PROJECT REPORT

ON

FM TRANSMITTER

BY

Ga bha

(Roll No. 78)

Under the supervision of

Tanu dutta

Submitted

In fulfilment of the requirements of the Bachelor of Technology degree

To the

Electronics and Communication Department

Session 2010-2011

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

Jaipur Engineering College Jaipur

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CERTIFICATE

This is to certify that the work, which is being presented in the Project, entitled “Three Stage FM Transmitter” submitted by “Da gha” student of B.Tech final year of branch Electronics and Communication from college Jaipur Engineering College Jaipur in partial fulfilment for the award of degree of Bachelor Of Technology is a record of work carried out by her under my guidance and supervision and has been found satisfactory and approved for submission during the academic year 2010-2011. This work has not been submitted elsewhere for the award of any other degree.

Mr. Sarkar Guided by: Head of the Department, ECE, Ms. Kumari JEC (Faculty Lecturer) Department of ECE,

JEC

Date:

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ACKNOWLEDGEMENT

Success can be achieved only by hard work and proper guidance. I would like to express my sincere gratitude to everyone who have been a source of information and inspiration to me during the development of my project on “Three Stage FM Transmitter”.

I am immensely grateful to Prof. Sarkar (Head of the department,ECE, JEC, Jaipur) for giving me constant guidance and valuable suggestions.

I am extremely grateful and remain indebted to my guide Ms. Kumari (Faculty Lecturer) for giving me the proper guidance which was required inevitably for the successful completion of my project. She gave me all the knowledge required to overcome the various problems. I know her guidelines will definitely help me in future.

I hope this project, which is a fruit of long dedicated hours and consistent dedication, will be appreciated. I acknowledge my sincere thanks to all the staff members for providing me the infrastructure and facilities to carry out my project successfully.

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ABSTRACT

The aim of the project is to develop a Miniaturised low power FM Transmitter to be used in specialised applications such as a hearing aid for a tour guiding system and room monitoring (such as a baby listening device). The overall module should be miniature to enable portability. Frequency modulation has several advantages over the system of amplitude modulation (AM) used in the alternate form of radio broadcasting. The most important of these advantages is that an FM system has greater freedom from interference and static. Various electrical disturbances, such as those caused by thunderstorms and car ignition systems; create amplitude modulated radio signals that are received as noise by AM receivers. A well-designed FM receiver is not sensitive to such disturbances when it is tuned to an FM signal of sufficient strength. Also, the signal-to-noise ratio in an FM system is much higher than that of an AM system. FM broadcasting stations can be operated in the very-high-frequency bands at which AM interference is frequently severe; commercial FM radio stations are assigned frequencies between 88 and 108 MHz and will be the intended frequency range of transmission.

The main report will reflect on 4 issues, background to frequency modulation, electronics component characteristics, basic transmitter building blocks and finally an analysis of the finished design as regards construction and performance.

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INDEX

S.No Topic No.

Topic Page No.

1 1 Frequency Modulation Background 62 1.1 Introduction 63 1.2 Technical Background 6-74 1.3 FM Theory 7-95 1.4 Technical terms associated with FM 96 2 Electronic Component and their Properties 107 2.1 Resistors 10-118 2.2 Capacitors 11-139 2.3 Inductors 1310 2.4 Resonant Circuits 13-1411 2.5 NPN Transistors 14-1512 2.6 Transistor Amplifiers 1613 2.7 Battery 1714 3 Basic Building Blocks for FM Transmitter 1815 3.1 Introduction 1816 3.2 General Overview 1817 3.3 The Microphone 1918 3.4 The Oscillator 1919 3.5 Reactance Modulator 1920 3.6 Buffer Amplifier 2021 3.7 Frequency Multiplier 20-2122 3.8 Driver Amplifier 2123 3.9 Power Amplifier 2124 3.10 Antenna 2125 4 Simple FM Transmitter 22-2326 5 Three Stage FM Transmitter 2427 5.1 Circuit Description 24-2628 5.2 Working in Brief 2629 5.3 Components 2730 6 Conclusions 2831 7 Recommendations 2832 8 References 28

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1 Frequency Modulation Background

1.1 Introduction

The comparatively low cost of equipment for an FM broadcasting station, resulted in rapid growth in the years following World War II. Within three years after the close of the war, 600 licensed FM stations were broadcasting in the United States and by the end of the 1980s there were over 4,000. Similar trends have occurred in Britain and other countries. Because of crowding in the AM broadcast band and the inability of standard AM receivers to eliminate noise, the tonal fidelity of standard stations is purposely limited. FM does not have these drawbacks and therefore can be used to transmit music, reproducing the original performance with a degree of fidelity that cannot be reached on AM bands. FM stereophonic broadcasting has drawn increasing numbers of listeners to popular as well as classical music, so that commercial FM stations draw higher audience ratings than AM stations. The integrated chip has also played its part in the wide proliferation of FM receivers, as circuits got smaller it became easier to make a modular electronic device called the “Walkman”, which enables the portability of a tape player and an AM/FM radio receiver. This has resulted in the portability of a miniature FM receiver, which is carried by most people when travelling on long trips.

