fm

FM GENERATION

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Index:

Abstract

Block diagram

Introduction

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Abstract:

The reason for choosing Frequency modulation is , in Amplitude modulation power consumption

and bandwidth is more. In order to reduce the bandwidth and power consumption we go for

Frequency Modulation (FM). FM involves minute frequency variations of the carrier, where as

AM results in large-scale variations of the carrier. Compared to AM, FM is more immune to

noise .

Frequency modulation is a system in which amplitude of the modulated carrier is kept constant,

while its frequency and rate of change are varied by the modulating signal. FM can be generated

in two ways they are Direct method and Indirect method. In this project we are using direct

method. In direct method voltage variable reactance which can be connected across the tank

circuit of an oscillator.

Audio frequency signal can be converted into electrical signal with the use of micro phone. The

reactance modulator which provides a variation of capacitance as a result of a voltage change.

The voltage variable reactance is placed across the tank and the tank is tuned to the oscillating

frequency is equal to the desired carrier frequency (in the absence of modulation). The

capacitance of the variable element is changed according to the modulating voltage so that

frequency is also varies. The output of the oscillator is connected to the transmitting antenna to

send message signal into longer distances.

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Block diagram:

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AF in (Micro phone)

Reactance Modulator

Oscillator Transmitter antenna

Power supply to all sections

Step down

T/F

Bridge Rectifier

Filter Circuit Regulator

Introduction to embedded systems

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INTRODUCTION OF EMBEDDED SYSTEM:

An Embedded System is a combination of computer hardware and software, and perhaps

additional mechanical or other parts, designed to perform a specific function. A good example is

the microwave oven. Almost every household has one, and tens of millions of them are used

everyday, but very few people realize that a processor and software are involved in the

preparation of their lunch or dinner.

This is in direct contrast to the personal computer in the family room. It too is

comprised of computer hardware and software and mechanical components (disk drives, for

example). However, a personal computer is not designed to perform a specific function rather; it

is able to do many different things. Many people use the term general-purpose computer to make

this distinction clear. As shipped, a general-purpose computer is a blank slate; the manufacturer

does not know what the customer will do wish it. One customer may use it for a network file

server another may use it exclusively for playing games, and a third may use it to write the next

great American novel.

Frequently, an embedded system is a component within some larger system. For

example, modern cars and trucks contain many embedded systems. One embedded system

controls the anti-lock brakes, other monitors and controls the vehicle's emissions, and a third

displays information on the dashboard. In some cases, these embedded systems are connected by

some sort of a communication network, but that is certainly not a requirement.

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At the possible risk of confusing you, it is important to point out that a general-purpose

computer is itself made up of numerous embedded systems. For example, my computer consists

of a keyboard, mouse, video card, modem, hard drive, floppy drive, and sound card-each of

which is an embedded system. Each of these devices contains a processor and software and is

designed to perform a specific function. For example, the modem is designed to send and receive

digital data over analog telephone line. That's it and all of the other devices can be summarized

in a single sentence as well.

If an embedded system is designed well, the existence of the processor and software

could be completely unnoticed by the user of the device. Such is the case for a microwave oven,

VCR, or alarm clock. In some cases, it would even be possible to build an equivalent device that

does not contain the processor and software. This could be done by replacing the combination

with a custom integrated circuit that performs the same functions in hardware. However, a lot of

flexibility is lost when a design is hard-cooled in this way. It is mush easier, and cheaper, to

change a few lines of software than to redesign a piece of custom hardware.

History and Future:

Given the definition of embedded systems earlier is this chapter; the first such systems

could not possibly have appeared before 1971. That was the year Intel introduced the world's

first microprocessor. This chip, the 4004, was designed for use in a line of business calculators

produced by the Japanese Company Busicom. In 1969, Busicom asked Intel to design a set of

custom integrated circuits-one for each of their new calculator models. The 4004 was Intel's

response rather than design custom hardware for each calculator, Intel proposed a general-

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purpose circuit that could be used throughout the entire line of calculators. Intel's idea was that

the software would give each calculator its unique set of features.

The microcontroller was an overnight success, and its use increased steadily over the

next decade. Early embedded applications included unmanned space probes, computerized traffic

lights, and aircraft flight control systems. In the 1980s, embedded systems quietly rode the

waves of the microcomputer age and brought microprocessors into every part of our kitchens

(bread machines, food processors, and microwave ovens), living rooms (televisions, stereos, and

remote controls), and workplaces (fax machines, pagers, laser printers, cash registers, and credit

card readers).

It seems inevitable hat the number of embedded systems will continue to increase

rapidly. Already there are promising new embedded devices that have enormous market

potential; light switches and thermostats that can be central computer, intelligent air-bag systems

that don't inflate when children or small adults are present, pal-sized electronic organizers and

personal digital assistants (PDAs), digital cameras, and dashboard navigation systems. Clearly,

individuals who possess the skills and desire to design the next generation of embedded systems

will be in demand for quite some time.

Real Time Systems:

One subclass of embedded is worthy of an introduction at this point. As commonly

defined, a real-time system is a computer system that has timing constraints. In other words, a

real-time system is partly specified in terms of its ability to make certain calculations or

decisions in a timely manner. These important calculations are said to have deadlines for

completion. And, for all practical purposes, a missed deadline is just as bad as a wrong answer.

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The issue of what if a deadline is missed is a crucial one. For example, if the real-time

system is part of an airplane's flight control system, it is possible for the lives of the passengers

and crew to be endangered by a single missed deadline. However, if instead the system is

involved in satellite communication, the damage could be limited to a single corrupt data packet.

The more severe the consequences, the more likely it will be said that the deadline is "hard" and

thus, the system is a hard real-time system. Real-time systems at the other end of this discussion

are said to have "soft" deadlines.

All of the topics and examples presented in this book are applicable to the designers of

real-time system who is more delight in his work. He must guarantee reliable operation of the

software and hardware under all the possible conditions and to the degree that human lives

depend upon three system's proper execution, engineering calculations and descriptive

paperwork.

Application Areas:

Nearly 99 per cent of the processors manufactured end up in embedded systems. The

embedded system market is one of the highest growth areas as these systems are used in very

market segment- consumer electronics, office automation, industrial automation, biomedical

engineering, wireless communication, data communication, telecommunications, transportation,

military and so on.

Consumer appliances:

At home we use a number of embedded systems which include digital camera, digital

diary, DVD player, electronic toys, microwave oven, remote controls for TV and air-conditioner,

VCO player, video game consoles, video recorders etc. Today’s high-tech car has about 20

embedded systems for transmission control, engine spark control, air-conditioning, navigation

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etc. Even wristwatches are now becoming embedded systems. The palmtops are powerful

embedded systems using which we can carry out many general-purpose tasks such as playing

games and word processing.

Office automation:

The office automation products using embedded systems are copying machine, fax

machine, key telephone, modem, printer, scanner etc.

Industrial automation:

Today a lot of industries use embedded systems for process control. These include

pharmaceutical, cement, sugar, oil exploration, nuclear energy, electricity generation and

transmission. The embedded systems for industrial use are designed to carry out specific tasks

such as monitoring the temperature, pressure, humidity, voltage, current etc., and then take

appropriate action based on the monitored levels to control other devices or to send information

to a centralized monitoring station. In hazardous industrial environment, where human presence

has to be avoided, robots are used, which are programmed to do specific jobs. The robots are

now becoming very powerful and carry out many interesting and complicated tasks such as

hardware assembly.

medical electronics:

Almost every medical equipment in the hospital is an embedded system. These

equipments include diagnostic aids such as ECG, EEG, blood pressure measuring devices, X-ray

scanners; equipment used in blood analysis, radiation, colonoscopy, endoscopy etc.

Developments in medical electronics have paved way for more accurate diagnosis of diseases.

Computer networking:

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Computer networking products such as bridges, routers, Integrated Services Digital

Networks (ISDN), Asynchronous Transfer Mode (ATM), X.25 and frame relay switches are

embedded systems which implement the necessary data communication protocols. For example,

a router interconnects two networks. The two networks may be running different protocol stacks.

