expressed as

em = Em cos wmt

ec = Ec sin wc t

A phase modulated signal is represented as:

e pm = Ec sin (wc t + m cos wmt) ----- 1 [11]

where m – Modulation index

The instantaneous angular frequency w p of the above phase modulated signal is given by:

w p = () ------------------------------ 2;

where θ (t) = wc t + m coswm t

w p = [wc t + m coswm t ] ------------- 3;

w p = wc – m sin wm t × wm -------------- 4;

In terms of linear frequencies above equation can be written as:

f p = f c – m f m sin(2πf mt) ------------------- 5;

The 2nd term in the equation represents the frequency shift with respect to centre frequency

i.e. f c + ∆f [11]

This shows that frequency of the phase modulated signal varies around the carrier frequency

f c with the deviation of ∆f = m f m sin(2πf mt). It can be seen that if modulating frequency f m


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remains constant then frequency deviation is directly proportional to m. Thus as long as the

modulation frequency does not change, phase modulation produces FM output. [11] This is

the basis of indirect modulation.

2.4.1

ADVANTAGE OF INDIRECT FM

The crystal oscillator can be used; hence there is better frequency stability.

2.4.2 DISADVANTAGE OF INDIRECT FM

There is limited phase deviation; hence low modulation index.

2.5 REVIEW OF PROJECT WORKS ON FM TRANSMITTERS

A quick review of some of the past works done in this field will be evaluated. The results

obtained and the method used will be described.

2.5.1 MULTICHANNEL FM TRANSMITTER BY F. MC_SWIGGAN. [12]

The circuit design of the Portable Miniaturised, Multichannel FM transmitter employed the

direct frequency modulation technique and implemented it using a 2 stage transistor circuit.

The first stage of the circuit was used as a pre-audio amplifier while the 2nd transistor stage acts

as an oscillator and modulator circuit. The circuit works based on the transistor reactance

modulator concept. The reactance modulator is an amplifier designed so that its output

impedance has a reactance that varies as a function of the amplitude of the applied input

voltage. The circuit was able to provide an effective tuning range of 6 MHz and an effective

range of 80 feet. The range achieved by this circuit is quite small and would limit its

applications.


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Figure 2.2 PORTABLE MULTICHANNEL FM TRANSMITTER BY F. Mc SWIGGAN

2.5.2 SINGLE TRANSISTOR FM TRANSMITTER BY D. MOHANKUMAR [13]

The single transistor FM transmitter is based on the transistor reactance modulator model. The

circuit is simplified by excluding a pre-amplifier stage, while the modulator and carrier

oscillator stage are implemented on a single 2N3904 or BC547 general purpose transistors. The

modulating effect is achieved by the specific arrangement of the input resistor R 1 = 4k7 and C1

= 1nF capacitor. The single transistor FM Transmitter had a very poor range of about 9 - 15

meters, and also the stability of the circuit was a bit poor, as the frequency often drifted off.


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Figure 2.3 SINGLE TRANSISTOR FM TRANSMITTER BY D. MOHANKUMAR

2.5.3 2 WATT FM TRANSMITTER BY SINNER[14]

This 2 Watt FM transmitter is reported to provide over 1 km range in good weather conditions

with a 9V supply. The transmitter can be tuned between 88 – 108 MHz. It was discovered that

this FM transmitter provided good quality audio signal, however this FM transmitter was

discovered to consume so much power that a 9V battery cell cannot power it, even when 2 or

3 banks of batteries are combined, the transistor generated excessive heat, and hence cooling

fans would be required to prevent damage to the transistors.


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Figure 2.4 2 WATT FM TRANSMITTER BY SINNER

The goal of this project is to build a low cost FM transmitter with good quality sound output at

the receiver and enough power to transmit over a radius of 1KM. A varactor diode modulator

and a cross-coupled LC oscillator design similar to the design employed by SINNER will be

adopted, as a basis for our design. The cross-coupled LC oscillator presents a relatively lower

phase noise compared to the other designs reviewed. [15]

The following modifications will be made to the 2 watt FM transmitter in order to address the

high collector current in the 2N3533 NPN transistor, which leads to excessive heat dissipation

but still transmit enough power for a 1KM range.

