A Transmitter is an Electronic Device Which

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Transmitter:

A transmitter is an electronic device which, usually with the aid of an antenna,

propagates an electromagnetic signal such as radio, television, or other

telecommunications.

Function

The radio transmitter encodes the sound into a sine wave. The sine wave is modulated

to carry information using either pulse modulation (PM), amplitude modulation (AM), or

frequency modulation (FM). This sine wave is then transmitted through an antenna.

Features

Radio transmitters allow communication of information over any distance without wires.

They also use many frequencies allowing multiple communications simultaneously.

They allow encoding and decoding that result in high quality sound for the

communications being sent.

Benefits

Radio transmitters have a range of benefits, including their life-saving capabilities, which

have been employed by ships since the early 1900s. With technologies developed for

the radio transmitter, we now have television, cell phones and the Internet.

The development of the radio transmitter created the modern communications age.

Without the radio transmitter, television, cell phones, computers, the Internet and the

radio would have developed in considerably different ways, if at all. The technologies

developed to make and improve radio transmitters and receivers led to all of these

innovations.



History

In 1888, Heinrich Hertz discovered that energy generated by a transmitting oscillator

could be detected from across his laboratory by using a gapped metal loop. Guglielmo

Marconi seized on this idea to build a wireless telegraph and started experiments in

1894. When the Italian Ministry of Post and Telegraphs said it was not interested in his

inventions, Marconi moved to England in 1896. The British were extremely interested

and gave him all the support he needed. Marconi obtained a patent and set up the

Wireless Telegraph and Signal Company. Ships communications were greatly improved

by his inventions, and many lives were saved using his wireless telegraph system,

including survivors of the Titanic, which had a Marconi wireless on board. He

succeeded in transmitting a transatlantic signal on Dec. 12, 1901, from Ireland to

Canada. A year before, Canadian inventor Reginald Fessenden succeeded in

transmitting the first voice over radio waves. With these two technologies in place,

development of the radio as we know it today began.

Spark gap transmitters were the earliest form of radio transmitter, but were limited in

range and could not transmit voice or music successfully. The Poulson arc generator,

patented in 1903, and the Alexanderson alternator, patented in 1911, superseded the

spark gap transmitter in the early 1900s as they had much greater control and range.

Vacuum tube transmitters took over in the 1920s and '30s. Modern radio uses

integrated circuits.



Receiver & Transmitter Types

Beltpack Transmitters

Beltpack transmitters are designed to be placed on the body of the user. This type of

transmitter is generally used with an external microphone, either a lapel, headworn, or

instrument pick-up. These transmitters will provide phantom power for most electret

devices directly from the internal batteries.

Plug-in Transmitters

Plug-in transmitters are designed to be inserted into the connector of a standard

dynamic microphone. This type of transmitter is generally popular with television ENG

crews working in the field.

Handheld Transmitters

Handheld transmitters are a combination microphone element, and transmitter built into

the same unitized assembly. These devices are popular for vocalists, and speakers that

prefer to hold on to the microphone. Handhelds generally are available with a variety of

different microphone capsules in both electret condenser, and dynamic elements.

Sound performance on most good wireless handhelds will duplicate the performance of

a wired counterpart without coloration or distortion.



The basic receiver types:

Stand Alone Receivers

Stand alone or rack mountable receivers are the most common for use in applications

where the receiver is usually in a fixed location. These are also popular for rental

applications. These receivers vary in complexity and performance, and generally run the

full gambit on pricing from very inexpensive to very expensive depending on

sophistication, and features. Some less expensive units will have permanently attached

telescoping antennas which makes them less desirable for permanent installation.

ENG/EFPR

eceivers

ENG/EFP (Electronic News Gathering, Electronic Field Production) portable type

receivers. These receivers are typically used in remote work for broadcast applications

where a small lightweight receiver is either attached directly to the camera, or placed on

the body of the camera person like a beltpack. These receivers are also very popular

with videographers for use in weddings, and other special events. Due to their compact

size, some ENG type receivers do not have the filtering or advanced RF circuitry of their

larger stand alone cousins. When they are equipped with advanced circuitry, they are

generally very costly. These devices will generally run on internal batteries as well as

external power sources.

Card Frame Receivers

Card frame style receivers are typically used in large multi-system venues where

numerous systems are operating simultaneously. There are two types of receiver

construction in these systems. One type uses a downconverter scheme that actually

converts the operating frequency to a lower band, either UHF to VHF, or VHF to HF.



The second type is actually a series of self contained plug-in receivers that simply share

a common built-in antenna combiner and audio output section.

The subcategories of receivers:

Non-Diversity which includes single antenna recievers.

Diversity which includes twin antenna receivers.

Diversity Reception

There has been a great amount of marketing hype publicized in recent years on various

names for diversity reception in wireless microphones. Diversity reception is a method

of minimizing the effects of multipath during the reception of the radio signal. This is

done by combing or selecting two or more sources of received-signal energy which

carry the same intelligence, but differ in strength or signal-to-noise ratio in order to

produce a usable signal. This always requires more than one antenna, but not

necessarily multiple receivers. The term "true" diversity is merely a marketing term, as

there are several ways in which diversity can be employed in a wireless microphone

system. One particular design is no more "true" than any of the other methods, as this

would imply that the numerous other methods are "false." Each type of design has its

advantages and disadvantages. However, all of the methods described can be effective

in combating the ill effects of phase cancellation.

Phase Cancellation or Multipath

Phase cancellation or multipath is a phenomenon where a direct radio signal and a

reflected radio signal combine in the receiver. The two signals are out of phase from



each other due to the reflection of the second signal off a nearby object. This reflection

adds a slight time imbalance to the second signal which causes it to partially or even

totally cancel the primary transmission.

