Design and Construction of Fm Transmitter Report

DESIGN AND CONSTRUCTION OF FM TRANSMITTER 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) ELECTRONICS AND COMMUNICATIONS ENGINEERING

MARCH 2016

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.

i

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

ii

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)

iii

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.

iv

TABLE OF CONTENTS ABSTRACT................................................................................................................................ i DECLARATION .......................................................................................................................ii CERTIFICATION ................................................................................................................... iii ACKNOWLEDGEMENTS ...................................................................................................... iv LIST OF TABLES ...................................................................................................................vii LIST OF FIGURES .................................................................................................................vii CHAPTER 1 .............................................................................................................................. 1

2

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

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

2.4.1

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

2.4.2

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

2.5

3

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

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

CHAPTER 3 DESIGN AND IMPLEMENTATION ....................................................... 13 3.1

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

3.2

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

3.2.1 3.3

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

3.3.1 3.4

4

5

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

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

3.4.1 3.5

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

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

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

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

CHAPTER 5 CONCLUSION AND RECOMMENDATION ......................................... 30 5.1

CONCLUSION ......................................................................................................... 30

5.2

LIMITATION ........................................................................................................... 30

5.3

RECOMMENDATION ............................................................................................ 30

REFERENCES ........................................................................................................................ 32 6

APPENDIX ...................................................................................................................... 33

vi

LIST OF TABLES Table 4.1 RESISTORS .................................................................................................................... 25 Table 4.2 CAPACITORS ................................................................................................................ 26 Table 4.3 INDUCTORS................................................................................................................... 26 Table 4.4 TRANSISTORS .............................................................................................................. 26 Table 4.5 DIODES ........................................................................................................................... 27 Table 4.6 VOLTAGE AND CURRENT MEASUREMENT........................................................... 29

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

vii

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 audiofrequency 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.

1

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 and signal 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

Microphone

Audio Amplifier

Modulator

Antenna

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 on the microphone generates an electrical signal representation of the sound, which 2

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. 3

The objectives of this project are: I. II.

To generate an electrical signal representation of an audio message using a transducer. 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 4

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.

5

2

2.1

CHAPTER 2 LITERATURE REVIEW

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

6

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

Modulating Signal

Carrier Oscillator

Modulator

FM Signal

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

7

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: epm = Ec sin (wc t + m cos wmt) ----- 1 [11] where m – Modulation index The instantaneous angular frequency wp of the above phase modulated signal is given by: wp =


 ()


------------------------------ 2;

where θ (t) = wc t + m coswm t


wp =
 [wc t + m coswm t ] ------------- 3; wp = wc – m sin wm t × wm -------------- 4; In terms of linear frequencies above equation can be written as: fp = fc – m fm sin(2πfmt) ------------------- 5;

The 2nd term in the equation represents the frequency shift with respect to centre frequency i.e. fc + ∆f [11]

This shows that frequency of the phase modulated signal varies around the carrier frequency fc with the deviation of ∆f = m fm sin(2πfmt). It can be seen that if modulating frequency fm 8

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.

9

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 R1 = 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.

10

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.

11

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.

12

3

3.1

CHAPTER 3 DESIGN AND IMPLEMENTATION

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.

SOUND SENSOR

AUDIO AMPLIFIER

VARACTOR MODULATOR

CARRIER OSCILLATOR

ANTENNA

Frequency Modulator

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.

13

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 R1 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: ImR1 + Vmic = Vcc R1 =

  



----- (I) ----- (II)

We choose Im = 0.1mA ; Vmic = 2V  

R1 = . ×  = 70 × 103 Ω

14

Standard value chosen for R1 = 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 15

Taking KVL across the circuit, the following equations are obtained: VC = VCC – (IC + IB) RC

----- (I)

VE = 0

----- (II)

VBE = VB – VE

----- (III)

 

----- (IV)

IB =



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 RC =

  

 

≈ IC RC

------ (VI)

.