The router’s function is to obtain the data packets from incoming pores, analyze the packets and

send them towards the destination after doing necessary protocol conversion. Most networking

equipments, other than the end systems (desktop computers) we use to access the networks, are

embedded systems

Telecommunications:

In the field of telecommunications, the embedded systems can be categorized as

subscriber terminals and network equipment. The subscriber terminals such as key telephones,

ISDN phones, terminal adapters, web cameras are embedded systems. The network equipment

includes multiplexers, multiple access systems, Packet Assemblers Dissemblers (PADs),

sate11ite modems etc. IP phone, IP gateway, IP gatekeeper etc. are the latest embedded systems

that provide very low-cost voice communication over the Internet.

Wireless technologies:

Advances in mobile communications are paving way for many interesting applications

using embedded systems. The mobile phone is one of the marvels of the last decade of the 20’h

century. It is a very powerful embedded system that provides voice communication while we are

on the move. The Personal Digital Assistants and the palmtops can now be used to access

multimedia services over the Internet. Mobile communication infrastructure such as base station

controllers, mobile switching centers are also powerful embedded systems.

Finance:

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Financial dealing through cash and cheques are now slowly paving way for transactions

using smart cards and ATM (Automatic Teller Machine, also expanded as Any Time Money)

machines. Smart card, of the size of a credit card, has a small micro-controller and memory; and

it interacts with the smart card reader! ATM machine and acts as an electronic wallet. Smart card

technology has the capability of ushering in a cashless society. Well, the list goes on. It is no

exaggeration to say that eyes wherever you go, you can see, or at least feel, the work of an

embedded system!

Overview of Embedded System Architecture:

Every embedded system consists of custom-built hardware built around a Central

Processing Unit (CPU). This hardware also contains memory chips onto which the software is

loaded. The software residing on the memory chip is also called the ‘firmware’. The embedded

system architecture can be represented as a layered architecture as shown in Fig.

The operating system runs above the hardware, and the application software runs above

the operating system. The same architecture is applicable to any computer including a desktop

computer. However, there are significant differences. It is not compulsory to have an operating

system in every embedded system. For small appliances such as remote control units, air

conditioners, toys etc., there is no need for an operating system and you can write only the

software specific to that application. For applications involving complex processing, it is

advisable to have an operating system. In such a case, you need to integrate the application

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software with the operating system and then transfer the entire software on to the memory chip.

Once the software is transferred to the memory chip, the software will continue to run for a long

time you don’t need to reload new software.

Now, let us see the details of the various building blocks of the hardware of an embedded

system. As shown in Fig. the building blocks are:

Input devices:

Unlike the desktops, the input devices to an embedded system have very limited

capability. There will be no keyboard or a mouse, and hence interacting with the embedded

system is no easy task. Many embedded systems will have a small keypad-you press one key to

give a specific command. A keypad may be used to input only the digits. Many embedded

systems used in process control do not have any input device for user interaction; they take

inputs from sensors or transducers 1’fnd produce electrical signals that are in turn fed to other

systems.

Output devices:

The output devices of the embedded systems also have very limited capability. Some

embedded systems will have a few Light Emitting Diodes (LEDs) to indicate the health status of

the system modules, or for visual indication of alarms. A small Liquid Crystal Display (LCD)

may also be used to display some important parameters.

Communication interfaces:

The embedded systems may need to, interact with other embedded systems at they may

have to transmit data to a desktop. To facilitate this, the embedded systems are provided with one

or a few communication interfaces such as RS232, RS422, RS485, Universal Serial Bus (USB),

IEEE 1394, Ethernet etc.

Application-specific circuitry:

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Sensors, transducers, special processing and control circuitry may be required fat an

embedded system, depending on its application. This circuitry interacts with the processor to

carry out the necessary work. The entire hardware has to be given power supply either through

the 230 volts main supply or through a battery. The hardware has to design in such a way that the

power consumption is minimized.

Conclusions:

Embedded Systems plays a vital role in our day today life. They are used for household

appliances like microwave oven to the satellite applications. They provide good man to machine

interface.

Automation is the further step in the world of Embedded Systems, which includes the

elimination of the human being in the mundane applications. They are cost effective, accurate

and can work in any conditions and round the clock.

FM transmitter

An FM transmitter is a portable device that plugs into the headphone jack or proprietary output

port of a portable audio or video device, such as a portable media player, CD player, or satellite

radio system. The sound is then broadcast through the transmitter, and plays through an FM

broadcast band frequency. Purposes for an FM transmitter include playing music from a device

through a car stereo, or any radio.

The FM-transmitter plugs into the audio output of audio devices and converts the audio output

into an FM radio signal, which can then be picked up by appliances such as car or portable

radios. Most devices on the market typically have a short range of up to 30 feet (9 meters) with 14

any average radio (up to about 75 feet (23 meters) with a very good radio under perfect

conditions) and can broadcast on any FM frequency from 88.0 to 108.0 MHz (or 88.1 to 107.9 in

the US). Some lower-cost transmitters are hard-wired to the 87.7–91.9 MHz band allocated to

educational broadcasts in the United States, or a certain other smaller range of frequencies.

FM transmitters are usually battery driven, but some use the cigarette lighter socket in cars, or

draw their power from the device itself. They are typically used with portable audio devices such

as Nokia Phones or MP3 players, but are also used to broadcast other outputs (such as that from

a computer sound card) throughout a home or other building.

WHYFM?

Frequency modulation is a type of modulation where the frequency of the carrier is aried in

accordance with the modulating signal. The amplitude of the carrier remains constant.

The information-bearing signal (the modulating signal) changes the instantaneous frequency of

the carrier. Since the amplitude is kept constant, FM modulation is a low-noise process and

provides a high quality modulation technique which is used for music and speech in hi-fidelity

broadcasts For FM: the carrier’s frequency is proportional to the baseband’s amplitude, the

carrier increases frequency proportional to the positive agnitude of the baseband and decreases

frequency proportional to the negative magnitude of the baseband.

ADVANTAGESFREQUENCYMODULATION

• Frequency modulation have some advantages over amplitude modulation, FM is not sensitive

to the amplitude noise, and have high efficient use of transmitted power

• Lesser distortion. Frequency modulated wave is less susceptible to intereferences from

buildings, traffic etc which provides improved signal to noise ratio (about 25dB) w.r.t. to

manmadeinterference.

• Waves at higher frequencies can carry more data than the waves at low frequency.

• Smaller geographical interference between neighboring stations.

• Less radiated power.

• Well defined service areas for given transmitter power

FM-TRANSMITTER:

Transmitter which generates frequency modulated signal is known as FM-Transmitter

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APPLICATIONS:

• Non-commercial broadcasting from 88 – 90 MHz (WFM)

• Commercial broadcasting from 90 – 108 MHz (WFM)

• Television audio

• Public Service communications

• Radio Service Communications

• Point-to-point microwave links used by telecommunications companies

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Circuit Diagram

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Microphone

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How Do Microphones Work?

The Basics

Microphones are a type of transducer - a device which converts energy from one form to another.

Microphones convert acoustical energy (sound waves) into electrical energy (the audio signal).

Different types of microphone have different ways of converting energy but they all share one

thing in common: The diaphragm. This is a thin piece of material (such as paper, plastic or

aluminium) which vibrates when it is struck by sound waves. In a typical hand-held mic like the

one below, the diaphragm is located in the head of the microphone.

Location of Microphone Diaphragm

When the diaphragm vibrates, it causes other components in the microphone to vibrate. These

vibrations are converted into an electrical current which becomes the audio signal.

Note: At the other end of the audio chain, the loudspeaker is also a transducer - it converts the

electrical energy back into acoustical energy.