1.2 Technical Background

Frequency Designation Abbreviation Wavelength

3 - 30 kHz

30 - 300 kHz

300- 3,000 kHz

30 - 30MHz

30 - 300 MHz

300- 3,000 MHz

3 - 30 GHz

30 - 300 GHz

Very Low frequency

Low frequency

Medium frequency

High frequency

Very High frequency

Ultra-high frequency

Super-high frequency

Ex-high frequency

VLF

LF

MF

HF

VHF

UHF

SHF

EHF

100,000-10,000 m

10,000 - 1,000 m

1,000 - 100 m

100 - 10 m

10 - 1m

1m – 10cm

10cm - 1cm

1cm - 1mm

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The main frequencies of interest are from 88MHz to 108MHz with wavelengths between 3.4 and 2.77 meters respectively.With a bandwidth of 200Khz for one station, up to 100 stations can be fitted between 88 & 108Mhz.Station 88Mhz to 91.2Mhz are for non-commercial stations (educational) which could be a good area to transmit in, but in recent years the band from 88MHz to 103Mhz has been filled by a lot of commercial channels, making the lower frequencies very congested indeed.

1.2.1 Radio Frequency and Wavelength Ranges

Radio waves have a wide range of applications, including communication during emergency rescues (transistor and short-wave radios), international broadcasts (satellites), and cooking food (microwaves). A radio wave is described by its wavelength (the distance from one crest to the next) or its frequency (the number of crests that move past a point in one second). Wavelengths of radio waves range from 100,000 m (270,000 ft) to 1 mm (.004 in). Frequencies range from 3 kilohertz to 300 Giga-hertz.

1.3 FM theory

Angle and Amplitude Modulation are techniques used in Communication to transmit Data or Voice over a particular medium, whether it is over wire cable, fibre optic or air (the atmosphere). A wave that is proportional to the original baseband (a real time property, such as amplitude) information is used to vary the angle or amplitude of a higher frequency wave (the carrier).

Carrier = A Cos Ф(t)Ф(t) = 2πfct +α

Where A is the amplitude of the carrier and Ф(t) is the angle of the carrier, which constitutes the frequency (fc ) and the phase (α) of the carrier. Angle modulation arise the angle of the carrier by an amount proportional to the information signal. Angle modulation can be broken into 2 distinct categories, frequency modulation and phase modulation. Formal definitions are given below :

Phase Modulation (PM) : angle modulation in which the phase of a carrier is caused to depart from its reference value by an amount proportional to the modulating signal amplitude.

Frequency Modulation (FM): angle modulation in which the instantaneous frequency of a sine wave carrier is caused to depart from the carrier frequency by an amount proportional to the instantaneous value of the modulator or intelligence wave.

Phase modulation differs from Frequency modulation in one important way. Take a carrier of the form A Cos (ωct +θ) = Re{A.e ^j(ωct +θ)}

PM will have the carrier phasor in between the + and - excursions of the modulating signal. FM modulation also has the carrier in the middle but the fact that when you integrate the modulating signal and put it through a phase modulator you get fm, and if the modulating wave were put through a differentiator before a frequency modulator you get a phase modulated wave. This may seem confusing at this point, but the above concept will be reinforced further in the sections to

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follow.1.3.1 Theory

Suppose the baseband data signal (the message) to be transmitted is

And is restricted in amplitude to be

And the sinusoidal carrier is

Where fc is the carrier's base frequency and ac is the carrier's amplitude. The modulator combines the carrier with the baseband data signal to get the transmitted signal,

Xm(t)= Vpk Cos(2πfmt)VFM = A Cosθ(t) = A Cos[2πfct +Mf sin(2πfmt)]

1.3.2 Modulation index

As with other modulation indices, this quantity indicates by how much the modulated variable varies around its unmodulated level. It relates to the variations in the frequency of the carrier signal:

Where is the highest frequency component present in the modulating signal xm(t), and is the Peak frequency-deviation, i.e. The maximum deviation of the instantaneous frequency from the carrier frequency. If , the modulation is called narrowband FM, and its bandwidth is approximately . If , the modulation is called wideband FM and its bandwidth is approximately . While wideband FM uses more bandwidth, it can improve signal-to-noise ratio significantly.