I.

The base current will be reduced so that the current drawn by each transistor will be

limited such that a 9V battery will be enough to power the circuit without excessive

heat dissipation.

II. A yagi antenna will be used for increased directivity gain and better transmission

range.


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3 CHAPTER 3 DESIGN AND IMPLEMENTATION

3.1

BASIC BUILDING BLOCKS OF AN FM TRANSMITTER

The FM Audio transmission system consists of different basic building blocks, which have to

be designed to fit our goals. Figure 3.1 shows the building blocks adopted for the design of

this FM transmitter.

Figure 3.1 BUILDING BLOCK OF THE FM TRANSMITTER

3.2 SOUND SENSOR

A Sound sensor is a device that converts sound into an electrical signal. The most common

sound sensor is a microphone, it produces an electrical analogue output signal either in the

form of a voltage or current which is proportional to the actual sound wave. The most common

types of microphones available as sound transducers are Dynamic, Electret

Condenser, Ribbon and the newer Piezo-electric Crystal types.[16]

For the purpose of this design an electret condenser microphone will be used. It is a small

cylindrical device that contains 2 plates which form a capacitor. One of the plates is made of a

very light material and acts as a diaphragm while the other plate is fixed. The diaphragm

vibrates when impinged by sound waves, thereby changing the distance between the two plates

and therefore changing the capacitance. The change in capacitance causes a variable electric

current flow proportional to the sound wave. Ordinarily an electret microphone would not

require an external bias power, however for better sensitivity most electret microphone contain

a JFET pre-amplifier which would require power.

SOUND

SENSOR

VARACTOR

MODULATOR

AUDIO

AMPLIFIER

CARRIER

OSCILLATORANTENNA


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3.2.1 BIAS DESIGN FOR THE ELECTRET CONDENSER MICROPHONE

Specification from Pro-Signal ABM-713 RC Datasheet:

Standard Operating Point = [2V, 0.1mA]

Max. Current Consumption = 0.5 mA

Max. Operating Voltage = 10V

To provide appropriate bias conditions for the electret microphone, a resistor R 1 will be

connected in series with the electret microphone as shown in Fig 3.1

Figure 3.2 ELECTRET MICROPHONE BIAS

Where Im – Standard current flowing through resistor and electret mic

Vmic – Standard Voltage across electret mic (desired voltage = 2V)

Vcc – Source Voltage (9V for this circuit)

From Ohm’s law; the relating equation is:

ImR 1 + Vmic = Vcc ----- (I)

R 1 =

----- (II)

We choose Im = 0.1mA ; Vmic = 2V

R 1 =

. = 70 × 103 Ω


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Standard value chosen for R 1 = 68 KΩ

3.3 AUDIO AMPLIFIER

The output waveform from the electret microphone is typically between 3 – 30mV, depending

on how close it is to the source of the sound. This is too low to provide the desired level of

modulation. In order to produce a good signal to noise ratio a larger frequency deviation of the

carrier signal is desired[3], since the amount of frequency deviation produced during

modulation is proportional to the amplitude of the modulating signal, it is desirable to increase

the amplitude of the produced audio signal before modulation. A voltage divider bias transistor

amplifier will be designed for this purpose.

