School Intercom

8/10/2019 School Intercom

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Fabrication of a Low-Cost Wireless Intercom Unit

_________________________

A Thesis

Presented to the

Graduate School of

Uni ersit! of Saint Louis Tu"ue"arao

Tu"ue"arao Cit!

_________________________

Sherwin #$ Catolos

%&&'

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Chapter I

The Problem and Its Background

Introduction

In broad sense, the term communication refers to the

sending, receiving and processing of information by

electronic means. Communications started with wire

telegraphy in the eighteen forties, developing with

telephony some decades later and radio at the beginning of

this century (Kennedy, 1995 . It became even more widely

used and refined through the invention and use of the

transistors, integrated circuits and other semiconductor

devices.

!enerally, electronic communication systems have basic

components such as transmitter, a communications channel or

medium, and a receiver. In most systems, a human generates

medium, and a receiver. In most systems, a human generates a

message which contains information, or intelligence, signal.

"his signal is inputted to the transmitter which then

transmits the message over the communication channel. "he

message is pic#up by the receiver and is relayed to another

human ($oddy % Coolen, 1995 .

In communication systems, all pieces of communications

e&uipment were individually pac#aged in units based on their

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function. In general, transmitters and receives were always

separate units. "oday, however, most two way communications

e&uipment is pac#aged so that all the functions are

contained within a single housing. "ypically, both the

transmitter and receiver are in the same pac#age. 'or this

reason, this combination of units is refereed to as a

transceiver. "ransceiver vary in si e and comple)ity from

very large, high power, des#top units to the very small,

poc#et*si e, hand*held wal#ie*tal#ies ('ren el, 1995 .

Intercom is an e)ample where the transmitter and

receiver are pac#aged in a single housing that has made it

#nown for its mobility, lower cost, and in some cases,

smaller si e. It is a private telecommunication system that

allows typically two or more locations to communicate with

each other li#e telephone.

+any productions which needs co*operation of more than

a few people need special intercoms that cover many users.

Intercom systems used in " and stage productions are

usually headset type intercoms connected to one line using

party line arrangement. "he primary use of this type of

system is in live or media productions where (for e)ample

the video director spea#s to the camera operators, or where

the stage manager spea#s to the stage hands and lighting

operator, etc.

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Intercom systems, by definition, may be comprised of

many different types of intercoms and subsystems. "he most

basic intercom circuit consists of two intercom stations

lin#ed to each other with a push to tal# (-"" switch. "his

#ind of circuits are simple and generally consist of only

one or two amplifiers and generally use the spea#er as

normal spea#er and microphone (how it is used depends on

tal# switch position . epending on the circuit design there

could be one -"" switch on one end, or separate -""s on both

stations. /hen one wants to get rid of push to tal# switch

and want full duple), things get more complicated to build.

( http0 www.epanorama.et lin#s intercom html2generalin#s ,

3ct 4 , 4 5

/ith the above set*up, limitations and disadvantages

can be encountered. 6sing wire can contribute to costly set*

up and eventually area covered by the system will be

limited. 7lso, its applications can no longer be benefited

when endeavors with mobility are re&uired.

"wo way radios were first to arrive in the mar#et, and

have been used and wor#ed for some applications such as pre*

show setup and post*show teardown. 8ut this did not,

however, do as well for the rigors of line television

production. (" : ;


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directional conversation, only one user may communicate at a

time and all other users must listen until the person who is

communicating is finished. -roblems then would seem to be

evident in communicating one from the other, say, during a

show.

"wo*way radios have higher operating power which

affords substantially increased operating range of over a

mile or more in some cases.(" : ;


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"hus, the need to fabricate a prototype of wireless

intercom unit yet at low cost is conceived.

Project Objectives

"he ob=ectives of the study were0

1. to design and fabricate a wireless intercom unit

4. to test its functionality and performance

>. to determine and assess its interference effect to

e)isting communication systems available in the locale of

the study

?. to compare between the cost of the pro=ect with the

commercially available device.

Scope and Delimitation

"he study was limited in the fabrication of a wireless

intercom system to be used by the !eodetic ngineering

students in their field survey activities of the College of

ngineering, 7rchitecture and 'ine 7rts at 6niversity of

@aint :ouis, "uguegarao City.

"he pro=ect would be operating at the


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range that could be covered would be within the vicinity of

the locale of the study.

"he pro=ect would be tested within the campus with

regards to its functionality and its interference effect to

e)isting communication systems within the campus.

Significance of the Study

"he main contribution of this study was to develop a

prototype wireless intercom unit to be used by students.

"he following were benefited in this study0

dministration! "his design would enable the

administration to provide students with state*of*the art

means of communication gadget yet at low cost.

"esearcher! "his pro=ect would provide the researcher

the essential procedures, details and other set*up to

enhance the #nowledge and s#ills of the researcher in the

design of electronic communication systems.

Students . "his pro=ect design would provide students

opportunities to enhance their #nowledge in communication

system.

#uture "esearchers! "his study would serve as basis for

future researchers in the fabrication of similar electronic

devices.

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Chapter II

"$% T$D %IT$" T&"$ 'D ST&DI$S

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In this chapter, the &uotations, summaries, discussion

of the principles, theories, concepts, and findings from

those literatures which were found to be significant and

valuable to the study are presented.

"elated %iterature

Development of Communication

Communications between human beings probably begun in

the form of human gestures and facial e)pression, which

gradually evolved in the verbal grunts and groans. erbal

communications using sound waves however was limited by how

loud a person could yell. :ong*distance communications

probably began with smo#e signals or tom*tom drums, and that

using electricity began in 1B> when @amuel 'inley 8reese

+orse invented the first wor#able telegraph. +orse applied

for a patent in 1B>B and was finally granted in 1B?B.