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1.3.3 Differences of Phase over Frequency modulation

The main difference is in the modulation index, PM uses a constant modulation index, whereas FM varies (Max frequency deviation over the instantaneous baseband frequency). Because of this the demodulation S/N ratio of PM is far better than FM. The reason why PM is not used in the commercial frequencies is because of the fact that PM need a coherent local oscillator to demodulate the signal, this demands a phase lock loop, back in the early years the circuitry for a PLL couldn’t be integrated and therefore FM, without the need for coherent demodulation was the first on the market. One of the advantages of FM over PM is that the FM VCO can produce high-index frequency modulation, whereas PM requires multipliers to produce high-index phase modulation. PM circuitry can be used today because of very large scale integration used in electronic chips, as stated before to get an FM signal from a phase modulator the baseband can be integrated, this is the modern approach taken in the development of high quality FM transmitters.

For miniaturisation and transmission in the commercial bandwidth to be aims for the transmitter, PM cannot be even considered, even though Narrow Band PM can be used to produce Wide band FM (Armstrong Method).

1.4 Technical terms associated with FM

Now that Fm has been established as a scheme of high quality baseband transmission, some of the general properties of FM will be looked at.

1.4.1 Capture Effect

Simply put means that if 2 stations or more are transmitting at near the same frequency FM has the ability t pick up the stronger signal and attenuated the unwanted signal pickup.

1.4.2 Carrier Swing

The carrier swing is twice the instantaneous deviation from the carrier frequency.FCS = 2.∆FC

The frequency swing in theory can be anything from 0Hz to 150KHz.

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2 Electronic Components and their properties

Electronic component are classed into either being Passive devices or Active devices. A Passive Device is one that contributes no power gain (amplification) to a circuit or system. It has no control action and does not require any input other than a signal to perform its function. In other words, it is called “A components with no brains!” Examples are Resistors, Capacitors and Inductors.

2.1 Resistors

A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current through it in accordance with Ohm's law:

V = IR

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).

The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance depends upon the materials constituting the resistor as well as its physical dimensions; it's determined by design.

Colour coding of resistors

Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.

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Colour 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. CoefficientBlack 0 0 ×100

Brown 1 1 ×101 ±1% (F) 100 ppmRed 2 2 ×102 ±2% (G) 50 ppmOrange 3 3 ×103 15 ppmYellow 4 4 ×104 25 ppmGreen 5 5 ×105 ±0.5% (D)Blue 6 6 ×106 ±0.25% (C)Violet 7 7 ×107 ±0.1% (B)Gray 8 8 ×108 ±0.05% (A)White 9 9 ×109

Gold ×10-1 ±5% (J)Silver ×10-2 ±10% (K)None ±20% (M)

The three main factors when choosing a resistor for an intended application are: Tolerance Power Rating Stability

2.2 Capacitors

2.2.1 Ceramic Capacitors

In electronics ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric.

A ceramic capacitor is a two-terminal, non-polar device. The classical ceramic capacitor is the "disc capacitor". This device pre-dates the transistor and was used extensively in vacuum-tube equipment (e.g., radio receivers) from about 1930 through the 1950s, and in discrete transistor equipment from the 1950s through the 1980s. As of 2007, ceramic disc capacitors are in widespread use in electronic equipment, providing high capacity & small size at low price compared to other low value capacitor types.

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There is a three digit code printed on a ceramic capacitor specifying its value. The first two digits are the two significant figures and the third digit is a base 10 multiplier. The value is given in Pico farads (pF). A letter suffix indicates the tolerance:

C ± 0.25pF M ± 20%

D ± 0.5pF P +100 -0%

J ± 5% Y -20 +50%

K ± 10% Z -20 + 80%

Example: a label of "104K" indicates 10×104 pF = 100,000 pF = 100 nF = 0.1uF (aka 0.1mF) ± 10%

2.2.2 Electrolytic Capacitor

An electrolytic capacitor is a type of capacitor that uses an ionic conducting liquid as one of its plates with a larger capacitance per unit volume than other types. They are valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations in rectifier output. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not.

Electrolytic capacitors can have a very high capacitance, allowing filters made with them to have very low corner frequencies.