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Types of Microphone

There are a number of different types of microphone in common use. The differences can be

divided into two areas:

(1) The type of conversion technology they use

This refers to the technical method the mic uses to convert sound into electricity. The most

common technologies are dynamic, condenser, ribbon and crystal. Each has advantages and

disadvantages, and each is generally more suited to certain types of application. The following

pages will provide details.

(2) The type of application they are designed for

Some mics are designed for general use and can be used effectively in many different situations.

Others are very specialised and are only really useful for their intended purpose. Characteristics

to look for include directional properties, frequency response and impedance (more on these

later).

Mic Level & Line Level

The electrical current generated by a microphone is very small. Referred to as mic level, this

signal is typically measured in millivolts. Before it can be used for anything serious the signal

needs to be amplified, usually to line level (typically 0.5 -2V). Being a stronger and more robust

signal, line level is the standard signal strength used by audio processing equipment and common

domestic equipment such as CD players, tape machines, VCRs, etc.

This amplification is achieved in one or more of the following ways:

Some microphones have tiny built-in amplifiers which boost the signal to a high mic

level or line level.

The mic can be fed through a small boosting amplifier, often called a line amp.

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Sound mixers have small amplifiers in each channel. Attenuators can accommodate mics

of varying levels and adjust them all to an even line level.

The audio signal is fed to a power amplifier - a specialised amp which boosts the signal

enough to be fed to loudspeakers

"Microphones" redirects here. For the indie band, see The Microphones.

A Neumann U87 condenser microphone with shock mount

A Sennheiser Microphone

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A microphone (colloquially called a mic or mike; both pronounced /ˈmaɪk/[1]) is an acoustic-to-

electric transducer or sensor that converts sound into an electrical signal. In 1876, Emile Berliner

invented the first microphone used as a telephone voice transmitter. Microphones are used in

many applications such as telephones, tape recorders, karaoke systems, hearing aids, motion

picture production, live and recorded audio engineering, FRS radios, megaphones, in radio and

television broadcasting and in computers for recording voice, speech recognition, VoIP, and for

non-acoustic purposes such as ultrasonic checking or knock sensors.

Most microphones today use electromagnetic induction (dynamic microphone), capacitance

change (condenser microphone), piezoelectric generation, or light modulation to produce an

electrical voltage signal from mechanical vibration.

Components

The sensitive transducer element of a microphone is called its element or capsule. A complete

microphone also includes a housing, some means of bringing the signal from the element to other

equipment, and often an electronic circuit to adapt the output of the capsule to the equipment

being driven. A wireless microphone contains a radio transmitter.

Varieties

Microphones are referred to by their transducer principle, such as condenser, dynamic, etc., and

by their directional characteristics. Sometimes other characteristics such as diaphragm size,

intended use or orientation of the principal sound input to the principal axis (end- or side-

address) of the microphone are used to describe the microphone.

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Condenser microphone

Inside the Oktava 319 condenser microphone

The condenser microphone, invented at Bell Labs in 1916 by E. C. Wente[2] is also called a

capacitor microphone or electrostatic microphone. Here, the diaphragm acts as one plate of a

capacitor, and the vibrations produce changes in the distance between the plates. There are two

types, depending on the method of extracting the audio signal from the transducer: DC-biased

and radio frequency (RF) or high frequency (HF) condenser microphones. With a DC-biased

microphone, the plates are biased with a fixed charge (Q). The voltage maintained across the

capacitor plates changes with the vibrations in the air, according to the capacitance equation (C = Q⁄V), where Q = charge in coulombs, C = capacitance in farads and V = potential difference in

volts. The capacitance of the plates is inversely proportional to the distance between them for a

parallel-plate capacitor. (See capacitance for details.) The assembly of fixed and movable plates

is called an "element" or "capsule".

A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge

across the capacitor does change very slightly, but at audible frequencies it is sensibly constant.

The capacitance of the capsule (around 5 to 100 pF) and the value of the bias resistor (100 MΩ to

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tens of GΩ) form a filter that is high-pass for the audio signal, and low-pass for the bias voltage.

Note that the time constant of an RC circuit equals the product of the resistance and capacitance.

Within the time-frame of the capacitance change (as much as 50 ms at 20 Hz audio signal), the

charge is practically constant and the voltage across the capacitor changes instantaneously to

reflect the change in capacitance. The voltage across the capacitor varies above and below the

bias voltage. The voltage difference between the bias and the capacitor is seen across the series

resistor. The voltage across the resistor is amplified for performance or recording. In most cases,

the electronics in the microphone itself contribute no voltage gain as the voltage differential is

quite significant, up to several volts for high sound levels. Since this is a very high impedance

circuit, current gain only is usually needed with the voltage remaining constant.

AKG C451B small-diaphragm condenser microphone

RF condenser microphones use a comparatively low RF voltage, generated by a low-noise

oscillator. The signal from the oscillator may either be amplitude modulated by the capacitance

changes produced by the sound waves moving the capsule diaphragm, or the capsule may be part

of a resonant circuit that modulates the frequency of the oscillator signal. Demodulation yields a

low-noise audio frequency signal with a very low source impedance. The absence of a high bias

voltage permits the use of a diaphragm with looser tension, which may be used to achieve wider

frequency response due to higher compliance. The RF biasing process results in a lower

electrical impedance capsule, a useful by-product of which is that RF condenser microphones

can be operated in damp weather conditions that could create problems in DC-biased

microphones with contaminated insulating surfaces. The Sennheiser "MKH" series of

microphones use the RF biasing technique.

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Condenser microphones span the range from telephone transmitters through inexpensive karaoke

microphones to high-fidelity recording microphones. They generally produce a high-quality

audio signal and are now the popular choice in laboratory and recording studio applications. The

inherent suitability of this technology is due to the very small mass that must be moved by the

incident sound wave, unlike other microphone types that require the sound wave to do more

work. They require a power source, provided either via microphone inputs on equipment as

phantom power or from a small battery. Power is necessary for establishing the capacitor plate

voltage, and is also needed to power the microphone electronics (impedance conversion in the

case of electret and DC-polarized microphones, demodulation or detection in the case of RF/HF

microphones). Condenser microphones are also available with two diaphragms that can be

electrically connected to provide a range of polar patterns (see below), such as cardioid,

omnidirectional, and figure-eight. It is also possible to vary the pattern continuously with some

microphones, for example the Røde NT2000 or CAD M179.

Electret condenser microphone

An electret microphone is a relatively new type of capacitor microphone invented at Bell

laboratories in 1962 by Gerhard Sessler and Jim West.[3] The externally applied charge described

above under condenser microphones is replaced by a permanent charge in an electret material.

An electret is a ferroelectric material that has been permanently electrically charged or polarized.

The name comes from electrostatic and magnet; a static charge is embedded in an electret by

alignment of the static charges in the material, much the way a magnet is made by aligning the

magnetic domains in a piece of iron.

Due to their good performance and ease of manufacture, hence low cost, the vast majority of

microphones made today are electret microphones; a semiconductor manufacturer[4] estimates

annual production at over one billion units. Nearly all cell-phone, computer, PDA and headset

microphones are electret types. They are used in many applications, from high-quality recording

and lavalier use to built-in microphones in small sound recording devices and telephones.

Though electret microphones were once considered low quality, the best ones can now rival

traditional condenser microphones in every respect and can even offer the long-term stability and

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ultra-flat response needed for a measurement microphone. Unlike other capacitor microphones,

they require no polarizing voltage, but often contain an integrated preamplifier that does require

power (often incorrectly called polarizing power or bias). This preamplifier is frequently

phantom powered in sound reinforcement and studio applications. Microphones designed for

personal computer (PC) use, sometimes called multimedia microphones, use a stereo 3.5 mm

plug (though a mono source) with the ring receiving power via a resistor from (normally) a 5 V

supply in the computer; unfortunately, a number of incompatible dynamic microphones are fitted

with 3.5 mm plugs too. While few electret microphones rival the best DC-polarized units in

terms of noise level, this is not due to any inherent limitation of the electret. Rather, mass

production techniques needed to produce microphones cheaply don't lend themselves to the

precision needed to produce the highest quality microphones, due to the tight tolerances required

in internal dimensions. These tolerances are the same for all condenser microphones, whether the

DC, RF or electret technology is used.