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metallic*wire communications system using a device tey

called the telephone. ("omasi, 4 ?

In 1B9?, +archese !uglielmo +arconi successfully

transmitted the first wireless radio signal through earthEs

atmosphere, and in 19 D, :ee e'orest invented the triode

vacuum tube, which provided the first practical means of

amplifying electrical signals. Commercial radio broadcasting

began in 194 when radio station K K7 began broadcasting

amplitude*modulated (7+ signals out of -ittsburgh,

-ennsylvania. In 19>1, +a=or dwin 5. ("omasi, 4 ? /ireless

communications seem to grow rapidly and becoming more

sophisticated due to the state*of*the*art development of

electronic devices and integrated circuits (ICs and also

to the unending desire of human to create and develop

something that suffice their needs of communication

#&'D ($'T %S O# CO((&'IC TIO' S)ST$(

"he fundamental re&uirement of a communication system

is to change data between two parties. It consists of an

information source and destination connected by a

communication channel (medium , which transfers message from

transmitter to receiver.

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In fact telecommunications involve the conversion of

messages may be in the form of words or coded symbols, into

electrical voltage or current which varies with the time and

is used to carry information from sending and to receiving

end. @uch electrical &uantities are termed as signals. "his

conversion process is referred to as encoding. "his encoded

signal is then fed to the modulation where the signal is

mi)ed with the carrier for transmission. "he process of

encoding and modulation ta#es place in the transmitter.

(Kumar and Fagannathan, 4 ?

In addition to their normal function of message

(information to signal (data conversion, modern encoders

also employ various data processing to protect the data from

channel disturbances so that, the information can be easily

retrieved bac# at the receiver by simple decoding process.

"he communication channel is the path or medium for

electrical or electromagnetic transmission between the

transmitter and the receiver. "his maybe either a guided

transmission line such as a single wire, pair of wires,

waveguides, fiber optic cables and coa)ial lines or a home

10

TxSource Rx Destination

Medium


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guided atmosphere or space channels such as radio wave,

microwave and laser beam.

7t the receiving end, it is re&uired that a

corresponding bac#*mapping is done to reconnect the signal

with the original message. "his process is referred to as

decoding (demodulation . "he associated module with this

process is the receiver. (Kumar and Fagannathan, 4 ?

*istory of +ireless Intercoms

In the beginning there was wire, and the wire was good.

@oon engineers reali ed if they could cut the wires and move

the audio, video and communications signals around the

television venue without encumbering cables, they would have

tremendous freedom to accommodate ever*increasing production

challenges. "hey also believed that wireless transmission of

signals would ma#e their =ob easier by not having to run

miles of cable for large remote productions. It turned out

not to be so simple. eveloping wireless microphones,

wireless cameras and wireless intercom systems would be a

trial and error adventure that has spanned the last > years

or more ( www.tele).com intercom features , 3ctober 45, 4 5

"he original Gwireless intercomH consisted of two*way

radios and a headset. "he advantage of the technology was

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http://www.telex.com/intercom/featureshttp://www.telex.com/intercom/features


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readily available and it was relatively ine)pensive to use.

"wo ways wor#ed well for some applications, such as pre*show

setup and post*show teardown where they are still used today

in much the same way they were > years ago. "wo*ways (now

often called


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@oon after it become apparent that a half*duple)

communications system would never satisfy the needs of on*

air production, a vast array of new


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ach user station in the system consisted of two

beltpac#s, one for transmitting and one for receiving. "wo

beltpac#s were necessary to combat the phenomenon #nown as

desensing, where a transmitter in close pro)imity to a

receiver causes the receiver to have greatly reduced

sensitivity. ach wireless userEs transmitter was on a

uni&ue fre&uency which corresponded to receiverEs fre&uency

in the base station. 7ll of the wireless usersE receivers

were tuned to the same fre&uency which corresponded to the

single base transmitter. 7 single headset with a split feed

cable eliminated the need for an e)ternal headset interface

bo). 8y utili ing this system, each wireless user could

communicate to both hardwired and wireless intercom users in

a full duple) mode (htt www.tele).com, 3ct. 45, 4 5

"his system, at long last, provided engineers with a

reliable and functional solution to the wireless

communications problem. 'uture systems would combine receive

and transmit beltpac#s and incorporate numerous interfacing

and operational advantages.

(odern Intercom Systems

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"odayEs wireless intercom systems are technological

giants compared to their earlier predecessors. "hey allow

users to Gcut the cableH of hardwired party*line systems and

move about freely within the systemEs operational range.

+odern wireless intercoms can be either party*line intercom

systems or individual beltpac# systems that allow users to

operate independently from other wireless users. !ood

&uality systems can be seamlessly attached to e)isting

hardwired communications systems commonly used in broadcast

and other facilities. +odern, high &uality wireless

intercoms offer a distinct advantage over traditional, two*

way radios in that they offer a more natural full*duple)

operation. "his enables all users on the system to spea# and

hear other users simultaneously without GcoveringH other

usersE transmissions.

"he demands of modern broadcast productions ma#e the

full*duple) operation of wireless intercom systems an

absolute necessity for stage managers, lighting and audio

technician, or any professional who has to deal with the

brea#nec# speed and comple)ity of television productions.