The principle of the electrolytic capacitor was discovered in 1886 by Charles Pollak, as part of his research into anodizing of aluminum and other metals. Pollack discovered that due to the thinness of the aluminum oxide layer produced, there was a very high capacitance between the aluminum and the electrolyte solution. A major problem was that most electrolytes tended to dissolve the oxide layer again when the power is removed, but he eventually found that sodium perforate (borax) would allow the layer to be formed and not attack it afterwards. He was granted a patent for the borax-solution aluminum electrolytic capacitor in 1897.

The first application of the technology was in making starting capacitors for single-phase alternating current (AC) motors. Although most electrolytic capacitors are polarized, that is, they can only be operated with direct current (DC), by separately anodizing aluminum plates and then interleaving them in a borax bath, it is possible to make a capacitor that can be used in AC systems.

The first major application of DC versions of this type of capacitor was in large telephone exchanges, to reduce relay hash (noise) on the 48 volt DC power supply. The development of AC-

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operated domestic radio receivers in the late 1920s created a demand for large capacitance (for the time) high voltage capacitors, typically at least 4 microfarads and rated at around 500 volts DC. Waxed paper and oiled silk capacitors were available but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive.

2.3 Inductors:

You may remember from science class that adding electrical current to a coil of wire produces a magnetic field around itself. This is how the inductor works. It is charged with a magnetic field and when that field collapses it produces current in the opposite direction. Inductors are used in Alternating Current circuits to oppose changes in the existing current. Most inductors can be identified by the "coil" appearance. Others actually look like a resistor but are usually green in color. A. Air Core, B. Iron Core, C. Powered Metal Core.

2.4 Resonant Circuits

In the last section the resistor, inductor & capacitor were looked at briefly from a voltage, current and impedance point of view. These components will be the basic building blocks used in any radio frequency section of any transmitter/receiver. What makes them important is there response at certain frequencies. At low frequency the impedance of an inductor is small and the impedance of a capacitor is quite high. At high frequency the inductor’s impedance becomes quite high and the capacitor’s impedance drops. The resistor in theory maintains its resistive impedance at low & high impedance. At a certain frequency the capacitor’s impedance will equal that of an inductor’s. This is called the resonant frequency and can be calculated by letting the impedance of a capacitor to that of the inductor’s and then solving for ω (angular velocity in radians per seconds) and then finding the resonant frequency Fc (it is normally represented as Fo, but in relation to FM it essentially represents the oscillator carrier frequency) in Hertz.ωc = 1/√LCfc = 1/2π√LCThere are two configurations of RLC circuits, the series and parallel arrangements, which will now be looked at below.

2.4.1 Series resonant circuit

Figure 2.4-1

At low frequencies the capacitor impedance will dominate the overall impedance of the series circuit and the current is low. At high frequencies the inductor impedance will dominate and the

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current will also be low. But at the resonant frequency the complex impedance of the capacitor will cancel that of the inductor’s and only the resistance of the resistor will remain effective, this is when the current through the circuit will be at a maximum.

2.4.2 Parallel resonant circuit

Figure 2.4-2

The parallel circuit above (known as an LC tank) takes the same advantage of the resonant frequency but this time the impedance will be at a maximum and the current will be at a minimum at FC. This is due to the fact that the minimum impedance in a parallel circuit dominates the overall impedance of the tank.

Active Devices are components that are capable of controlling voltages or currents and can create a switching action in the circuit. In other words, we can say that “Devices with smarts!” Examples are Diodes, Transistors and Integrated circuits. Most active components are semiconductors.

2.5 NPN Transistor

Figure 2.5-1PNP bipolar and P channel J-Fets are widely used at low frequencies, the preference for high frequency systems lies with the NPN and N channel J-Fets. This is due to the electrons being the

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majority carriers in both the BJT’s and J-Fet’s conduction channel.

The NPN BJT is the most commonly used and for the rest of this discussion will be the transistor that will be focused on.

Transistors are non-linear especially when biased in the saturation region. The bias current acts as a controlled flow source which steadily opens up the collector

emitter channel enabling charge carriers to flow, this can be analogous to a slues gate, this rate of flow is controlled by the current gain = IC/IB .

The Input impedance drops as the biasing current being sinked to the collector increases. As the base current increases to allow more collector current through, the current gain also

increases. The collector-emitter voltage has a maximum value that cannot be exceeded at an instant

in time.

2.5.1 High Frequency Response

The most interesting property is the junction capacitance from the base to emitter and base-collector, the Figure 2.9-2 shows that for the 2N3904, the base-emitter capacitance is larger than the base-collector, because of heavier extrinsic doping and it’s forward biasing the depletion region is naturally smaller than the base-collector’s. As the frequencies are increased the two capacitances will drop. Because the capacitors are effectively in series, the smaller one dominates (base-collector capacitance). The capacitance is also influenced by the rate of change in base current magnitudes.