3.3.1

AUDIO AMPLIFIER DESIGN

Figure 3.3 AUDIO-AMPLIFIER CIRCUIT

Specification from Fairchild BC547 Datasheet:

β = 110 – 220

VBE = 0.7 V

Chosen Design Parameters:

IC = 0.5mA; VCC = 9V; β = 150


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Taking KVL across the circuit, the following equations are obtained:

VC = VCC – (IC + IB) R C ----- (I)

VE = 0 ----- (II)

VBE = VB – VE ----- (III)

IB =

----- (IV)

IC = βIB ----- (V)

Where:

VC – Collector Voltage

VB – Base Voltage

The transistor quiescent collector voltage needs to be about half of VCC so that the output

signal can swing by equal amounts above and below this value without driving the transistor

into saturation.[17]

Therefore ≈ IC R C ------ (VI)R C =

=

.∗.∗ = 9KΩ

From eqn (V); IB = =

.∗ = 3.33∗10 A

From eqn (I) VC = 9 – (0.5 × 10-3

+ 3.33 × 10-6

) (9 × 103 ) = 4.47 V

From eqn (III) VB = VBE = 0.7 V

From eqn (IV): R B =

R B =. .

. = 1.04 × 106Ω

Practical Values chosen: R C = 10 KΩ; R B = 1M Ω

The primary function of the coupling capacitors C1, C2 is to allow A-C signals to pass whilst

blocking DC at the input and output so that voltages present in circuits before or after it, will

not upset the bias condition for this amplifier.


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The major consideration is to ensure that the capacitive reactance is low enough compared

with the input impedance of the amplifier or any load connected to the input.

Practical value used to couple electret mic (C1): 22nF

Practical values used for audio coupling (C2): 100nF

3.4 VARACTOR-DIODE FREQUENCY MODULATOR

The basic concept of FM is to vary the carrier frequency in accordance with the modulating

signal. The carrier signal can be generated by an LC oscillator, whose frequency is determined

by the components of a tank circuit (i.e. parallel connection of inductor and capacitor). The

carrier frequency can be varied by varying either the inductance or the capacitance of the tank

circuit. It is however desired that the variation should be as a result of the modulating signal

and proportional to it. In order to achieve this we would require a circuit that converts the

modulating voltage into a corresponding change in capacitance of the oscillator tank circuit.

The design employed is a varactor modulator as seen in fig 3.4, which is a cross-coupled BJT

transistor setup in parallel with a LC tank circuit. A varactor diode which produces the

modulating effect due to changes in its capacitance as a result of the modulating signal is also

placed across the LC tank.

3.4.1 DESIGN OF VARACTOR-DIODE TANK CIRCUIT

Figure 3.4 VARACTOR TANK CIRCUIT


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(a) Design for Oscillator Tank Circuit

Chosen Inductance (L) = 1 H

Using the equation for calculating the inductance of a single layer air-core coil

L = ( )2 ×

. H [18] ----- (I)

D – Diameter of the core in mm

n – No. of turns

l – Length of Coil in mm

d – thickness of wire

l = d × n ----- (II)

Inductor Design Parameters Chosen:

Inductance (L) = 1 H

Core Diameter (D) = 10 mm

Length of Coil (l) = 5 mm

Thickness of wire (d) = 0.5 mm

From equation (I) and (II) , the number of turns required is calculated as:

n=10 ×√ (. )

n=10 ×√() (.()() )

= 9.74 turns

n ≈ 10 turns

The carrier frequency f c generated by the LC tank is given as

f c =

√ ----- (III)

Desired operating Frequency f c = 80 MHz

From equation (IV) C5 is calculated as:


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C5 =

----- (IV)

C5 =

( ) = 12.43 × 10-12 F

Practical Value chosen for C5 = 5 - 40 pF variable capacitor.

b) VARACTOR DIODE BIAS

The back-to-back varactor diode configuration will be employed in order to overcome the

problem of the RF altering the applied modulating voltage.[19] As the RF voltage rises the

capacitance on one diode will increase and the other will decrease, essentially cancelling out

the effect of the RF voltage on the capacitance of the varactor diode. The variable bias Resistor

R 5 must be high enough to isolate the tank circuit from the modulating signal, a typical starting

value is 10 KΩ. [19]

c) DC – ANALYSIS

Under typical geographical conditions a 1 watt transmitter can be received up to 3 KM away

