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:

Reactance Modulator

AF in (Micro phone)

Step down T/F

Bridge Rectifier

Oscillator

Filter Circuit

Regulato r

Introduction

Transmitter antenna

Power supply to all sections

to embedded

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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 generalpurpose 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. 7

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 etc. Even wristwatches are now

becoming embedded systems. The palmtops are powerful

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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: Computer networking products such as bridges, routers, Integrated Services Digital Networks (ISDN), Asynchronous Transfer Mode (ATM), X.25 and frame relay switches are 10

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: 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 11

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 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. 12

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: 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. 13

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 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.

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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 APPLICATIONS: • Non-commercial broadcasting from 88 – 90 MHz (WFM) • Commercial broadcasting from 90 – 108 MHz (WFM) • Television audio • Public Service communications 15

• Radio Service Communications • Point-to-point microwave links used by telecommunications companies

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Circuit Diagram

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Microphone .

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.

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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.

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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 A microphone (colloquially called a mic or mike; both pronounced /ˈmaɪk/[1]) is an acoustic-toelectric 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 21

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 sideaddress) 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 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. 23

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. 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 24

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 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 25

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 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 26

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 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.

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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.

Fiber optic microphone

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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 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, highfidelity recording and law enforcement.

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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.

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 30

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.

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 31

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 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. 32

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. 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 33

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 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 lowercost 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. 34

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 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 35

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, the amplitude of these sidebands approaches a negligible level the farther away they are from the carrier, which allow FM transmission within finite bandwidths.

36

Figure: FM Signal Representation

The Bessel functions solution to the FM equation is

in figure is an example of a FM spectrum

37

Figure: FM Spectrum

Center Frequency 38

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).

39

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

40

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 41

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 42

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 Deviation: 5 kHz 43

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, RC 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.

44

Fig. 19 H. An RC phase shifter

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.

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 45

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 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 46

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.

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 47

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.

Electronic oscillator

48

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

49

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 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 50

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

•

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 51

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 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

52

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. 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 53

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

54

x

Antenna

gain.

Below is picture of the antenna dimension .

How to Build an FM Transmitter Antenna 55

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

Colpitts Oscillator 56

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. 

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

57

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.

58

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.

59

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.

60

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.

61

Applications:  Television audio  Public Service communications  Radio Service Communications  Point-to-point microwave links used by telecommunications companies

Conclusions: 62

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.

63

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.

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