"he spread of digital television ( " and the ever*

increasing number of wireless users has made the available

fre&uency spectrum a more difficult place in which to find

available channels for wireless intercoms. "he spectrum has

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also become a lot smaller, especially considering that four

television channels (4? +< of spectrum have been

reallocated for public safety use and the upcoming

reallocation of 6


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"he answer to the fre&uency problem is to utili e a

digitally synthesi ed, fre&uency agile system. "hat may

sound simple enough in theory, but in reality, designing

such a product is a totally different matter. 7 digitally

synthesi ed, fre&uency agile system must not only

incorporate a superior design with high*&uality filtering to

withstand the rigors of an overcrowded fre&uency spectrum,

but it must also offer an ergonomically designed user

interface that allows ease of fre&uency selection and

operation. nd users must e)perience the same ease of

operation they get from their e)isting two*wire beltpac#s.

"o date, the chief limitation to most wireless

intercoms (other than finding available spectrum has been

inherently one*channel in nature while the most common

hardwired intercom system from $"@ (used in virtually all "

broadcast truc#s and facilities is two*channel. "wo*channel

operation allows users to switch easily from one intercom

channel to another. "his allows a stage manager, for

instance, to communicate with the producer and then switch

over to the director circuit as necessary. "wo*channel

operation has become the hardwired industry standard and

users who have increasingly relied on wireless intercoms

form without having to deal with huge rac#s full of

e&uipment.

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/ireless intercom systems that can operate in high $'

environments must not only offer interference resistant

operation, but must utili e design techni&ues that will not

interfere with other wireless e&uipment li#e wireless

microphones. 7nother #ey to a wireless intercomEs successful

operation and coe)istence with " is its ability to avoid

strong local " stations, as well as, coordinate multiple

system fre&uencies. "his holds true whether the system is


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7s wireless intercom applications for broadcast

professionals continue to grow more comple) and challenging,

the need for products that can meet these challenges will

also grow accordingly (" : ;


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spectrum is a good way for the commission to move from an

e)pense center to a profit center, and they are pursuing it

with a passion.

/ireless intercoms, li#e any other wireless system,

re&uire at least one transmitter to function. 6nder 'CC

rules, all transmitters must be licensed prior to operation

(there are some very low power transmitters that can operate

and do not need to be licensed, but that doesnEt apply to

any modern $' intercom systems . "here are different forms

to obtain various types of licenses depending on what area

of the spectrum your system will operate in, who will be

operating the system, and what the system will be used for.

"he law is very clear in that no one is permitted to operate

a transmitter typically used for wireless intercom systems

without first obtaining an 'CC license.

/ireless e&uipment often operates in areas of the $'

spectrum that are designated for " channels, but are unused

in a given area. In all cases low power transmitters used by

wireless intercoms and wireless microphones must operate on

a secondary, non*interfering basis. "his means that wireless

users must not cause harmful interference to television or

other receivers, and must accept all interference sources.

In #eeping with this, the 'CC rules state that


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transmitter occupying a similar spectrum. "he rules further

state that 6


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/hereas sound needs some physical mass to move, $' signals

do not. "he electromagnetic spectrum e)ists everywhere and

enables $' signals to move through the vast vacuum of space

where sound waves could never go. ($oddy and Coolen, 1995

"he name electromagnetic is really a combination of two

words, electron (or electronic and magnet (or magnetic .

"he reason for this is that waves that propagate in the

electromagnetic spectrum have two separate and distinct

components, an electrical and magnetic. "hese two components

e)ist at right angles to each other, as well as, to the

direction of propagation. "he electrical component, or field

as it is called, is represented by the letter and the

magnetic field by the letter


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the ratio of the magnitudes of the electrical and magnetic

components of an $' wave vary dramatically as fre&uency

changes. !enerally spea#ing, the magnetic component of an $'

wave is much greater than the electrical component at very

low fre&uencies. 7s the fre&uency increases, the electrical

component increases and the magnetic component decreases,

until, at very high fre&uencies, the electrical component is

much greater than the magnetic. "he different ma#eup of $'

waves at different fre&uencies is what allows us to use the

signals for different and sometimes unusual applications.

'or instance, at super low fre&uencies, such as 5


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In its most basic form, an $' system puts information

on an $' signal, sends it to a remora location and retrieves

the information in e)actly the same form as it originally

e)isted. "he transmitter is a device that has an input for

information, audio, data, or some other form of intelligence

called a source signal, that needs to get from here to

there. "he transmitter then ta#es that information and puts

it onto an $' signal. "he $' signal is called a carrier

because it, in effect, carries the source signal as it

propagates. "he process of actually putting the source

signal onto the carrier is called modulating the carrier,

which normally is referred to simply as modulation. "he

carrier which has had the source signal applied is then

broadcast into the air (actually the electromagnetic

spectrum via an antenna. "he antenna is a transducer that

allows the carrier to be efficiently broadcast or received.

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3nce the signal is broadcast into the air, it

propagates out away from the transmit antenna and eventually

reaches the receive antenna. "he area between the transmit

antenna and the receive antenna is called the propagation

path, or =ust path. 7t the receive antenna, the signal,

which is now much wea#er, is collected and enters into the

receiver. "he receiverEs =ob is to find the one uni&ue

carrier from the transmitter and strip off the source signal

so it e)actly matches the original information. "his process

is called demodulation.