Dynamic microphone

Patti Smith singing into a Shure SM58 (dynamic cardioid type) microphone

Dynamic microphones work via electromagnetic induction. They are robust, relatively

inexpensive and resistant to moisture. This, coupled with their potentially high gain before

feedback, makes them ideal for on-stage use.

Moving-coil microphones use the same dynamic principle as in a loudspeaker, only reversed. A

small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached

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to the diaphragm. When sound enters through the windscreen of the microphone, the sound wave

moves the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field,

producing a varying current in the coil through electromagnetic induction. A single dynamic

membrane does not respond linearly to all audio frequencies. Some microphones for this reason

utilize multiple membranes for the different parts of the audio spectrum and then combine the

resulting signals. Combining the multiple signals correctly is difficult and designs that do this are

rare and tend to be expensive. There are on the other hand several designs that are more

specifically aimed towards isolated parts of the audio spectrum. The AKG D 112, for example, is

designed for bass response rather than treble.[5] In audio engineering several kinds of

microphones are often used at the same time to get the best result.

Carbon microphone

A carbon microphone, also known as a carbon button microphone (or sometimes just a button

microphone), use a capsule or button containing carbon granules pressed between two metal

plates like the Berliner and Edison microphones. A voltage is applied across the metal plates,

causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in

sympathy with incident sound waves, applying a varying pressure to the carbon. The changing

pressure deforms the granules, causing the contact area between each pair of adjacent granules to

change, and this causes the electrical resistance of the mass of granules to change. The changes

in resistance cause a corresponding change in the current flowing through the microphone,

producing the electrical signal. Carbon microphones were once commonly used in telephones;

they have extremely low-quality sound reproduction and a very limited frequency response

range, but are very robust devices. The Boudet microphone, which used relatively large carbon

balls, was similar to the granule carbon button microphones.[7]

Unlike other microphone types, the carbon microphone can also be used as a type of amplifier,

using a small amount of sound energy to control a larger amount of electrical energy. Carbon

microphones found use as early telephone repeaters, making long distance phone calls possible

in the era before vacuum tubes. These repeaters worked by mechanically coupling a magnetic

telephone receiver to a carbon microphone: the faint signal from the receiver was transferred to

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the microphone, with a resulting stronger electrical signal to send down the line. One illustration

of this amplifier effect was the oscillation caused by feedback, resulting in an audible squeal

from the old "candlestick" telephone if its earphone was placed near the carbon microphone.

Piezoelectric microphone

A crystal microphone or piezo microphone uses the phenomenon of piezoelectricity—the

ability of some materials to produce a voltage when subjected to pressure—to convert vibrations

into an electrical signal. An example of this is potassium sodium tartrate, which is a piezoelectric

crystal that works as a transducer, both as a microphone and as a slimline loudspeaker

component. Crystal microphones were once commonly supplied with vacuum tube (valve)

equipment, such as domestic tape recorders. Their high output impedance matched the high input

impedance (typically about 10 megohms) of the vacuum tube input stage well. They were

difficult to match to early transistor equipment, and were quickly supplanted by dynamic

microphones for a time, and later small electret condenser devices. The high impedance of the

crystal microphone made it very susceptible to handling noise, both from the microphone itself

and from the connecting cable.

Piezoelectric transducers are often used as contact microphones to amplify sound from acoustic

musical instruments, to sense drum hits, for triggering electronic samples, and to record sound in

challenging environments, such as underwater under high pressure. Saddle-mounted pickups on

acoustic guitars are generally piezoelectric devices that contact the strings passing over the

saddle. This type of microphone is different from magnetic coil pickups commonly visible on

typical electric guitars, which use magnetic induction, rather than mechanical coupling, to pick

up vibration.

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Fiber optic microphone

A fiber optic microphone converts acoustic waves into electrical signals by sensing changes in

light intensity, instead of sensing changes in capacitance or magnetic fields as with conventional

microphones.

During operation, light from a laser source travels through an optical fiber to illuminate the

surface of a reflective diaphragm. Sound vibrations of the diaphragm modulate the intensity of

light reflecting off the diaphragm in a specific direction. The modulated light is then transmitted

over a second optical fiber to a photo detector, which transforms the intensity-modulated light

into analog or digital audio for transmission or recording. Fiber optic microphones possess high

dynamic and frequency range, similar to the best high fidelity conventional microphones.

Fiber optic microphones do not react to or influence any electrical, magnetic, electrostatic or

radioactive fields (this is called EMI/RFI immunity). The fiber optic microphone design is

therefore ideal for use in areas where conventional microphones are ineffective or dangerous,

such as inside industrial turbines or in magnetic resonance imaging (MRI) equipment

environments.

Fiber optic microphones are robust, resistant to environmental changes in heat and moisture, and

can be produced for any directionality or impedance matching. The distance between the

29

microphone's light source and its photo detector may be up to several kilometers without need

for any preamplifier and/or other electrical device, making fiber optic microphones suitable for

industrial and surveillance acoustic monitoring.

Fiber optic microphones are used in very specific application areas such as for infrasound

monitoring and noise-canceling. They have proven especially useful in medical applications,

such as allowing radiologists, staff and patients within the powerful and noisy magnetic field to

converse normally, inside the MRI suites as well as in remote control rooms. [10]) Other uses

include industrial equipment monitoring and sensing, audio calibration and measurement, high-

fidelity recording and law enforcement.

Stereo

Stereo sound is an illusion of spaciousness produced by playing a recording back through two

speakers. The success of this illusion is referred to as the image. A good image is one in which

each instrument is a natural size, has a distinct location within the sound space, and does not

move around. The main factors that establish the image are the relative strength of an

instrument's sound in each speaker, and the timing of arrival of the sounds at the listener's ear. In

a studio recording, the stereo image is produced artificially. Each instrument has its own

microphone, and the various signals are balanced in the console as the producer desires. In a

concert recording, where the point is to document reality, and where individual microphones

would be awkward at best, it is most common to use two mics, one for each speaker.

30

Laser microphone

Laser microphones are often portrayed in movies as spy gadgets. A laser beam is aimed at the

surface of a window or other plane surface that is affected by sound. The slight vibrations of this

surface displace the returned beam, causing it to trace the sound wave. The vibrating laser spot is

then converted back to sound. In a more robust and expensive implementation, the returned light

is split and fed to an interferometer, which detects movement of the surface. The former

implementation is a tabletop experiment; the latter requires an extremely stable laser and precise

optics.

A new type of laser microphone is a device that uses a laser beam and smoke or vapor to detect

sound vibrations in free air. On 25 August 2009, U.S. patent 7,580,533 issued for a Particulate

Flow Detection Microphone based on a laser-photocell pair with a moving stream of smoke or

vapor in the laser beam's path. Sound pressure waves cause disturbances in the smoke that in turn

cause variations in the amount of laser light reaching the photo detector. A prototype of the

device was demonstrated at the 127th Audio Engineering Society convention in New York City

from 9 through 12 October 2009.31

Liquid microphone

Early microphones did not produce intelligible speech, until Alexander Graham Bell made

improvements including a variable resistance microphone/transmitter. Bell's liquid transmitter

consisted of a metal cup filled with water with a small amount of sulfuric acid added. A sound

wave caused the diaphragm to move, forcing a needle to move up and down in the water. The

electrical resistance between the wire and the cup was then inversely proportional to the size of

the water meniscus around the submerged needle. Elisha Gray filed a caveat for a version using a

brass rod instead of the needle. Other minor variations and improvements were made to the

liquid microphone by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version

was patented by Reginald Fessenden in 1903. These were the first working microphones, but

they were not practical for commercial application. The famous first phone conversation between

Bell and Watson took place using a liquid microphone.