The result is a noise-up or sometimes a complete loss of signal often referred to as a

drop-out.

A very common example of phase cancellation or multipath has occurred to most

people at one time or another. If you have ever driven your car listening to your favorite

FM station and pulled-up to a stoplight and noticed that your FM radio station became

fuzzy and full of whooshing sounds, you have experienced phase cancellation or

multipath. Notice that when you pulled your car up just a few feet the station came back

to perfect reception.

Technically, the direct radio signal, and a reflected signal bounced off a nearby object

such as the stoplight are combining in your car stereo¶s receiver at the same instant.

Since the reflected signal or multipath is out of phase anywhere from 0 to 180° from the

original signal, the two signals mix in the receiver and either partially or totally cancel

each other out. The result manifests itself as noise and either partial or complete loss of

the signal commonly called a ³drop-out.´

The phase cancellation will only occur in those physical locations where the direct and

indirect reflected waves interact. Moving away just a few feet away will often cause the

multipath to disappear. A diversity system overcomes the multipath problem by the

virtue of having two antennas in separate locations. The possibility of having a multipath

cancellation at both locations simultaneously is very slim.



When direct radio signals and signals reflected off of a metal object, such as a steel

stud mix, they produce a condition known as multipath. This condition will cause a null

in the signal (drop-out) in the physical location that the mixing occurs. Diversity

reception helps to overcome this phenomenon in wireless microphone systems.

Diversity reception is a common way to eliminate noise and reception problems that can

occur in a radio link. Practical applications typically apply only to wireless microphones

because noise-ups and complete loss of signal can be very undesirable when used in a

sound reinforcement application. Similar problems in wireless personal hearing

assistance systems and intercom systems are not as great of a concern because they

will typically affect only a few users and not an entire auditorium of listeners.

At the beginning of the 20th century, there were four chief methods of arranging the

transmitting circuits:

1. The transmitting system consists of two tuned circuits such that the one

containing the spark-gap is a persistent oscillator; the other, containing the aerial

structure, is a free radiator maintained in oscillation by being coupled to the first

(Nikola Tesla and Guglielmo Marconi).

2. The oscillating system, including the aerial structure with its associated

inductance-coils and condensers, is designed to be both a sufficiently persistent

oscillator and a sufficiently active radiator (Oliver Joseph Lodge).

3. The transmitting system consists of two electrically coupled circuits, one of

which, containing the air-gap, is a powerful but not persistent oscillator, being

provided with a device for quenching the spark so soon as it has imparted



sufficient energy to the other circuit containing the aerial structure, this second

circuit then independently radiating the train of slightly damped waves at its own

period (Oliver Joseph Lodge and Wilhelm Wien).

4. The transmitting system, by means either of an oscillating arc (Valdemar

Poulsen) or a high-frequency alternator (Rudolf Goldschmidt), emits a persistent

train of undamped waves interrupted only by being broken up into long and short

groups by the operator's key.

The essential part of tramsmitter:

The oscillator is the heart of the transmitter. It has four leads, but we only use three of

them. When the power is connected to two of the leads, the voltage on third lead starts

jumping between 0 volts and 5 volts, one million times each second.

The oscillator is built into a metal can. The corners of the can are rounded, except for

the lower left corner, which is sharp. This indicates the where the unused lead is. The

lead is there to help hold the can down firmly on the printed circuit board, but it is not

connected to anything inside the can.

The other main part is the audio transformer . In this circuit it is used as a modulator .

The modulator changes the strength of the radio waves to match the loudness of the

music or voice we want to transmit.

A pictorial diagram of the transmitter looks like this:



How does it work?

The oscillator is connected to one end of a long wire antenna. It alternately applies 9

volts of electricity to the end of the wire, and then 0 volts, over and over again, a million

times each second.

The electric charge travels up and down the wire antenna, causing radio waves to be

emitted from the wire. These radio waves are picked up by the FM radio, amplified, and

used to make the speaker cone move back and forth, creating sound.

The sound source (your CD player, or tape recorder with our project we microphone) is

normally connected to drive a speaker or earphone. It drives the speaker by emitting

electricity that goes up and down in power to match the up and down pressure of the

sound waves that were recorded. This moves the speaker in and out, recreating the

sound waves by pushing the air in and out of your ears.



In our transmitter, the sound source is connected to the transformer instead of to a

speaker.

The transformer is connected to the power supply of the oscillator. The sound source

causes the transformer to add and subtract power from the oscillator, just as it would

have pushed and pulled on the speaker.

As the power to the oscillator goes up and down, the power of the electricity in the

antenna goes up and down also. The voltage is no longer simply 9 volts. It is now

varying between 0 volts and 10 volts, because the power from the transformer adds and

subtracts from the power of the battery.

The varying power in the antenna causes radio waves to be emitted. The radio waves

follow the same curves as the waves in the antenna. However, because the transmitter

and the receiver are not connected, the receiver does not know what the transmitter is



using for the value of zero. All the receiver sees is a radio wave whose amplitude is

varying. In the receiver, zero is the average power of the wave. This makes the wave

look like this:

If we sent this wave to the earphone, we would hear nothing, because the average

power is zero. This is why our crystal radio has a diode.

The diode does a neat little trick. A diode only lets electricity flow in one direction. This

means that the part of the graph where the power is rising up from zero can get through

the diode, but the part where the power is going down from zero is blocked.

All those little peaks of power happening a million times per second are too fast for

human ears, and too fast for the earphone to reproduce. But since they are all pushing

on the earphone diaphragm, all those little pushes add up, and the earphone moves.

Since some of the little pushes are stronger than others (taller blue bars in the

illustration) they move the earphone more than the weaker ones. We hear this variation

as sound.



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A Transmitter is an Electronic Device Which