It is #nown that $' propagates or moves from one point

to another, and that propagation can be affected by the

fre&uency of the wave. "he energy carried by the wave moves

away from the original point in all directions e&ually and

each vector that can be drawn from the center point

represents $' energy traveling away from the point of origin

in a straight line. (" : ;


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wave becomes wea#er can be calculated via the inverse s&uare

law 1 where L distance traveled by the wave. "his is a

very important concept because it shows why a wave that

travels twice as far as another wave of e&ual magnitude is

not half as strong. 'or instance, two transmitters ";7 and

";8 both emit signals that are e)actly the same at 1 /att of

power. "he signal from ";7 travels 1 units. -ower at that

point can be calculated by1 1 4 ) 1/ or . 1 ) 1/. "hat

means there is . 1/ of the ";7 signal left after it has

traveled 1 units. Aow letEs say that the ";8 signal travels

twice the distance of ";7 or 4 units. -ower at that point

can be calculated by 1 4 4 ) 1/ or . 45 ) 1/. "hat means

there is . 45/ of the ";8 signal left after it has

traveled 4 units. "hat is, the signal that traveled twice

as far was not M the power, but N the power of the first

signal. (" : ;


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"he theoretical range of an $' system is important to

#now, but it is the functional range that must be more

concerned with. "he functional range of a system ta#es into

account a certain cushion factor called fade margin that

will ensure the signal coming from the transmitter to the

receiver will not only be detectable, but will also be

usable. "his is less of a concern in communications systems

as you can tolerate less fade margin than in an on*air

wireless microphone system, because a small momentary

dropout will not critically affect communications as

compared to audio.

$' waves travel away from the source in a straight line

until that path is interrupted or disturbed by some outside

influence. $' waves are being reflected and this changes the

path of some of the $' energy. "his phenomenon is called

reflection.

#igure! "eflection of Signals

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-olari ation is the term that describes the orientation

of an $' wave. "here are two components that ma#e up an $'

wave, the electrical and the magnetic. "he field was the

electrical component and that the < field was the magnetic

component. "he polari ation of an $' signal is determined by

the orientation of the field. If the field is

perpendicular to the plane of the arth, the wave is said to

be vertically polari e. If the filed is parallel to the

plane of the arth, the wave is said to be hori ontally

polari ed.

"ransmit and receive antennas of the same system must

be oriented in the same direction (plane to have a proper

transfer of the carrier. In theory, if a transmit antenna is

oriented vertically, thus producing a vertically polari ed

carrier, and the corresponding receive antenna is oriented

hori ontally, the receive antenna will not be able to see

the vertically polari ed wave at all. In practice, there

will always be some polari ation shift in the path and the

receiver will see a very small signal if it is close enough

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to the transmitter. "o avoid this problem, antennae in a

given $' system should always have similar orientation.

"here are other forms of polari ation, such as circular

polari ation, which can be used to help counteract the

effect of multipath, but for now we will use hori ontal and

vertical polari ation for our discussion. It is important to

note here the difference between polari ation and phase, as

the two terms are often confused. -hase refers to the

relationship of the sinusoidal energy of two or more waves,

not to the orientation of the electrical component. "wo

identical waves that are in phase, and are combined, add to

ma#e a larger wave. "wo identical waves that are out of

phase by e)actly 1B , and are combined, cancel each other

out. /aves that are not e)actly identical in fre&uency,

amplitude, or phase will have a composite sum that may

increase the overall amplitude at some points, and either

reduce or eliminate the overall amplitude at others.

Interference

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+ultipath can be described as a form of self

interference caused when a reflected $' carrier arrives at

the receive antenna along with an $' carrier that has ta#en

a direct path. "he reason multipath is so detrimental to the

successful operation of an $' system has to do with the

nature of the relationship of the reflected signal to the

direct path signal.

"he direct path carrier ta#es the most direct, and

conse&uently, the shortest path from transmitter to

receiver. "he reflected carrier, on the other hand, ta#es a

longer path, from the transmitter to the reflective surface,

and from the reflective surface to the receiver. "he waves

leaving the transmitter antenna are all in phase, but

because the direct carrier and the reflected carrier travel

different distances, thus ta#ing slightly different lengths

of time, the two carriers are out of phase, and of different

amplitudes (remember the inverse s&uare law , when they

reach the receive antenna. "he two carriers are combined at

the receive antenna and, being out of phase, they cancel

each other out so that little or nothing can be detected by

the receiver. "his causes a momentary interruption in the $'

wave, which is called a dropout. ropouts are manifested in

audio $' systems by a loud clic# or pop surrounded by noise.

-roper system design and careful antenna placement can go a

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long way to reducing the effects of multipath on a wireless

communications system.

"he ne)t concept that must be familiari ed with in the

design of wireless intercom system is receiver

desensiti ation or desensing. 7s mentioned, desensing

happens when transmitter is in close pro)imity to a

receiver, even if that transmitter is not on or near the

receiverEs operating fre&uency. $eceiver decentrali ation

happens because receivers must maintain critical voltage and

current levels throughout the front end stages, and a strong

(i.e. close by transmitter can cause these levels to vary

greatly. 7s those levels are changed over a wide range, the

receiver performance will greatly degrade. "he greater the

physical distance between transmitter and receiver, the less

the receiver will be affected. :i#ewise, the greater the

fre&uency separation between the two, the less the receiver

performance will be affected. @electing fre&uencies that are

Gclean,H or free from the effects of intermodulation

products, is essential to good wireless communications.

Intermodulation is often one of the prevalent sources of

system interference. 7s stated, intermodulaton, or I+ as it

is often called, happens when two or more fre&uencies mi) in

a non*linear device and produce a number of related

different fre&uencies #nown as intermodulation products.

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"hese I+ products can cause severe, harmful interference to

a wireless intercom system if they fall on or near of the

operating fre&uencies of that system (" : ;


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idea of how the calculations wor#, but for comprehensive

fre&uency selection, an advanced computer program must be

used.