MEMS microphone

The MEMS (MicroElectrical-Mechanical System) microphone is also called a microphone chip

or silicon microphone. The pressure-sensitive diaphragm is etched directly into a silicon chip by

MEMS techniques, and is usually accompanied with integrated preamplifier. Most MEMS

microphones are variants of the condenser microphone design. Often MEMS microphones have

built in analog-to-digital converter (ADC) circuits on the same CMOS chip making the chip a

digital microphone and so more readily integrated with modern digital products. Major

manufacturers producing MEMS silicon microphones are Wolfson Microelectronics (WM7xxx),

Analog Devices, Akustica (AKU200x), Infineon (SMM310 product), Knowles Electronics,

Memstech (MSMx), NXP Semiconductors, Sonion MEMS, AAC Acoustic Technologies,and

Omron.

Speakers as microphones

A loudspeaker, a transducer that turns an electrical signal into sound waves, is the functional

opposite of a microphone. Since a conventional speaker is constructed much like a dynamic

microphone (with a diaphragm, coil and magnet), speakers can actually work "in reverse" as 32

microphones. The result, though, is a microphone with poor quality, limited frequency response

(particularly at the high end), and poor sensitivity. In practical use, speakers are sometimes used

as microphones in applications where high quality and sensitivity are not needed such as

intercoms, walkie-talkies or Video game voice chat peripherals, or when conventional

microphones are in short supply.

However, there is at least one other practical application of this principle: Using a medium-size

woofer placed closely in front of a "kick" (bass drum) in a drum set to act as a microphone. The

use of relatively large speakers to transduce low frequency sound sources, especially in music

production, is becoming fairly common. A product example of this type of device is the Yamaha

Subkick, a 6.5-inch (170 mm) woofer shock-mounted it into a 10" drum shell used in front of

kick drums. Since a relatively massive membrane is unable to transduce high frequencies,

placing a speaker in front of a kick drum is often ideal for reducing cymbal and snare bleed into

the kick drum sound. Less commonly, microphones themselves can be used as speakers, almost

always as tweeters. Microphones, however, are not designed to handle the power that speaker

components are routinely required to cope with. One instance of such an application was the

STC microphone-derived 4001 super-tweeter, which was successfully used in a number of high

quality loudspeaker systems from the late 1960s to the mid-70s.

FM Radio

The built-in FM radio tuner is now considered a basic feature. It allows the user to listen to most

of the live-broadcasted FM radio stations. Almost all phones with FM radio tuner require a wired

headset to be connected to the unit as it’s used as an antenna.

Most FM radio tuners can receive basic radio station info over RDS. The use of FM radio does

not interfere with the network carrier and it’s free.

Nokia enhance their FM radio interfaces with the Visual Radio enhancement that adds visuals

and text as an additional info layer to normal radio broadcasts.

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A presentation of graphics and text, synchronized to the audio programming, gets downloaded to

the phone over a data connection; the FM transmission chain is unaffected by the addition of

Visual Radio.

Here's the type of content that Visual Radio can offer:

Information on the song and artist currently playing on air

View images related to presenters or news stories

A weather map during the weather broadcast

News, weather and traffic alerts while songs are playing

Listen in to a talk show and see what has been discussed so far

Join in audience votes, Big Brother style

Participate in on-air competitions

This article is concerned with low powered transmitters used in some countries for interfacing

personal audio devices. "FM transmitter" can also refer to high powered broadcast equipment

used by pirate radio and licensed broadcast stations

An FM transmitter is a portable device that plugs into the headphone jack or proprietary output

port of a portable audio or video device, such as a portable media player, CD player, or satellite

radio system. The sound is then broadcast through the transmitter, and plays through an FM

broadcast band frequency. Purposes for an FM transmitter include playing music from a device

through a car stereo, or any radio.

The FM-transmitter plugs into the audio output of audio devices and converts the audio output

into an FM radio signal, which can then be picked up by appliances such as car or portable

radios. Most devices on the market typically have a short range of up to 100 feet (30 metres)

with any average radio (up to about 300feet (100 metres) with a very good radio under perfect

34

conditions) this range can also be enhanced if operated in fixed locations of good high elevation,

such as a multi-story apartment or tall building and can broadcast on any FM frequency from

87.5 to 108.0 MHz in most of the world, (or 88.1 to 107.9 in the US and Canada). Some lower-

cost transmitters are hard-wired to the 87.7–91.9 MHz band allocated to educational broadcasts

in the United States, or a certain other smaller range of frequencies.

FM transmitters are usually battery driven, but some use the cigarette lighter socket in cars, or

draw their power from a mains powered wall socket or the device itself. They are typically used

with portable audio devices such as MP3 players, as well as hifi systems, message systems, etc..

They are also used to broadcast other outputs (such as that from a computer sound card)

throughout a home or other building.

Limitations

The relatively low power output of FM transmitters sometimes makes it unsuitable for use in

some large urban areas because of the number of other radio signals. This is compounded by the

fact that strong FM signals can bleed over into neighboring frequencies making the frequencies

unusable with the transmitter. Removing a car's radio antenna has been found to significantly

improve transmitter reception.

Some models which connect via ports other than the headphone jack have no means of

controlling the volume, which can force the sound to transmit out from the device harshly

(causing over modulation, audio distortion and possible radio interference), or too low. In theory

a device could use an automatic level control or audio limiter circuit to overcome this problem

although there are few (if any) devices with such a facility available out on the market yet.

Modulation is a process that causes a shift of the range of frequencies in a signal. It is used to

facilitate transmission over a given channel. Baseband signals produced by various information

sources are not always suitable for direct transmission over a given channel. These signals are

usually further modified to facilitate transmission. This conversion process is known as

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modulation. In this process the baseband signal is used to modify some parameter of a high

frequency carrier signal.

A carrier is a sinusoid of high frequency, and one of the parameters such as amplitude, frequency

or phase is varied in proportion to the baseband signal m(t). In FM modulation the frequency of

the carrier is modify in proportion to the baseband signal. A baseband signal is used to designate

the band of frequencies of the signal delivered bythe source. Another advantages of modulation

are the ease of radiation of electromagnetic energy allowing antenna sizes to be reasonable and

simultaneous transmission of several signals over the same channel.

Frequency Modulation

Another means of encoding a wave of information is by producing a complex wave whose

frequency is varied in proportion to the instantaneous amplitude of the information wave. Such

an encoding is frequency modulation. The result of this encoding or modulation process is a

complex modulated wave whose instantaneous frequency is a function of the amplitude of the

modulating wave and differs from the frequency of the carrier from instant to instant as the

amplitude of the modulating wave varies.

The following equation provides the equivalent formula for FM: v(t) Asin( t M sin( t)) c f i

where

v(t) is the instantaneous voltage

A is the peak value of the carrier

c is the carrier angular velocity

Mf is the modulation index

i is the modulating signal angular velocity

The FM formula is really complex. In figure 1 is the waveform of a FM signal. To solve for the

frequency components of an FM wave requires the use of the Bessel functions. They show that

frequency-modulating carrier with a pure sine actually generates an infinite number of sidebands

spaced at multiples of the intelligence frequency, fm, above and below the carrier. Fortunately,

36

the amplitude of these sidebands approaches a negligible level the farther away they are from the

carrier, which allow FM transmission within finite bandwidths.

Figure: FM Signal Representation

37

The Bessel functions solution to the FM equation is

in figure is an example of a FM spectrum

38

Figure: FM Spectrum

39

Center Frequency

The center frequency is that frequency assigned to the carrier, but during modulation the carrier

is not always present in the complex modulated wave and the instantaneous frequency of the

complex modulated wave varies above and below the frequency assigned to the carrier. This

assigned frequency, then, is the “center” about which the instantaneous frequencies of the

modulated wave vary.