A resistance exists of typically in the order of tens of ohms at the base, this parasitic is caused by impure contact between the base’s polysilicon to silicon junction. This coupled with the r’e resistance and the current gain makes up the input resistance of the transistor. Rin = ( Rbase + r’e) ; as stated previously the r’e will inevitably drop as the frequency increases, therefore Rin (base) will inevitably be equal to (Rbase). Thismakes the system rather unstable, as Rbase is essentially parasitic impedance. To increase stability RE, (which is normally RF bypassed), will have to be introduced.

Another inherent flaw which might be used to some advantage in the high frequency response of the NPN model, is that of output collector signals are be fed back to thebase. This increases the likelihood of continuous oscillation at high frequencies.

2.6 Transistor Amplifiers

Now that the basic electronic components have been considered, a look at the 3 transistor

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amplifiers is worthwhile prelude to the next section, which contains references and examples of these amplifiers. The three amplifiers are called Common Emitter, Common collector and Common Base.

2.6.1 Common Emitter

Figure 2.6-1

r’c and r’e are the junction resistances at the collector and emitter respectively. r’c is seen as infinite (reverse bias junction), r’c is equal to the threshold voltage VT divided by the emitter current.

IC = IB + IE, IB is relatively small compared to IB the base current⇒ IC≈ IE .

All capacitor’s used here are DC opens and AC shorts. The supply ideally has noimpedance and therefore no voltage dropped across it. So it is an AC ground

2.7 BATTERY

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It’s used to supply power. The battery that we have used is a 9 volts general purpose battery i.e., it provides voltage of 9 volts that is dc in nature.

Classification: Alkaline

Chemical System: Zinc-Manganese Dioxide (Zn/ MnO2 )

Designation: ANSI-1604A, IEC-6LR61

Nominal Voltage: 9.0 volts

Operating Temp: -18°C to 55°C (0°F to 130°F)

Typical Weight: 45.6 grams (1.6 oz.)

Typical Volume: 21.1 cubic centimeters (1.3 cubic inch)

Jacket: Metal

Shelf Life: 5 years at 21°C (80% of initial capacity)

Terminal: Miniature Snap

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3 Basic Building blocks for an FM transmitter

3.1 Introduction

When creating a system for transmitting a frequency modulated wave a number of basic building blocks have to be considered, the diagram below gives a very broad impression of the transmitter and it’s individual parts.

To Antenna

Audio InputExciter/modulator frequency multiplier power output section

3.2 General Overview

3.2.1 Exciter /Modulator

Carrier Oscillator generates a stable sine wave for the carrier wave. Linear frequency even when modulated with little or no amplitude change.

Buffer amplifier acts as a high impedance load on oscillator to help stabilise frequency. The Modulator deviates the audio input about the carrier frequency. The peak + of audio

will give a decreased frequency & the peak - of the audio will give an increase of frequency.

3.2.2 Frequency Multipliers

Frequency multipliers tuned-input, tuned-output RF amplifiers. In which the output resonance circuit is tuned to a multiple of the input .Commonly they are *2 *3*4 & *5.

3.2.3 Power output section

This develops the final carrier power to be transmitter. Also included here is an impedance matching network, in which the output impedance is

the same as that on the load (antenna).

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Carrier oscillator

Buffer amplifier

Frequency multipliers

Driver amplifier

Power output amplifier

Reactance modulator

3.3 The Microphone

Microphones are acoustic to electrical transducers. The four best known variations of these are the moving coil (‘dynamic’), ribbon, piezoelectric (‘crystal’), and electrets (‘capacitor’). The electrets type will be discussed because of their incredibly small size and high performance at audio frequencies.

3.4 The Oscillator

The carrier oscillator is used to generate a stable sine-wave at the carrier frequency, when no modulating signal is applied to it . When fully modulated it must change frequency linearly like a voltage controlled oscillator. At frequencies higher than 1MHz a Colpitts (split capacitor configuration) or Hartley oscillator (split inductor configuration) may be deployed.

A parallel LC circuit is at the heart of the oscillator with an amplifier and a feedback network (positive feedback). The Barkhausen criteria of oscillation require that the loop gain be unity and that the total phase shift through the system is 360o. In that way an impulse or noise applied to the LC circuit is fed back and is amplified (due to the fact that in practice the loop gain is slightly greater than unity) and sustains a ripple through the network at a resonant frequency of 1/2π√LC.