It is important to note that intermodulation products

are not created in the airP they are the result of the

mi)ing of signals in non*linear devices such as amplifiers

or other usually active elements. "he most common place for

this mi)ing to ta#e place is in the active receiver $'

circuitry. 3nce $' signals get past the receiver front end

and get to the first $' amplifier and beyond, mi)ing of and

get to the first $' amplifier and beyond, mi)ing of those

signals can and will ta#e place. If the intermodulation

products that are generated fall on or near the operating

fre&uency of the receiver, harmful interference will be

heard. !ood &uality receivers have front ends that are

passive, linear devices that limit the range of fre&uencies

that will enter the rest of the receiver circuitry. +a#ing

sure you pic# wireless intercoms with well designed front

ends, is critical to proper operation in hostile $'

environments.

"he ne)t most common place for I+ products to be

generated is in the final amplifier of a transmitter.

8ecause the transmit antenna can and does also act as a

receive antenna, strong $' signals from nearby transmitters

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can ma#e their way into the non*linear, active, final

amplifier and produce intermodulation products. "hese

products can then be broadcast out with the intended signal

and cause harmful interference. It is important to note that

I+ products do not have to end up e)actly at a receive

fre&uency. @ometimes, they can be of sufficient power at

relatively close fre&uencies to create a desensing

situation. $educing the effect that intermodulation can have

on wireless intercom system comes down to a few important

principles. 'irst, and foremost, one must pic# fre&uencies

that are intermodulation free with each other and with

surrounding transmitters. @econd, pic# wireless intercom

systems that have well designed receivers and transmitters

with appropriate passive filtering. "hird, manage the

positioning of antennas and beltpac#s within the system to

optimi e operational potential (" : ;


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deliver it to the transmit antenna for broadcast into the

electromagnetic spectrum.

'irst, an audio signal is brought in and any necessary

audio amplification is done via the +ic :ine Input section.

Ae)t, the signal is sent through a Compressor circuit to

ensure the levels of the input signal are held within

acceptable limits. "he signal is then mi)ed with a reference

fre&uency in the +odulator. "his reference fre&uency can be

the main carrier fre&uency, or (as in most cases it is a

base fre&uency that results in a composite signal. "he

signal is then sent to the 7mplifier +ultiplier. If the

signal is already on the desired transmit fre&uency, it is

only further amplified. If, however, the signal is only a

composite signal, then it is fre&uency multiplied to reach

the desired operating fre&uency. "he signal is then sent to

a 'inal 7mplifier where it reaches its ma)imum power level.

6sually this is slightly more than the actual output power

as measured at the output connector. "he reason for this is

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to ma#e up for the losses induced by the 3utput 'ilter and

Impedance +atching circuits.

"he 3utput filter and Impedance +atching circuits are

generally passive and therefore, do not provide any means of

amplification. 7s such, they can only reduce the output

signal levels. "he 3utput 'ilter is a very narrow bandpass

filter that removes any unwanted harmonics from the signal.

"he Impendance +atcher provides the necessary interface

between the transmitter and the 7ntenna "ransmission :ine to

ensure ma)imum power transfer. If the 7ntenna "ransmission

:ine are not properly matched, significant loss can occur.

In some situations, it is possible for this to cause damage

to the transmitter, transmission line, and or antenna.

(" : ;


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"he receiver starts with the front*end filter. "he

front*end filter is e)tremely important to successful

operation in high $' level environments. "he front end

filter is e)tremely important to successful operation in

high $' level environments. "he front end filter is the

first line of defense. Its =ob is to limit the number of

potential interfering fre&uencies that could affect the

receiver. It is usually a passive, linear section and it

must be impedance matched tot eh antenna for proper signal

transfer. :inearity is the most important factor in a front

end, even more so than how tight or narrow the section is. 7

high degree of linearity will ensure that no intermodulation

products are generated in the front end before entraneous $'

signals are filtered out.


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"he ne)t section of the receiver is the first $'

amplifier. "he first $' ampEs =ob is to ta#e the e)tremely

low level $' signal coming through from the front end and

bring it up to a usable level. "he incoming $' signal at the

first $' amp can vary dramatically from less than .5 to

almost the value of the transmitter output. "he #ey for the

first $' amp is that is should be able to handle very small,

as well as, relatively large incoming signals within its

linear region of operation. "o maintain a good linear

region, $' amps normally re&uire a high current drain which

can negatively impact battery life. 7 compromise between

linearity and effective battery life must be managed

carefully.

"he ne)t receiver section is the first local oscillator

(:3 . "he =ob of the first :3 is to provide a reference

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signal that is a fi)ed distance from the operating fre&uency

of the system. It is very important the first :3 be stable

over a wide range of temperatures. In fi)ed crystal systems,

one or more crystals cut to a specific relationship of the

operating fre&uency are use to generate this highly accurate

reference signal. 7 different crystal is necessary for each

operating fre&uency. In synthesi ed units, a single

reference crystal is used in a phase*loc#*loop to provide

the signal for any operating fre&uency needed by the

receiver.