[20]. Therefore we would choose that the transmitted power would be equal to 1 watt (i.e Pt =

1 watt).

d =

[21] ----- (I)

Typical Receiver Sensitivity (E) = 50 V/m [21]

Transmitter distance (d) = 1000m

Pt = ( ) = ( ) = 83.33 × 10-6 W

Considering only about 1 % of the power in the tank circuit get transmitted in small wire

antenna. [21] Therefore the required Ptank will be:

Ptank = 100 × 83.33 × 10-6 = 8.3 mW

The impedance Z of the tank circuit ≈ R inductor

R inductor = 1Ω


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Power in the tank circuit (Ptank ) = I2Z ----- (II) [21]

I = = .

= 0.091 A

Since the two cross-coupled transistors will supply the current, hence the collector current of

each 2N3533 transistor will be:

Ic = = 0.046A

Parameter Specification from 2N3553 Datasheet [22]:

hfe = β = 150; VBE = 0.7v

Design Parameters:

IC = 46mA; VCC = 9v; R 3 = R 4 = 4.7 KΩ

VR1 = VR2 = VBE = 0.7 V ----- (I)

VR3 = VR4 = Vcc – VR1 ----- (II)

IC = βIB ----- (III)

R 1 =

----- (IV)

I1 = I2 = I3 – IB ----- (V)

VR3 = I3 R 3 ----- (VI)

From eqn (II) : VR3 = VR4 = 9 – 0.7 = 8.3 V

From eqn (VI) : I3 = =

.. = 0.00177 A

From eqn (III): IB = =

= 0.31 × 10

-3 A

From eqn (V) : I1 = I2 = 1.77mA – 0.31mA = 1.46 mA

From eqn (IV): R 1 =.

. = 479 Ω


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Standard values chosen: R 2 = R 1 = 470 Ω; R 4 = R 3 = 4.7 KΩ

A BJT cross-coupled oscillator has limited voltage swing determined by the differential pair

non-linearity. The voltage swing is further enhanced by providing feedback capacitors (i.e C1

& C2)[6]. C1 = C2 = 22 pF was used. [23]

Figure 3.5 COMPLETE FM TRANSMITTER CIRCUIT WITH DESIGN VALUES

3.5 YAGI ANTENNA DESIGN

The antenna parameters element lengths and spacing are given in terms of wavelength, so an

antenna for a given frequency can be easily designed. The lengths of various antenna

elements are related to the frequency (f=106 MHz) is as follows:

Planned frequency of transmission f = 100MHz

The following equations will be used to derive the appropriate length of the elements that will

make up the yagi antenna and the spacing between them. Fig 3.7 will be used as the

reference.

The equations for length of the elements are: [24]

First Director Length =

() ----- (I)


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Second Director Length =

() ----- (II)

Third Director Length =

() ----- (III)

Fourth Director Length = () ----- (IV)

Dipole Length =

() ----- (V)

Reflector Length =

() ----- (VI)

The Spacing between the elements can be found from the following equations: [24]

A = () ----- (VII)

B =

() ----- (VIII)

C =

() ----- (IX)

D =

() ----- (X)

E = () ----- (XI)

First Director Length = = 1.2 meters

Second Director Length = = 1.25 meters

Third Director Length = = 1.3 meters

Fourth Director Length =

= 1.38 meters

Dipole Length = = 1.43 meters

Reflector Length = = 1.50 meters

A =

= 0.6 meters

B =

= 0.45 meters


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C =

= 0.3 meters

D =

= 0.3 meters

E = = 0.48 meters

Figure 3.6 YAGI ANTENNA STRUCTURE [24]


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4 CHAPTER 4 TEST AND RESULTS

4.1

INTRODUCTION

This section will discuss tests carried out on the final circuit and the results obtained. Measured

waveforms from the oscilloscope will be used to illustrate the performance at each stage of the

circuit and the method used to evaluate the obtained result will be described.