Frequency Deviation

In FM the shift in frequency is proportional to the amplitude of the modulating wave. A weak

modulating wave will have a small peak amplitude, which will produce a small peak frequency

variation. A strong modulating wave whose peak amplitude is the maximum that the modulating

system is designed to handle will produce the maximum peak frequency variation. Any of these

peak variations from the center frequency is called the frequency deviation (fd).

The maximum value of the frequency deviation (fD) is a system constant, and when it is

intended that the modulated wave be radiated, its magnitude is established by law.

Frequency Swing

The overall extreme of the excursion of instantaneous frequencies from maximum negative to

maximum positive is called the frequency swing. Frequency swing, therefore, is equal to twice

the maximum design frequency deviation. In FM the frequency swing is a system constant and is

usually expressed in terms of the maximum frequency deviation as ±fD.

Deviation Ratio

Any modulating system is intended to accommodate some specific band of modulating

frequencies. Therefore the lower limit and especially the upper limit of this band are of

importance in the design of the equipment. The upper frequency is the most important because it

determines the maximum bandwidth requirements. In FM, the highest modulating frequency

(fM) is a system constant. The ratio of the maximum frequency deviation (fD) to the highest

modulating frequency (fM) is called the deviation ratio (D).

40

The deviation ratio is strictly an equipment characteristic and is a quantity used to set the circuit

bandwidth of a modulating system.

Modulation Index

There is a very important signal characteristic that resembles the deviation ratio and under

certain circumstances is equal to it. It is the ratio,

Notice, however, that fd is any frequency deviation, not necessarily maximum and fm is any

modulating frequency, not necessarily the highest. There are system restrictions that apply; for

example fd cannot exceed fD and fm cannot be greater than fM.

Bandwidth Calculations

Since the sidebands are separated by multiples of the modulating frequency, the frequency of the

highest important sidebands is given by

The overall bandwidth from the highest to the lowest side frequency whose amplitude is 15 %

(or greater) of the unmodulated carrier is twice this value. In figure 3 is the commercial FM

bandwidth allocation for two adjacent stations

Figure : Commercial FM bandwidth allocation for two adjacent stations

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Reactance Modulator

The reactance modulator is a very popular means of FM generation and is shown in figure 4. The

reactance modulator is an amplifier designed so that its input impedance has a reactance that

varies as a function of the amplitude of the applied input voltage modulating signal).

Figure 4: FM Reactance Modulator

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PROCEDURE

1. Connect the power supply to the trainer with all equipment turned off. Follow the

following diagram.

2. Select the following conditions in the trainer

a. Mixer/Amplifier Amplitude dial : Maximum clockwise direction

b. VCO switch (PLL detector block): OFF position

c. Modulation switch: Reactance

3. Turn on the power supply. Connect the oscilloscope to test point 1. Turn Audio Oscillator

amplitude dial to the maximum position. Using the oscilloscope adjust the Audio Oscillator

frequency to 2 kHz (fm = 2 kHz). This signal is the modulating signal.

4. using a cable jumper, connect the Audio Oscillator output to the Modulator Circuits audio

input.

5. The output of the Reactance modulator can be measured in test point 13, but to avoid the

capacitive effect of the probe, connect the oscilloscope’s probe to test point 34. The input

impedance of the mixer/amplifier block is high enough to get a good measurement.

6. Turn the REACTANCE MODULATOR CARRIER FREQUENCY dial to the middle

position.

7. Turn the Audio Oscillator amplitude dial to the minimum position.

8. Connect the oscilloscope to test point 34 and measure the frequency, peak to peak voltage and

average voltage of the carrier. Record in table 1.

9. Disconnect the oscilloscope and connect the universal (frequency) counter to test point 34 to

measure the carrier frequency. Start changing the modulating signal amplitude and determine the

maximum and minimum frequency measured in the frequency counter. What’s the frequency

deviation (fd) Did you see a significant change in the carrier frequency? What’s the signal

bandwidth? Record in table 2 43

10. Disconnect the frequency counter and connect the spectrum analyzer to test point 34. Set the

spectrum analyzer center frequency with the frequency determined in step 8 and span of 20 kHz.

Start changing the modulating signal amplitude and see what happen with the FM spectrum.

Print out the frequency spectrum with

a) Audio Oscillator amplitude dial in the minimum position,

b) middle position and

c) maximum position. What happen with the frequency spectrum?

11. Turn the power off and disconnect the Audio Oscillator from the modulator, then connect the

microphone Audio Input Module’s output to the Audio Input socket in the Modulator Circuits

block. Turn the power on and connect the oscilloscope to test point 34 and start talking into the

microphone. See how your frequency modulated voice looks like, then connect the spectrum

analyzer at the same point. Examine the signal spectrum.

12. Connect the waveform generator to the 50as illustrated in the diagram.

13. Run HP-VEE program in the computer and call up the function generator control panel from

the I/O instruments menu. Set the following parameters in the instrument using HP-VEE.

Main Panel:

Function: Sinusoid

Freq: 15 kHz

Amplitude: 5 Vp-p

Load: 50

Modulation Pan:

Frequency Panel

Function: Sinusoid

Freq: 1 Hz

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Deviation: 5 kHz

FM State: ON

A reactance modulator changes the frequency of the tank circuit of the oscillator by changing its

reactance. This is accomplished by a combination of a resistor, a condenser, and a vacuum tube

(the modulator) connected across the tank circuit of the oscillator as in Fig. 33 A, and so adjusted

as to act as a variable inductance or capacitance.

Fig. 33 A. Principle of a reactance modulator

The net result is to change the resonant frequency of the LC circuit by amounts proportional to

the instantaneous a.f. voltages applied to the grid of the modulator tube, without changing the

resistance of the LC circuit or the amplitude of the oscillations. A modulator circuit is shown in

Fig. 33 B.

The voltages supplied to both the modulator and oscillator must be carefully stabilized to prevent

undesired frequency changes. The speech amplifier (Fig. 33 A) does not have to deliver any

power and need supply only a small output voltage, say 10 or 15 volts. A pentode and triode, R-

C coupled, will be sufficient even with a sensitive microphone and a high-powered oscillator.

The frequency change of LC per volt change on the a.f. grid of the modulator tube will be greater

when C1, Fig. 33 B, is made smaller. The blocking condenser C2 has a comparatively high value,

and hence offers but small reactance to r.f. currents.

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In Fig. 33 B, the radio-frequency voltages which are developed across the tank in the oscillator

circuit also appear across the RC1 circuit and across the parallel 6L7 modulator tube. Now look

up the phase-shifting circuit of Fig. 19 H. The resistance r has been replaced by the internal

resistance of the modulator tube of Fig. 33 B. The voltage drop across C1 is 90° out of phase with

the tank voltage. It is applied to the control grid of the 6L7 whose r.f. plate current responds in

the same phase. Thus this current is made to lag 90° behind the tank voltage. The r.f. plate

current flows through the tank circuit and, combined with the current therein, is equivalent to a

new current whose phase differs from the normal value just as though an additional reactance

(not resistance) had been connected in with L and C. This, of course, changes the frequency of

the LC circuit and hence of the transmitter. When a.f. is fed into the modulator tube, it causes

proportionate changes in the r.f. plate current and hence in the equivalent reactance of the LC

circuit.

A technical analysis tool that is banded between two extreme values and built with the results

from a trend indicator for discovering short-term overbought or oversold conditions. As the value

of the oscillator approaches the upper extreme value the asset is deemed to be overbought, and as

it approaches the lower extreme it is deemed to be oversold.

46

Fig. 19 H. An RC phase shifter

Oscillators:

Oscillation is the repetitive variation, typically in time, of some measure about a central value

(often a point of equilibrium) or between two or more different states. Familiar examples include

a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to

mean a mechanical oscillation but sometimes is used to be synonymous with "oscillation".

Oscillations occur not only in physical systems but also in biological systems and in human

society.