"he 'irst :3 feeds the reference signal to the +i)er

where the incoming $' carrier is mi)ed, or beat with the

reference signal, to produce the 'irst Intermediate

're&uency (I' . "he fre&uency of the 'irst I' is the

difference in fre&uency between the incoming $' carrier and

the 'irst :3 reference signal. 6nfortunately, what comes out

of the +i)er is not a =ust the 'irst I', it is the algebraic

sum and difference of the two signals being mi)ed plus

numerous other harmonic =un#. "o get to the point where a

clean 'irst I' consisting of =ust the desired fre&uency, the

signal is passed through to the 'irst I' 'ilter. "he 'irst

I' 'ilter is e)tremely important to proper receiver

operation. It is a passive, very narrow (often 5 to45

K< , and precise filter that eliminates the vast ma=ority

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of unwanted signals so the true 'irst I' can be processed

correctly. It is very important that the 'irst I' 'ilter be

sharp, as well as, very linear. 7ny non*linearity in the

filter will cause unwanted distortion of the demodulated

source signal.

Ae)t, the signal is sent to the second +i)er where a

second I' fre&uency is produced in the same way the 'irst I'

was obtained. "he @econd :3 is the same fre&uency for any $'

carrier fre&uency the receiver is capable of because the

first :3 ta#es care of the fre&uency differences and

produces an always*constant 'irst I' 're&uency differences

and produces an always*constant 'irst I' fre&uency for the

@econd I' to handle. 7gain, the @econd I' signal as it

leaves the @econd +i)er is full of harmonic =un# and needs

to be filtered by the @econd I' 'ilter. "he @econd I' filter

eliminates unwanted harmonic energy and prepared the signal

to be demodulated.

"he ne)t phase of the receiver is the emodulator.

"here are several types of demodulators used by wireless

manufacturers today and it would be beyond the scope of this

boo# to discuss them all in detail. @uffice to say, through

a type specific process the emodulator e)tracts the source

signal from the @econd I' carrier. "he &uality of the

emodulator circuit is critical to good audio &uality. 7ny

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type of signal distortion or modification that ta#es place

in the demodulation process will cause the final signal to

be a less than perfect reproduction of the original source

signal.

ntenna Considerations

7ntennas and cables (transmission lines are one of the

least thought about aspects of a wireless system among $'

novices. !ood &uality antennas, however, are some of the

most important aspects to establishing and maintaining a

&uality $' lin#. In addition, because antennas are more

easily changed and in general are less e)pensive than other

system components, they can be a G&uic# fi)H for many $'

problems found in common wireless communications systems.

In a transmitter, the antenna ta#es electrical energy

and allows it to be propagated out into the electromagnetic

spectrum. In a receiver the antenna GgathersH the $' signal

and converts it bac# into electrical voltages and currents.

In either case, the antenna acts as a transducer to change

the form of the $' energy.

7ll real world antennas have a pattern or specific

shape with which the $' energy is released or captured.

"here is no such thing as an antenna that sends energy out

e&ually in all directions. "he primary reason for this is

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that you have to get the signal to the antenna via a

transmission line and that line must be connected to the

antenna some how. "hat connection will always cause a

disruption or altering of $' propagation in some direction.

In theory though, it is nice to tal# about a perfect

antenna. "his perfect antenna radiates e&ually in all

directions and is called an isotropic radiator.

"he isotropic radiator is said to have ero antenna

gain. 7ntenna gain is an often misunderstood term. 7 passive

antenna is not an amplifier and cannot increase to total $'

energy being emitted or received.


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isotropic radiator located in the balloonEs center. 7ll of

the $' energy is e&ually dispersed in all directions. If you

s&uee ed the balloonEs center with your hands, a

corresponding bulge would appear on either end. "he balloon

is not any larger or smaller than it was, it has only

changed shape. "his is how a real world antenna wor#s. /hen

energy is focused in one direction, it must always be at the

e)pense of energy going in another direction.

"he most basic form of real world antenna is the

dipole. "he dipole has 4.15 d8i of antenna gain over an

isotropic radiator. "hat means there is 4.15 d8 more signal

in the direction that the energy is focused than there would

be if the antenna were an isotropic radiator. 7ntenna gain

is specified in one of two ways0 d8i or d8d. It is very

important to #now which specification is being used when

comparing antennas. d8i, as stated above, is referenced to

the uniform radiation of an isotropic radiator. d8d, on the

other hand, is referenced to a dipole. +ost antenna

manufacturers li#e the d8i spec because the number is bigger

but since there is no real world antenna that represents the

d8 mar#, many engineers prefer d8d. In reality, either

specification is fine as long as you are comparing apples to

apples. In the remainder of this boo# all antenna gain

references will be in d8d, referenced to a dipole, unless

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otherwise specified. Important0 "o convert from d8i to d8d,

simply subtract 4.15 d8 from the d8i number. "o convert from

d8d to d8i, simply add 4.15 db to the d8d number

( http0 tele).com intercoms features , 3ct. 45, 4 5 .

"here are two basic groups of antennas, omni

directional and directional antennas. 3mnidirectional

antennas are generally low gain antennas used in the center

of operational areas. 8ecause the $' energy in omni

directional antennas is in >D and not in one specific

direction, the antenna gain must always be low. "he

isotropic radiator and dipole antennas are both e)amples of

omni directional antennas. Aormally, omni directional

antennas will be fond with antenna gain less that 5 d8d.

!ain in omni directional antennas is achieved by flattening

the vertical angle of the pattern.

'or proper propagation to ta#e place, the length of an

omni directional is critical. "he theoretical minimum length

for an omni directional antenna is M the wavelength of the

$' carrier to be served. In many cases this M wave length is

too long to be practical so a wave antenna is used instead.