4.2 TEST EQUIPMENT

At various stages of the circuit different test were required to confirm the performance of the

stages. The following test tools were used:

a) Digital Multimeter: This is an electronic device used to measure continuity, voltage

and current. The multimeter was particularly useful for measuring the base-emitter

voltage of each transistor in order to verify if it was within the voltage range (i.e 0.6V

to 0.7V) of the transistor active region.

b) Oscilloscope: This is a type of electronic test instrument that allows observation of

constantly varying signal voltages with respect to time. It allows the observation of

signal amplitude and the period of the signal. The oscilloscope was used to check if the

oscillator part of the circuit was oscillating as desired. Also the performance of the

audio amplifier and the output of the electret microphone was evaluated with the

oscilloscope.

c) Analogue FM Radio Receiver: An analog FM receiver was required to tune to the

transmitting frequency of the transmitter. The FM receiver will intercept the transmitted

FM signal and demodulate it to reproduce the original sound input. With the FM radio

receiver it was possible to determine the range of the FM transmitter and also its sound

quality.

4.3 CONSTRUCTION AND ASSEMBLY TOOLS

a)

Cutting Plier

b) Flat Nose Plier

c)

Digital Multimeter


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d) Soldering Iron and Lead

e)

Small flat screw driver

f) Drill Bit

4.4 CONSTRUCTION AND ASSEMBLY

The FM transmitter was built using discrete electronic components (such as resistors,

capacitors, transistors) soldered on a vero board. The vero board was made up several vertical

conducting strips, on which components were soldered. A drill bit was used to etch out sections

of the strips where an electrical bridge was not wanted. The inductor was fabricated by winding

4 turns of a 2mm gauge copper wire on a threaded bolt; while the yagi antenna was constructed by cutting the elements of a ready-made yagi antenna to fit the design specification.

The circuit assembled on the vero board is placed into a handheld instrumentation case 90 ×

65 × 25 cm in dimension. A hole is drilled at the top to accommodate the electret microphone,

another hole is drilled by its side with an audio jack fitted for the purpose of accepting an

external audio signal source. An output for the yagi antenna connection is made on the right

side of the case while the power switch is mounted on the reverse side.

4.5 COMPONENT LIST

a) Electret Microphone

b) Resistor

Table 4.1 RESISTORS

Component Type Quantity Use

68 KΩ Carbon Film 1 Bias for electret

microphone

4.7 KΩ Carbon Film 4 Voltage divider DC-Bias

for carrier Oscillator


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10 KΩ Carbon Film 1 Provide Modulating

voltage

c) Capacitors

Table 4.2 CAPACITORS

Component Value Type Quantity Use

47 nF Ceramic 2 For stabilising D-C input

voltage

22 pF Ceramic 2 Feedback Capacitor to

enhance voltage swing of

Oscillator

22 F Ceramic 1 Audio Coupling Capacitor

2 – 10 pF Variable

capacitor

1 Capacitance for tank

circuit

d) Inductor

Table 4.3 INDUCTORS

Component

Value

Type Quantity Use

0.1 H Air – Core Wound

Inductor

1 Inductance for tank circuit

e) Transistor

Table 4.4 TRANSISTORS



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2N3553 BJT

Transistor

2 Carrier Oscillator

f) Diode

Table 4.5 DIODES


BB204 Variable Capacitance

Diode

2 Carrier Oscillator

Diode PN Diode 1 To provide protection

against reverse DC

polarity

f) Yagi Antenna

g) Vero Board

h) 9.0V Battery

4.6 TEST RESULT

The following tests were carried out to evaluate the performance of the circuit.

I. Waveform Measurement

II. Voltage and current measurement

III.