Simple harmonic oscillator

The simplest mechanical oscillating system is a mass attached to a linear spring subject to no

other forces. Such a system may be approximated on an air table or ice surface. The system is in

an equilibrium state when the spring is static. If the system is displaced from the equilibrium,

there is a net restoring force on the mass, tending to bring it back to equilibrium. However, in

moving the mass back to the equilibrium position, it has acquired momentum which keeps it

moving beyond that position, establishing a new restoring force in the opposite sense. If a

47

constant force such as gravity is added to the system, the point of equilibrium is shifted. The time

taken for an oscillation to occur is often referred to as the oscillatory period.

The specific dynamics of this spring-mass system are described mathematically by the simple

harmonic oscillator and the regular periodic motion is known as simple harmonic motion. In the

spring-mass system, oscillations occur because, at the static equilibrium displacement, the mass

has kinetic energy which is converted into potential energy stored in the spring at the extremes of

its path. The spring-mass system illustrates some common features of oscillation, namely the

existence of equilibrium and the presence of a restoring force which grows stronger the further

the system deviates from equilibrium.

Damped and driven oscillations

All real-world oscillator systems are thermodynamically irreversible. This means there are

dissipative processes such as friction or electrical resistance which continually convert some of

the energy stored in the oscillator into heat in the environment. This is called damping. Thus,

oscillations tend to decay with time unless there is some net source of energy into the system.

The simplest description of this decay process can be illustrated by oscillation decay of the

harmonic oscillator.

In addition, an oscillating system may be subject to some external force, as when an AC circuit is

connected to an outside power source. In this case the oscillation is said to be driven.

Some systems can be excited by energy transfer from the environment. This transfer typically

occurs where systems are embedded in some fluid flow. For example, the phenomenon of flutter

in aerodynamics occurs when an arbitrarily small displacement of an aircraft wing (from its

equilibrium) results in an increase in the angle of attack of the wing on the air flow and a

consequential increase in lift coefficient, leading to a still greater displacement. At sufficiently

large displacements, the stiffness of the wing dominates to provide the restoring force that

enables an oscillation.

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Coupled oscillations

The harmonic oscillator and the systems it models have a single degree of freedom. More

complicated systems have more degrees of freedom, for example two masses and three springs

(each mass being attached to fixed points and to each other). In such cases, the behavior of each

variable influences that of the others. This leads to a coupling of the oscillations of the individual

degrees of freedom. For example, two pendulum clocks (of identical frequency) mounted on a

common wall will tend to synchronies. This phenomenon was first observed by Christian

Huygens in 1665.[1] The apparent motions of the compound oscillations typically appears very

complicated but a more economic, computationally simpler and conceptually deeper description

is given by resolving the motion into normal modes.

More special cases are the coupled oscillators where the energy alternates between two forms of

oscillation. Well-known is the Wilberforce pendulum, where the oscillation alternates between

an elongation of a vertical spring and the rotation of an object at the end of that spring.

How Oscillators Work

Oscillators are important in many different types of electronic equipment. For example, a quartz

watch uses a quartz oscillator to keep track of what time it is. An AM radio transmitter uses an

oscillator to create the carrier wave for the station, and an AM radio receiver uses a special form

of oscillator called a resonator to tune in a station. There are oscillators in computers, metal

detectors and even stun guns.

To understand how electronic oscillators work, it is helpful to look at examples from the

physical world. In this article, you'll learn the basic idea behind oscillators and how they're used

in electronics.

49

Electronic oscillator

A popular op-amp relaxation oscillator.

An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often

a sine wave or a square wave. They are widely used in innumerable electronic devices. Common

examples of signals generated by oscillators include signals broadcast by radio and television

transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by

electronic beepers and video games.

A low-frequency oscillator (LFO) is an electronic oscillator that generates an AC waveform at a

frequency below ≈20 Hz. This term is typically used in the field of audio synthesizers, to

distinguish it from an audio frequency oscillator.

Oscillators designed to produce a high-power AC output from a DC supply are usually called

inverters.

50

There are two main types of electronic oscillator: the harmonic oscillator and the relaxation

oscillator.

Harmonic oscillator

Block diagram of a harmonic oscillator; an amplifier A with its output vo fed back into its input

vf through a filter, β(jω).The harmonic, or linear, oscillator produces a sinusoidal output.

The basic form of a harmonic oscillator is an electronic amplifier connected in a positive

feedback loop with its output fed back into its input through a filter. When the power supply to

the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels

around the loop and is filtered and re-amplified until it increasingly resembles a sine wave at a

single frequency.

In inductive-capacitive or LC oscillators, the filter is a tuned circuit (often called a tank circuit)

consisting of an inductor (L) and capacitor (C) connected together. Charge flows back and forth

between the capacitor's plates through the inductor, so the tuned circuit can store electrical

energy oscillating at its resonant frequency. There are small losses in the tank circuit, but the

amplifier compensates for those losses and supplies the power for the output signal. LC

oscillators are typically used when a tunable frequency source is necessary, such as in signal

generators, tunable radio transmitters and the local oscillators in radio receivers. Typical LC

51

oscillator circuits are the Hartley, Colpitts and Clapp circuits. On-chip inductors usually don't

have a high enough Q-factor to use in the tuned circuit. A piezoelectric crystal (commonly

quartz) may take the place of the filter to stabilize the frequency of oscillation, this is called a

crystal oscillator. These kinds of oscillators contain quartz crystals that mechanically vibrate as

resonators, and their vibration determines the oscillation frequency. Crystals have very high Q-

factor and also better temperature stability than tuned circuits, so crystal oscillators have much

better frequency stability than LC or RC oscillators. They are used to stabilize the frequency of

most radio transmitters, and to generate the clock signal in computers. The Pierce oscillator

circuit is often used for crystal oscillators. Because the crystal is an off-chip component, it adds

some cost and complexity to the system design, but the crystal itself is generally quite

inexpensive.Surface acoustic wave (SAW) devices are a kind of crystal oscillator, but achieve

much higher frequencies by establishing standing waves on the surface of the quartz crystal. [citation

needed] These are more expensive than crystal oscillators, and are used in specialized applications

which require a direct and very accurate high frequency reference, for example, in cellular

telephones.

There are many ways to implement harmonic oscillators, because there are different ways to

amplify and filter. Some of the different circuits are:

Armstrong oscillator

Hartley oscillator

Colpitts oscillator

Clapp oscillator

Delay line oscillator

Pierce oscillator (crystal)

Phase-shift oscillator

RC oscillator (Wien Bridge and "Twin-T")

Cross-coupled LC oscillator

52

Vackář oscillator

Opto-Electronic Oscillator.

Relaxation oscillator

A relaxation oscillator produces a non-sinusoidal output, such as a square, saw tooth or triangle

wave. It contains an energy-storing element (a capacitor or, more rarely, an inductor) and a

nonlinear trigger circuit (a latch, Schmitt trigger, or negative resistance element) that periodically

charges and discharges the energy stored in the storage element thus causing abrupt changes in

the output waveform.

Square-wave relaxation oscillators are used to provide the clock signal for sequential logic

circuits such as timers and counters, although crystal oscillators are often preferred for their

greater stability. Triangle wave or sawtooth oscillators are used in the timebase circuits that

generate the horizontal deflection signals for cathode ray tubes in analogue oscilloscopes and

television sets. In function generators, this triangle wave may then be further shaped into a close

approximation of a sine wave.

Ring oscillators are built of a ring of active delay stages. Generally the ring has an odd number

of inverting stages, so that there is no single stable state for the internal ring voltages. Instead, a

single transition propagates endlessly around the ring.

An oscillator is a mechanical or electronic device that works on the principles of oscillation: a

periodic fluctuation between two things based on changes in energy. Computers, clocks,

watches, radios, and metal detectors are among the many devices that use oscillators.

A clock pendulum is a simple type of mechanical oscillator. The most accurate timepiece in the

world, the atomic clock, keeps time according to the oscillation within atoms. Electronic

oscillators are used to generate signals in computers, wireless

receivers and transmitters, and audio-frequency equipment, particularly music synthesizers.