= ∗.∗ = 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): RB =

 

#

$.$% .%

RB = &.&& × ' = 1.04 × 106Ω Practical Values chosen: RC = 10 KΩ; RB = 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. 16

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

17

(a)

Design for Oscillator Tank Circuit

Chosen Inductance (L) = 1 µH Using the equation for calculating the inductance of a single layer air-core coil L = (

(

)2 ×



)*

$.(+,

µH [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 µH 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 fc generated by the LC tank is given as 

fc = 1√/2

3

----- (III)

Desired operating Frequency fc = 80 MHz From equation (IV) C5 is calculated as:

18



C5 =

$1/45*

----- (IV)

C5 =

$1 ×  × ' ×(6 × ' )*



= 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 R5 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=

7& × 89

[21]

:

----- (I)

Typical Receiver Sensitivity (E) = 50 µV/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 ≈ Rinductor Rinductor = 1Ω

19

Power in the tank circuit (Ptank) = I2Z 89<=>

I=;

?

6.& × 

=;



----- (II) [21]

= 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; R3 = R4 = 4.7 KΩ VR1 = VR2 = VBE = 0.7 V

----- (I)

VR3 = VR4 = Vcc – VR1

----- (II)

IC = βIB

----- (III)

@A

----- (IV)

R1 =

A

I1 = I2 = I3 – IB

----- (V)

VR3 = I3 R3

----- (VI)

From eqn (II) : VR3 = VR4 = 9 – 0.7 = 8.3 V From eqn (VI) : I3 =

@

From eqn (III): IB =

5





=

6.&

= $.% ×  = 0.00177 A $" ×  

= 0.31 × 10-3 A

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

From eqn (IV): R1 = .$" ×  = 479 Ω

20

Standard values chosen: R2 = R1 = 470 Ω; R4 = R3 = 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



= 4(BCD)

----- (I)

21



----- (II)

&

----- (III)

&6

----- (IV)

$&

----- (V)



----- (VI)

Second Director Length

=

4(BCD)

Third Director Length

=

4(BCD)

Fourth Director Length

= 4(BCD)

Dipole Length

=

Reflector Length

= 4(BCD)

4(BCD)

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

----- (VII)

$

----- (VIII)

&

----- (IX)

&

----- (X)

$6

----- (XI)

A = 4(BCD) B = 4(BCD) C = 4(BCD) D = 4(BCD) E = 4(BCD)



First Director Length

=

Second Director Length

=  = 1.25 meters



= 1.2 meters



&

Third Director Length

=  = 1.3 meters

Fourth Director Length

=  = 1.38 meters

Dipole Length

=  = 1.43 meters

Reflector Length

&6 $&



=  = 1.50 meters

"

A =  = 0.6 meters $

B =  = 0.45 meters

22

C=

&



= 0.3 meters

&

D =  = 0.3 meters E=

$6



= 0.48 meters

Figure 3.6 YAGI ANTENNA STRUCTURE [24]

23

4

4.1

CHAPTER 4 TEST AND RESULTS

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 24

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 Component 68 KΩ

Type Carbon Film

RESISTORS Quantity

1

Use Bias for electret microphone

4.7 KΩ

Carbon Film

4

Voltage divider DC-Bias for carrier Oscillator

25

10 KΩ

Carbon Film

1

Provide Modulating voltage

c)

Capacitors Table 4.2 Component Value

47 nF

CAPACITORS

Type

Quantity

Ceramic

2

Use For stabilising D-C input voltage

22 pF

Ceramic

2

Feedback Capacitor to enhance voltage swing of Oscillator

22 µF

Ceramic

1

Audio Coupling Capacitor

2 – 10 pF

Variable

1

Capacitance for tank

capacitor

d)

circuit

Inductor Table 4.3

Component

Type

INDUCTORS

Quantity

Use

Value 0.1 µH

Air – Core Wound

1

Inductance for tank circuit

Inductor

e)

Transistor Table 4.4 Component Value

Type

TRANSISTORS Quantity

26

Use

2N3553

BJT

2

Carrier Oscillator

Transistor

f)

Diode Table 4.5 Component Value

BB204

DIODES

Type

Quantity

Variable Capacitance

Use

2

Carrier Oscillator

1

To provide protection

Diode Diode

PN Diode

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. II. III.

Waveform Measurement Voltage and current measurement 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.

27

Volts

Time

/ Div

/ Div

50

1 ms

mV

Figure 4.1 PRE-AMPLIFIED VS AMPLIFIED AUDIO WAVEFORM

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.