Transmission Range

4.6.1 WAVEFORM MEASUREMENT

Fig 4.1 shows the combined waveform of the audio signal before amplification and after

amplification. The upper waveform is the waveform measured at the collector of the first

stage transistor, which is the output of the audio amplifier circuit. The bottom waveform is

the waveform measured at the output of the electret microphone. The time per division

setting was 1 milli-second; while the volts per division was 50 milli-volts.


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Figure 4.1 PRE-AMPLIFIED VS AMPLIFIED AUDIO WAVEFORM

Volts

/ Div

Time

/ Div

50

mV

1 ms

A comparison of the waveform shows a significant amplification of the audio signal, which is

very important to achieve a better modulation index.

4.6.2

VOLTAGE AND CURRENT MEASUREMENT

The voltage and current at key parts of the circuit was measured in order to derive the actual

power consumption of the circuit and also the amount of power generated in the tank circuit.


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Table 4.6 VOLTAGE AND CURRENT MEASUREMENT

Operational

Parameter

Voltage (V)

( V)

Current (mA)

( I )

Battery 9 165Transistor

1

VBE 0.67

IB 1.1

IC 97

Transistor

2

VBE 0.68

IB 1.1

IC 97

R 1 0.69R 2 0.69

R 3 8.31

R 4 8.31

From the measurements in table 4.6, we can calculate the following:

Power Consumption = V battery × Icurrent = 9 × 165mA = 1485 mW

Power in Tank circuit = 2 × Ic2 × R inductor

Power in Tank circuit = 2 × (97 × 10-3)2 × 1 = 18.8 mW

4.6.3 TRANSMISSION RANGE MEASUREMENT

A FM receiver was used to demodulate the transmitted FM signal; a good quality audible

message was received within a 30 meters radius of the FM transmitter. However the

transistor’s performance degraded significantly as the collector’s current rises; this

significantly limited the transmission power and consequently the distance covered was also

limited.


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5 CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1

CONCLUSION

A direct FM transmitter with a range up to 10 meters can be built using the varactor diode

modulator approach to generate frequency modulated signal. Within the 10 meters range the

quality of the sound produced was very good and the bandwidth of the generated FM signal

appeared to be within the ±75KHz. This was crucial in producing a good quality sound output.

The addition of a Yagi antenna to boost the transmitting distance did not yield a significantly

better result; it is suspected that the power generated by the circuit was insufficient to drive a

yagi antenna, as the transistor became excessively hot with the addition of a yagi antenna and

the FM signal produced degenerated.

5.2 LIMITATION

It was difficult to evaluate the generated frequency modulated signal, which is about 80MHz.

Measurement of the modulated waveform was not possible due to non-availability of an

oscilloscope capable of measuring up to the 80 MHz frequency range.

5.3 RECOMMENDATION

The FM transmitter is highly susceptible to frequency drift when touched or moved from one

place to another. It is recommended that the components on the circuit are closely put together,

as it was discovered that frequency drifting was reduced in this way.

The performance of the circuit can also be improved by building it on a Printed Circuit Board(PCB) or a well etched out vero board. It was found that the audio sound produced was clearer

when the unused conducting rails on the vero board were etched out or cut out. Vero boards

have relatively high parasitic capacitance between their conducting rails; these parasitic

capacitance do affect the general performance of the circuit.

It is believed that the performance of this circuit can also be improved, if a D-C power source

was used instead of a battery to power the circuit; however this would increase the power


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dissipated by the transistors and a cooling fan will be required to prevent the transistors from

getting damaged.


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[1] Ke-Lin Du, M.N.S., Wireless Communication Systems: From RF Subsystems to 4G enablingTechnologies. Swamy Cambridge University Press, 2010.

[2] Jerry C. Whitaker, The electronics Handbook , 2nd ed. CRC Press Taylor & Francis group, 2005.

[3] A.P Godse and U.A. Bakshi, Analog Communications. Technical Publications Pune, 2009.[4] V.K Mehta, Rohit Mehta, Principles of Electronics. S. CHAND & COMPANY, 2008.[5] Jeffrey Wheat, Randy Hiser, Jackie Tucker, Alicia Neely, and Andy McCullough, Designing a

Wireless Network . syngress.