There are many types of electronic oscillators, but they all operate according to the same basic

53

principle: an oscillator always employs a sensitive amplifier whose output is fed back to the

input in phase. Thus, the signal regenerates and sustains itself. This is known as positive

feedback. It is the same process that sometimes causes unwanted "howling" in public-address

systems.

The frequency at which an oscillator works is usually determined by a quartz crystal. When a

direct current is applied to such a crystal, it vibrates at a frequency that depends on its thickness,

and on the manner in which it is cut from the original mineral rock. Some oscillators employ

combinations of inductors, resistors, and/or capacitors to determine the frequency. However, the

best stability (constancy of frequency) is obtained in oscillators that use quartz crystals.

Transmitter Antenna

The transmitter antenna is part of the controller.

Definition: RC vehicles communicate through radio signals. The transmitter antenna is a part

of the radio system through which those signals travel. The transmitter antenna is a part of the

handheld transmitter or controller that sends messages to the receiver antenna on the RC.

54

The transmitter antenna might be a solid metal tube or a piece of flexible wire with an end cap

(which may or may not retract into the controller) or a telescoping antenna where the sections

nest inside each other when collapsed. With some radios you'll need to screw the antenna into the

controller. Antenna is final device for sending any information from your transmitter. In fm

transmitter you need signal transmitted to any direction so you need antenna type that transmitted

with polarized circularly.  Antenna gain -3.2 dB, bandwidth 500 KHz with maximum power

handling 500 Watts. In order to use antenna on 88-108 MHz, broadband antenna type must be

designed with no tune involved.

SignalAmplification

Amplification of signal to transmit based on antenna gain in Decible. Signal from transmitter be

able to doubled, tripled or even become more power. The value is called ERP (Emmittion

Radiating Power). For Doubled your transmitter power to be ERP Power need stacked 4

antennas (3.12 dB gain antenna), 6 antennas (5.12 dB), 8 antennas (6.4 dB).

ERP = Transmitter Power x Antenna gain.

55

Below is picture of the antenna dimension .

56

How to Build an FM Transmitter Antenna

Portable satellite receivers let you listen to music through your car's FM radio by using a built-in

FM transmitter. You can make an FM transmitter antenna, which will amplify and boost the FM

transmission capabilities of the satellite radio's FM transmitter. This will provide a better signal

for hearing the music being transmitted. You will need a few parts from an electronics supply

shop and expertise in using a heat gun and soldering iron. The satellite radio must have a plug to

accept an FM transmitter antenna or the antenna cannot be used.

Instructions

Unscrew the back off the phone plug. Unwind some copper wire from the copper wire

spool. Strip a half inch of insulation off the end of the copper wire with the wire strippers.

Push the end of the copper wire through the hole in the bottom of the back of the phone

plug.

Heat up the soldering iron for two minutes. Solder the end of the copper wire to the inside

metal lug attached to the phone plug. Let the solder cool for a minute.

Cut a piece of heat shrink tubing that is about the same length of the metal lug with the

utility knife.

Put the piece of heat shrink tubing over the metal lug. Hold the heat gun over the heat

shrink tubing. Turn on the heat gun.

Turn off the heat gun after a minute has gone by.

Screw the back of the phone plug back onto the phone plug.

Measure two feet of copper wire from the back of the phone plug. Cut the copper wire

from the copper wire spool at the two-foot mark with the wire cutters.

Plug the phone plug into the phone plug connection on the portable satellite radio

57

Colpitts Oscillator

A Colpitts oscillator, named after its inventor Edwin H. Colpitts, is one of a number of

designs for electronic circuits using the combination of an inductance (L) with

a capacitor (C) for frequency determination, thus also called LC oscillator. The

distinguishing feature of the Colpitts circuit is that the feedback signal is taken from

a voltage divider made by two capacitors in series. One of the key features of this type of

oscillator is its simplicity (needs only a single inductor) and robustness. The left picture

shows the schematic as used in the first publication, and the right shows the schematic

from the patent application.

The frequency is generally determined by the inductor and the two capacitors at the

bottom of the drawing.

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Figure 1: Simple common Colpitts oscillator (with simplified biasing)

A Colpitts oscillator is the electrical dual of a Hartley oscillator. Fig. 1 shows the basic

Colpitts circuit, where two capacitors and one inductor determine the frequency of

oscillation. The feedback needed for oscillation is taken from a voltage divider made by the

two capacitors, where in the Hartley oscillator the feedback is taken from a voltage divider

made by two inductors (or a tapped single inductor).

As with any oscillator, the amplification of the active component should be marginally larger

than the attenuation of the capacitive voltage divider, to obtain stable operation. Thus, using

the Colpitts oscillator for a variable frequency oscillator VFO is best done by using a variable

inductance for tuning, instead of tuning one of the two capacitors. If tuning by a variable

capacitor is needed, it should be a third one connected in parallel to the inductor (or in series

as in the Clapp oscillator).

Figure 2: Simple common Colpitts oscillator (with simplified biasing)

Fig. 2 shows an often preferred variant, where the inductor is also grounded (which

makes circuit layout easier for higher frequencies). Note that the feedback energy is now

fed into the connection between the two capacitors. The amplifier provides a current, not

a voltage amplification.

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Figure 3: Practical common base Colpitts oscillator (with an oscillation frequency of ~50

MHz)

Fig. 3 shows a working example with component values. Instead of bipolar junction

transistors, other active components like field effect transistors or vacuum tubes, capable of

producing gain at the desired oscillation frequency, could be used.

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The main advantages of FM over AM are:

1. Lesser distortion. Frequency modulated wave is less susceptible to intereferences from

buildings, traffic etc which provides improved signal to noise ratio (about 25dB) w.r.t. to

man made interference.

2. Waves at higher frequencies can carry more data than the waves at low frequency.

3. Smaller geographical interference between neighboring stations.

4. Less radiated power.

5. Well defined service areas for given transmitter power.

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Disadvantages of FM:

1. Since attenuation is directly proportional to frequency, so FM suffers more attenuation

tham AM signal.

2. FM signals propagate as line-of-sight phenomenon, so if there is a large hill between

transmitter and receiver, the receiver will not receive any signal. AM signals, on the other

hand, are at low frequencies which are reflected back by ionospehere and thus, using sky-

wave-propagation phenomenon, can travel farther distances. FM waves penetrate

ionosphere and are not refracted back.

3. Much more Bandwidth (as much as 20 times as much).

4. More complicated receiver and transmitter.

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Applications:

Television audio

Public Service communications

Radio Service Communications

Point-to-point microwave links used by telecommunications companies

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Conclusions:

In conclusion, when making trade-offs between transmitter power, antenna gain, coax loss, and

total system performance, it is the db difference between the options available to you that is

important. A difference of 3 db will not be apparent to the hams that you are communicating

with. They will hardly notice the difference if you run your transmitter at 50 watts instead of its

maximum 100 watt output power. A difference of 3 db or less between two antennas, two types

of coax, or two system implementations is usually not sufficient to justify higher costs. However,

a difference of 6 db may justify the more expensive approach.

64

References:

1. ^ Zimmer, Ben (29 July 2010). "How Should ‘Microphone’ be Abbreviated?".

The New York Times. Retrieved 10 September 2010.

2. ^ "Bell Laboratories and The Development of Electrical Recording".

Stokowski.org (Leopold Stokowski site).

3. ^ Sessler, G.M.; West, J.E. (1962). "Self-biased condenser microphone with high

capacitance". Journal of the Acoustical Society of America 34: 1787–1788.

doi:10.1121/1.1909130.

4. ^ http://www.national.com/nationaledge/dec02/article.html

5. ^ "AKG D 112 - Large-diaphragm dynamic microphone for bass instruments"

6. ^ "Local firms strum the chords of real music innovation". Mass High Tech: the

Journal of New England Technology. February 8, 2008.

7. ^ "Boudet's Microphone". Machine-History.com.

8. ^ Paritsky, Alexander; Kots, A. (1997). "Fiber optic

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