3.5 Reactance modulator

The nature of FM as described before is that when the baseband signal is Zero the carrier is at its “carrier” frequency, when it peaks the carrier deviation is at a maximum and when it troughs the deviation is at its minimum. This deviation is simply a quickening or slowing down of frequency around the carrier frequency by an amount proportional to the baseband signal. In order to convey the characteristic of FM on the carrier wave the inductance or capacitance (of the tank) must be varied by the baseband. Normally the capacitance of the tank is varied by a varactor diode. The

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varactor diode when in reverse bias has a capacitance across it proportional to the magnitude of the reverse bias applied to it. The formula for working out the instantaneous capacitance is shows that as the reverse bias is increased the capacitance is decreased.3.6 Buffer Amplifier

The buffer amplifier acts as a high input impedance with a low gain and low output impedance associated with it. The high input impedance prevents loading effects from the oscillator section, this high input impedance maybe looked upon as RL in the analysis of the Colpitts Oscillator. The High impedance RL helped to stabilise the oscillators frequency.

Looking at the Buffer amplifier as an electronic block circuit, it may resemble a common emitter with low voltage gain or simply an emitter follower transistor configuration.

3.7 Frequency Multipliers

Frequency modulation of the carrier by the baseband can be carried out with a high modulation index, but this is prone to frequency drift of the LC tank, to combat this drift, modulation can take place at lower frequencies where the Q factor of the tank circuit is quite high (i.e. low bandwidth or less carrier deviation) and the carrier can be created by a crystal controlled oscillator. At low frequency deviations the crystal oscillator can produce modulated signals that can keep an audio distortion under 1%. This narrow-band angle modulated wave can be then multiplied up to the required transmission frequency; the deviation brought about by the baseband is also multiplied up, which means that the percentage modulation and Q remain unchanged. This ensures a higher performance system that can produce a carrier deviation of 75Khz.

Frequency multipliers are tuned input, tuned output RF amplifiers, where the output resonant tank frequency is a multiple of the input frequency. The diagram of the simple multiplier below shows the output resonant parallel LC tank which is a multiple of the input frequency.

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3.8 Driver Amplifier

The driver amplifier can be seen to do the same function as the buffer amplifier, i.e. a high input impedance, low gain (close to unity) and low output impedance between the frequency multiplier and power output stages of the transmitter. The circuitry is the same as discussed in the Buffer amplifier description.

3.9 Power Output Amplifier

The power amplifier takes the energy drawn from the DC power supply and converts it to the AC signal power that is to be radiated. The efficiency or lack of it in most amplifiers is affected by heat being dissipated in the transistor and surrounding circuitry. For this reason , the final power amplifier is usually a Class-C amplifier for high powered modulation systems or just a Class B push-pull amplifier for use in a low-level power modulated transmitter. Therefore the choice of amplifier type depends greatly on the output power and intended range of the transmitter.

3.10 Antenna

The final stage of any transmitter is the Antenna; this is where the electronic FM signal is converted to electromagnetic waves, which are radiated into the atmosphere. Antennas can be vertically or horizontally polarised, which is determined by their relative position with the earth’s surface (i.e. antenna parallel with the ground is horizontally polarised). A transmitting antenna that is horizontally polarised transmits better to a receiving antenna that is also horizontally polarised, this is also true for vertically polarised antennas. One of the intended uses for the transmitter is as a tour guiding aid, where a walkman shall be used as the receiver, for a walkman the receiving antenna is the co-axial shielding around the earphone wire. The earphone wire is normally left vertical; therefore a vertically polarised whip antenna will be the chosen antenna for this particular application.

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4 Simple FM transmitter with a single transistor

Mini FM transmitters take place as one of the standard circuit types in an amateur electronics fan's beginning steps. When done right, they provide very clear wireless sound transmission through an ordinary FM radio over a remarkable distance. I've seen lots of designs through the years, some of them were so simple, some of them were powerful, some of them were hard to build etc.

Here is the last step of this evolution, the most stable, smallest, problem less, and energy saving champion of this race. Circuit given below will serve as a durable and versatile FM transmitter till you break or crush its PCB. Frequency is determined by a parallel L-C resonance circuit and shifts very slow as battery drains out.

Simple FM Transmitter involves on a single transistor oscillator

Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations :)

Transistor is not a critical part of the circuit, but selecting a high frequency / low noise one contributes the sound quality and range of the transmitter. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor.

Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Then note the value of the variable resistor and replace it with a fixed one.