It is e)tremely important to note that for a wave antenna to

wor# properly, it must have a corresponding ground plane

that is e&ual to or greater than the length of the antenna

itself. It is for this reason that a wave antenna that wor#s

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=ust fine when it is attached directly to the bac# of a

wireless receiver has very poor coverage when operated at

the end of a length of coa)ial cable. "he cable does not

provide the necessary ground plane for proper operation as

the receiver does. "his is a very common mista#e made by $'

novices who are trying to improve $' performance and end up

#illing it instead (" : ; D to for a

flashlight li#e coverage pattern. irectional antennas are

normally used on the edge of a coverage area. "hey can have

very high antenna gain factors in e)cess of 4 d8d. Aormally

though, in conventional wireless communications systems,

si e and cost limit directional antenna gain to less than 14

d8d. irectional antennas have the advantage of not only

focusing the $' energy in a given direction, but also

attenuating energy from undesired areas. "his is very

important for receive antennas in areas with high level of

$'. If positioned properly, a directional receive antenna

can increase the desired $' energy while attenuating

unwanted, potentially interfering $' energy from other areas

(Kennedy, 199D .

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"here are two very commonly used directional antennas

in wireless communications systems today, Qagi and :og

-eriodic antennas. irectional antennas must be tuned or

GcutH to a specific fre&uency range. "his is all well and

good when there is only one $' fre&uency, but if you are

using a range of fre&uencies through a single antenna, it is

important to ensure that all of the $' signals will be in

the effective range of that antenna. "he primary difference

of Qagi and :og periodic antennas normally handle a

relatively narrow range of $' fre&uencies, while of -eriodic

antennas can achieve much larger effective fre&uency ranges.

3n the surface it would appear the wide fre&uency range of

the :og -eriodic antenna would ma#e it the obvious choice,

especially when one consider that :og -eriodics are

generally also much smaller than Qagis. "his however, is not

always the case. Consider the application where there are

strong off fre&uency interference sources (virtually all

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high $' level applications . In these situations, the off

fre&uency re=ection of a Qagi antenna can greatly improve

system performance and decrease harmful interference. In

general, it is a good idea to choose an antenna that is =ust

wide enough to handle the desired operating fre&uencies.

3ne more note on directional antennas. 8ecause 'CC

rules concerning transmit power ( ffective $adiated -ower or

$- ta#e into account the antenna gain of the transmit

antenna, high gain transmit antennas may not be used on

transmitters in most wireless communications applications.

"he good news is that high gain antennas on the receive side

of an $' system are also very effective for increasing

system range and are commonly used.

"here is one more important antenna concept to be

considered. 7s stated earlier, antenna polari ation is

critical to proper system operation. "ransmit and receive

antennas of the same system must be oriented in the same

direction to have proper transfer of the carrier. In theory,

if a transmit antenna is oriented vertically, thus producing

a vertically polari ed carrier, and the corresponding

receive antenna is oriented hori ontally, the receive

antenna will not be able to see the vertically polari ed

wave at all. In practice, there will always be some

polari ation shift in the path and the receiver will see a

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very small signal if it is close enough to the transmitter,

but system range will be greatly reduced. "o avoid this

problem, antennas in a given $' system should always have

similar orientation.

6nless an antenna is attached directly to the receiver

or transmitter in an $' system, coa)ial cable is the usual

means used to span the gap. "he importance of choosing the

right coa)ial cable cannot be over*stressed. /hen choosing

cable to use in your $' system three main factors must be

considered0 (1 "he cable must be properly impedance matched

(correct characteristic impedance . +ost wireless systems

today are 5 ohm impedance system. "hat means the final

amplifier and filters in the transmitter, the front end of

the receiver and both transmit and receive antennas, air

designed to wor# using 5 ohms as the nominal impedance. It

is e)tremely important to choose coa)ial cable that is also

5 ohms. Coa)ial cable that is used in video applications is

normally 5ohms, not 5 ohms. (4 Consider the loss per foot

of coa)ial cable at your systemEs operating fre&uency. In


50/92

"his is a good rule of thumb that will #eep out of trouble

most of the time. (> Consider how the system is used. Is

this a fi)ed installation, or a mobile oneR If the system is

being moved fre&uently you want to use coa)ial cable that

has a stranded center conductor. Fust li#e other types of

wire, coa)ial cable with a stranded center conductor will

tolerate being fle)ed repeatedly without degradation in

performance ( http0 www.tele).com intercoms , 3ct. 45, 4 5

"elated Studies

$adioCom @ingle Channel @ystems ( @eries 6


51/92

applicationP however, these stand*alone systems can be

applied to churches, schools, auditoriums, or industrial

applications ( http0 www.tele).com intercom features.htm ,

3ct. 45, 4 5 .

8i#e Intercom

"his is wireless intercom prototype that has the

ability to support clear communication at highway speeds.

"he circuit is set up to use a noise*canceling microphone

design. "hat is two electret microphone capsules (can be as

small as match*head si e are glued into the helmet in

carefully chosen positions. "he general idea is that one of

these microphones can pic# up only the ambient noise in the

helmet, the other pic#s up both the e)act same noise and the

wearerEs speech. "he two resulting signals are

electronically subtracted resulting in a clean voice signal.

In practice, things are not so easy. "he bandwidth re&uired

for clear speech is from about > < to > < . 7t > #m (theoretical

"he . 1 factor compensates for the losses of a telescopic

antenna which is intended to be used as an antenna for the

transmitter design. "he computed distance varies depending

on the sensitivity of the receiver to be designed.