[6] Prof. Ali M. Niknejad, “Negative Resistance Oscillator, Differential Oscillator and VCOs.”[Online]. Available: http://rfic.eecs.berkeley.edu/~niknejad/ee142_fa05lects/pdf/lect23.pdf.[Accessed: 27-Feb-2016].

[7] Paul McLane, “Radio World: FM Signal Count Grew 38% in Ten Years.” [Online]. Available:http://www.radioworld.com/article/fm-signal-count-grew--in-ten-years/277873. [Accessed: 21-Feb-2016].

[8] Dale R. Patrick and Stephen W. Fardo, Electricity and Electronics Fundamentals, 2nd ed. The

Fairmont Press Inc.

[9] Kellejian, Robert, Applied electronic communication: Circuits, systems, transmission. ScienceResearch Associates, 1980.

[10] H. Ward Silver, The ARRL Extra Class License Manual for Ham Radio. .[11] Dr. J.S Chitode, Communication Theory, 5th ed. Technical Publications Pune, 2010.[12] “Miniaturised FM transmitter.” [Online]. Available: http://pe2bz.philpem.me.uk/Comm/-

%20Transmitters/-%20FMx/FMx-902-PortableMiniSterio/Index. [Accessed: 24-Jan-2016].[13] “Single Transistor FM Transmitter Design | electronics hobby.” [Online]. Available:

https://dmohankumar.wordpress.com/2011/04/23/single-transistor-fm-transmitter-design/.[Accessed: 24-Jan-2016].

[14] “2 Watt FM Transmitter.” [Online]. Available: http://electronics-diy.com/2-watt-fm-transmitter.php. [Accessed: 07-Mar-2016].

[15] Ali Hajimiri And Thomas H. Lee, “Design Issues In CMOS Differential LC Oscillators,IEEE Journal Of Solid - State Circuits,” vol. 34, May 1999.

[16] W. Storr, “Sound Transducer for Sensing and Generating Sounds,” Basic Electronics Tutorials,18-Aug-2013. [Online]. Available: http://www.electronics-tutorials.ws/io/io_8.html. [Accessed:18-Jan-2016].

[17] Eric Coates MA BSc. (Hons), “Amplifier Class A Biasing.” [Online]. Available:http://www.learnabout-electronics.org/Amplifiers/amplifiers12.php#stabilisation. [Accessed:18-Jan-2016].

[18] “How to calculate coil inductance (single-layer, cylindrical air core inductors) - OnlineJavascript.” [Online]. Available: http://zpostbox.ru/how_to_calculate_inductors.html.[Accessed: 18-Jan-2016].

[19] “Varactor Diode | Varicap Variable Capacitance Diode | Tutorial.” [Online]. Available:

http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basics-tutorial.php. [Accessed: 05-Mar-2016].[20] “Community Radio Frequently Asked Questions and Answers.” [Online]. Available:

http://radio.xtreamlab.net/faqa.html. [Accessed: 27-Feb-2016].[21] Andy Collinson, “Estimating Transmitter Distance.” [Online]. Available:

http://www.zen22142.zen.co.uk/Analysis/efftxd.htm. [Accessed: 27-Feb-2016].

[22] “2N3553 Datasheet.” [Online]. Available: http://www.futurlec.com/Transistors/2N3553.shtml.[Accessed: 27-Feb-2016].

[23] “2 Watt FM Transmitter.” [Online]. Available: http://electronics-diy.com/2-watt-fm-transmitter.php. [Accessed: 27-Feb-2016].

[24] “BUILD YOUR YAGI ANTENNA.” [Online]. Available:

http://radio.meteor.free.fr/us/antenna.html. [Accessed: 30-Jan-2016].


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6 APPENDIX

Documents

Design and Construction of Fm Transmitter Report