The critical part is the inductance L which should be handmade. Get an enameled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. Wire size may vary as well. Rest of the work is much dependent on your level of knowledge and experience on inductances:  Have an FM radio near the circuit and set frequency where there is no reception. Apply power to the circuit and put a iron rod into the inductance loops to chance its value. When

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we find the right point, adjust inductance's looseness and, if required, number of turns. Once it's OK, you may use trimmer capacitor to make further frequency adjustments. You may get help of a experienced person on this point. Do not forget to fix inductance by pouring some glue onto it against external forces. If the reception on the radio is lost in a few meters range, then it is probably caused by a wrong coil adjustment.

The problems with this one transistor circuit are –

The internal capacitances of the transistor also changes with the temperature of the transistor. The tuning capacitor also changes values slightly with temperature. So, one experiences a slow frequency drift till the transmitter reaches thermal equilibrium with its surroundings. This can take 5-10 minutes. If one does not use a closed box for the transmitter, a draft of air can shift the frequency of operation after thermal equilibrium is reached.

We already recognize that changing the transistor's operating point changes the internal capacitances. If one is using a transformer power supply, any slight hum will directly modulate the oscillator. If one is using batteries, the frequency of operation shifts as the battery runs down. This effect is called "Frequency Pushing".

The Antenna has to be connected either directly to the tank circuit or via a small sniffer capacitor. The Antenna now forms part of the tuning circuit. If you approach the antenna, the frequency of the oscillator shifts slightly. This effect is called "Frequency Pulling". (In fact some circuits use this feature to make a movement detector or metal detector)

Sometimes the Mic does not present sufficient Audio input to modulate the VCO to the maximum allowed 75 KHz. In this case, even if the carrier signal is strong enough to reach a receiver, the low modulation means a weak, noisy signal.

There is no "pre-emphasis" applied. The one transistor circuit can only output a miniscule 1-5mW RF signal that possibly travels

10-20 meters, due to limited power supply voltage, limited modulation, very loose coupling with the Antenna. (There are ways to make the One Transistor circuit go 200 meters, but then you land up with a highly unstable circuit that will forever keep on changing its output frequency)

After the initial adrenaline rush of hearing one's voice across the ether wears off, the more technically inclined get to work in removing the above deficiencies.

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5 Three-Stage FM-Transmitter:

This FM Transmitter has 3 R.F. stages. A variable frequency VHF Oscillator, a class C R.F. driver and a class C R.F. amplifier with harmonics filter.

This three stage, 9V FM transmitter (Tx) with a range of up to 1 meter in the open. It uses an RF transistor in its output stage and two BC547’s for the first two stages. Distance of transmission is critically dependent on the operating Conditions (in a building or out on the open), type of aerial used (single wire or dipole), operating voltage (12V is better than 6V) and if the circuit is peaked for maximum performance. This is constructed on a single-sided printed circuit board (PCB).

5.1 Circuit Description:

The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz Audio picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor T2. The tank circuit is connected in a Hartley oscillator circuit. The final stage built around T3 amplifies the output RF signal. Let us look at the individual blocks of the circuit more closely:

5.1.1 The electrets microphone:

Electrets are permanently charged dielectric. It is made by heating a ceramic material, placing it in a magnetic field and then allowing it to cool while still in the magnetic field. It is the electrostatic equivalent of a permanent magnet. In the electrets microphone a slice of this material is used as part of the dielectric of a capacitor in which the diaphragm of the microphone forms one plate. Sound pressure moves one of its plates. The movement of the plate changes the capacitance. The electrets capacitor is connected to an FET amplifier. These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost.

5.1.2 First amplification stage

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This is a standard self-biasing common emitter amplifier. The 22n capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current signals to pass.

5.1.3 Oscillator stage

Every transmitter needs an oscillator to generate the RF carrier waves. The tank circuit, the transistor and the feedback capacitor are the oscillator circuit here. An input signal is not needed to sustain the oscillation. The feedback signal makes the base-emitter current of the transistor vary at the resonant frequency. This causes the emitter-collector current to vary at the same frequency. This signal fed to the aerial and radiated as radio waves. The name 'tank' circuit comes from the ability of the LC circuit to store energy for oscillations. In a pure LC circuit (one with no resistance) energy cannot be lost. (In an AC network only the resistive elements will dissipate electrical energy. The purely reactive elements, the C and the L, just store energy to be returned to the system later.) Note that the tank circuit does not oscillate just by having a DC potential put across it. Positive feedback must be provided.

5.1.4 Final Amplification Stage

This RF stage adds amplification to the RF signal. It needs an RF transistor to do this efficiently. We use a Zetex ZTX320. L2 (an RFC -radio frequency choke) and the 10p capacitor in parallel with it are designed to reduce harmonics from the circuit. Output power from this stage will be max when it is tuned to oscillate at the same frequency as the previous stage. This can be done with the peaking circuit provided and described separately below. A small (10pF) coupling capacitor on the aerial is optional to minimize the effect of the aerial capacitance on the final stage LC circuit. (We have not used one in this circuit.)