28

Table 4.6 VOLTAGE AND CURRENT MEASUREMENT Operational

Voltage (V)

Current (mA)

Parameter

( V)

(I)

Battery

9

165

Transistor

VBE

1

IB

1.1

IC

97

Transistor VBE 2

0.67

0.68

IB

1.1

IC

97

R1

0.69

R2

0.69

R3

8.31

R4

8.31

From the measurements in table 4.6, we can calculate the following: Power Consumption = Vbattery × Icurrent = 9 × 165mA = 1485 mW Power in Tank circuit = 2 × Ic2 × Rinductor 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.

29

5

5.1

CHAPTER 5 CONCLUSION AND RECOMMENDATION

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

30

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

31

REFERENCES [1] [2] [3] [4] [5] [6]

[7]

[8] [9] [10] [11] [12] [13]

[14] [15] [16]

[17]

[18]

[19]

[20] [21] [22] [23] [24]

Ke-Lin Du, M.N.S., Wireless Communication Systems: From RF Subsystems to 4G enabling Technologies. Swamy Cambridge University Press, 2010. Jerry C. Whitaker, The electronics Handbook, 2nd ed. CRC Press Taylor & Francis group, 2005. A.P Godse and U.A. Bakshi, Analog Communications. Technical Publications Pune, 2009. V.K Mehta, Rohit Mehta, Principles of Electronics. S. CHAND & COMPANY, 2008. Jeffrey Wheat, Randy Hiser, Jackie Tucker, Alicia Neely, and Andy McCullough, Designing a Wireless Network. syngress. 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]. 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: 21Feb-2016]. Dale R. Patrick and Stephen W. Fardo, Electricity and Electronics Fundamentals, 2nd ed. The Fairmont Press Inc. Kellejian, Robert, Applied electronic communication: Circuits, systems, transmission. Science Research Associates, 1980. H. Ward Silver, The ARRL Extra Class License Manual for Ham Radio. . Dr. J.S Chitode, Communication Theory, 5th ed. Technical Publications Pune, 2010. “Miniaturised FM transmitter.” [Online]. Available: http://pe2bz.philpem.me.uk/Comm/%20Transmitters/-%20FMx/FMx-902-PortableMiniSterio/Index. [Accessed: 24-Jan-2016]. “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]. “2 Watt FM Transmitter.” [Online]. Available: http://electronics-diy.com/2-watt-fmtransmitter.php. [Accessed: 07-Mar-2016]. Ali Hajimiri And Thomas H. Lee, “Design Issues In CMOS Differential LC Oscillators, IEEE Journal Of Solid - State Circuits,” vol. 34, May 1999. 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]. Eric Coates MA BSc. (Hons), “Amplifier Class A Biasing.” [Online]. Available: http://www.learnabout-electronics.org/Amplifiers/amplifiers12.php#stabilisation. [Accessed: 18-Jan-2016]. “How to calculate coil inductance (single-layer, cylindrical air core inductors) - Online Javascript.” [Online]. Available: http://zpostbox.ru/how_to_calculate_inductors.html. [Accessed: 18-Jan-2016]. “Varactor Diode | Varicap Variable Capacitance Diode | Tutorial.” [Online]. Available: http://www.radio-electronics.com/info/data/semicond/varactor-varicap-diodes/basicstutorial.php. [Accessed: 05-Mar-2016]. “Community Radio Frequently Asked Questions and Answers.” [Online]. Available: http://radio.xtreamlab.net/faqa.html. [Accessed: 27-Feb-2016]. Andy Collinson, “Estimating Transmitter Distance.” [Online]. Available: http://www.zen22142.zen.co.uk/Analysis/efftxd.htm. [Accessed: 27-Feb-2016]. “2N3553 Datasheet.” [Online]. Available: http://www.futurlec.com/Transistors/2N3553.shtml. [Accessed: 27-Feb-2016]. “2 Watt FM Transmitter.” [Online]. Available: http://electronics-diy.com/2-watt-fmtransmitter.php. [Accessed: 27-Feb-2016]. “BUILD YOUR YAGI ANTENNA.” [Online]. Available: http://radio.meteor.free.fr/us/antenna.html. [Accessed: 30-Jan-2016].

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6

APPENDIX

33