5.1.5 Dipole Antenna

Greater range from the transmitter can be obtained by replacing the half-wave antenna (the length of wire about 160cm long) with a dipole antenna. This is basically two wires attached to two points in the circuit which are oscillating 180o out of phase with each other. Two such points are the antenna point and the positive rail (the +9V track.) You can experiment by cutting the antenna wire in half, leaving half soldered into the antenna point and soldering the other half to the +9V pad. Point the two wires in opposite directions.

5.1.6 Operating Voltage

Output power is also increased by using a higher operating voltage. 9V is better than 6V. The maximum operating voltage for this Kit is determined by the ZTX320. This is 15V but if you try this then the values of some resistances will have to change.

We can get more range as a trade off against stability by:

reducing R5 to 100R reducing R7 to 47R increasing C7 to 470p

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The range of this FM transmitter is around 1km. at 9V DC supply. This FM transmitter has 3R.F.stages. A variable frequency V.H.F.Oscilalator, a class C.R.F amplifier with harmonics filter.

Power supply is 9 to 12Volts.R.F. Output power is 150 mill watts. With telescopic antenna (75cm) range is up to 1km. Range can be extended upto3km, by using multi element Yagi antenna having dipole, reflector, and director elements.

Frequency of transmitter is to be set within 8 to 108MHz F.M.band by adjusting the first trimmer. Adjust output trimmers for maximum range of transmission.

To power this transmitter, use 9V battery. Do not use mains derived supply. Suggested Yagi antenna design using aluminium rods are shown here. Us 75 Ohm Co-axial cable between transmitter and antenna. Inner we o able connected to PCB.

The first stage is a microphone preamplifier built around BC548 transistor. The next stage is a VHF oscillator wired around another BC548.BC series transistors are generally used in low frequency stages. But this also works fine in RF stages as oscillator.

The third stage is a class ‘A’ tuned amplifier that boost signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter. Here coils L1=7turns 22SWG, L2=6tuns22SWG, L3=5turns22SWG, L4=5turns22SWG enamelled copper wire wound on a 4mm dia. Air core.

VC1 s a frequency-adjusting trimpot.VC2 should be adjusted for the maximum range. The transmitter unit is powered by a 9V PP3 battery. It combined with readily available FM receiver kit to make a walkie-talkie set.

5.2 Working in Brief:

Q1 is the oscillator, and is a conventional Colpitts design. L1 and C3 (in parallel with C2) tune the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver - usually ±75kHz.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarized. If polarized, the positive terminal must connect to the 10k resistor.

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5.3 Components:

5.3.1 Resistors, 5%, 1/4W:

100R brown black brown : 1470R yellow violet brown : 14K7 yellow violet red : 122K red red orange : 139K orange white orange : 147K yellow violet orange : 21M brown black green : 1

5.3.2 Capacitors:

Trim cap 5-20pF, red : 15.6 pF ceramic : 110p ceramic : 247pF ceramic : 31nF ceramic : 120nF or 22nF ceramic : 2100nF monoblock : 2RF transistor ZTX320 : 1Small signal transistor BC547 : 26 turn tinned copper coil : 16 turn enamelled coil : 18 turn enamelled coil : 19V battery snap : 1Electret microphone : 1PCB-mounted SPDT switch : 1Antenna wire : 1.6mk32 PCB : 1Peaking circuit : 1 packet of components

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6 Conclusions

The Miniaturised FM transmitter is essentially a Design and Implementation project. To approach a project like this a parallel path has to be taken in regards to the Theory and the practical circuitry, for a successful conclusion in any project these paths must meet, and this only happens when they are fully understood. This is why a good grounding in the basics of Communication theory and Analogue design must be achieved before ever approaching a project like this. To start off looking at block diagrams of basic transmitter was a must, even if it seemed abstract and obscure the underlying meaning of each block can be found out one by one which made the overall project challenging and rewarding.

7 Recommendations

The design used for this project is essentially quite a simple one, and it is this simplicity which partly brings it down when it comes to the overall reliable performance. The main area of instability is in the oscillator part of the circuit. Shielding (section 5.5) the oscillator helped in part to counteract this.

After learning a lot from this project, there would have been a few things that could have been done to the final design to improve its performance.

Use negative temperature coefficients to compensate for typically positive- temperature- coefficient tuned circuits.

Follow the oscillator with a buffer amplifier to reduce the effects of load changes.

8 References

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