The "eceiver Design

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Basic Processes in a "eceiver

7ll radio receivers perform these three basic

functions0

Selection J It allows the receiver to select one

fre&uency while re=ecting other fre&uencies. "his is

done by tuning the receiver to the fre&uency of the

desired carrier. "his function is performed through

combinations of inductances and capacitances where the

oscillating fre&uency can be determined by0

'o L (1 4 s&rt(:C

'o L 8/ ) T

/here0 : L inductance value

C L capacitance value

8/ L bandwidth of operation

T L Tuality 'actor

Detection J "he purpose of a detector or discriminator in

the receiver is to remove the desired information from

the carrier and convert it into a form that will actuate

the output device such as the spea#er for the case of

this study that deals with the transmission and reception

of audio signals.

mplification J the incoming signal may be wea#. 7s such,

amplification must ta#e place between the input of the

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receiver and its output. "his is usually called the gain

of the receiver.

Block Diagram of the "eceiver Design

"he researcher selected an '+ @uperheterodyne receiver.

"his is chosen in a way to get rid of encountering problems

regarding selectivity.

ntenna

"# mp

(i5er I# mplifier

Discriminator #and

Po1er mp

%ocalOscillator

ToSpeaker

"he "# amplifier stage increases the gain of the

received signal. Coupled with the $' amplifier is the

circuit for tuning. @electing the intended fre&uency to be

received is set in the tuning stage. 6sually, the capacitor

in the tuned circuit is made variable by rotating a #nob,

the capacitance is varied, thereby changing the resonant

fre&uency.

"he mi5er and the oscillator stages perform the actual

heterodyning (mi)ing function. "he oscillator stage is a

generator of an unmodulated $' signal necessary to convert

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the received signal to an intermediate fre&uency. "he

standard I' for '+ receiver is 1 . +< . "he mi)er is where

both $' and oscillator signals are heterodyned to produce a

new set of signals. "he output of the mi)er consists of two

original signals and the two new signals J the sum and

difference of the $' and oscillator signals. 7s the

receiver is tuned throughout the band, one of these

fre&uencies remains constant. It is the difference of the

two signals which is always 1 . +< and contains the same

audio modulation of the original $' signal at the antenna.

"his will be fed at the input of the I' amplifier stage.

"he I# 9Intermediate #re4uency: amplifier is fi)ed*

tuned to accept and amplify only the 1 . +< difference

signal. "he high gain provided by this stage remains

constant over the entire broadcast fre&uencies.

"he detector which can be called a discriminator

removes the intelligence from the carrier. "hus, the

remaining information will be the original transmitted audo

information. "his output will be amplified by the audio

amplifier!

In the case of the design, the researcher selected

headphones as the load in order not to have much

amplification. 7lso that in using headphones, ease of

communication can be attained.

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Selection of T ,60,BP Integrated Circuit

In an attempt to come up with the circuits for the

aforementioned bloc#s of the '+ receiver, it has been found

out that all active components in ma#ing an '+ receiver is

already pac#aged in a single integrated circuit (IC chip.

It comes from many variations depending on the manufacturer.

"he "741>48- IC has been chosen because it is

commercially available. "his consists of the basic stages

in the receiving of '+ signals such as the $' amplifier,

+i)er, I' amplifier, 7utomatic gain control stage, and the

detection stage.

7ctually, IC of this #ind is for 7+ '+ reception. 8ut

the concentration of the design is for '+ reception only.

/ith this IC, the tas# now of the researcher is to select

for few e)ternal components to tune to the fre&uency of

operation of the transmitter, which is at 9?.1 +< .

In addition, this receiver part of the intercom unit

has been patterned from the e)isting design elicited from

7le)an lectronics.

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Block Diagram of T ,60,BP IC

Selection of T ;048- involves

only the reception of '+ signals up to the demodulation

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process, there is a need to include the last stage which has

a function for audio amplification. "o complete the stages

of the receiver design, the researcher selected an audio

amplifier that is pac#aged also in an IC chip, specifically

"7 >DB-. "his is appropriate since it has only low power

output which is &uite good to drive the headphone.

Block Diagram of T 048-

'+ receiver. 3nly three e)ternal capacitors are needed.

"he capacitor in the supply acts as supply decoupling, the

capacitor in series with the load is

regarded as bypass capacitor for ripple filter and a

capacitor to prevent oscillation for the power amplifier.

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1. -rovide a chec#list for all the electronic

components with their corresponding values and the number of

pieces re&uired. "his is for both "ransmitter and $eceiver

circuits of the Intercom 6nit.

"eceiver Parts %istSemiconductors CapacitorsIC1 "741>48- C1 . ?Y'IC4 "7 >DB- C4 . 5Y'"esistors (7ll are N*watt,carbon film type

C> 1B['C? ?['C5 4 ['

$1 5 #U CD 4n'

$1,$4 1 #U C ,C1B .? Y' 1$> 51#U CB 1 Y' 1$? >> U C9 . 1Y'$5 1 U C1 44['$D 15U C11,C14 >.5['$ 4#U C1> 44Y' 1#ilter Capacitors C1? . 4Y'C'1 @' 1 . +@4 C15 . ? Y'C'4 @'6?55Q C1 >>Y' 1C'> C 71 . +C1 C1B,C19 44' 1Coils C4 . 44Y'

:1 '+ $' (?.5)> M C41 .1Y':> '+ 3sc. (>.B)> M C44 >>[':? '+ $' (?)> M C1 7+ '+ - C 4 :,CD 1n' $> ?. #UC? 14[' $? >> #UC5 4. [' $5 ? #UC 44p' 1 $D B4 U

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oltage @upply0 9* 8attery 7ntenna0 "elescopic 7ntenna

4. -repare and purchase the necessary materials and

instruments needed in the fabrication.

>. Chec# the materials and instruments prepared to

determine whether the materials and instruments wor#

properly and in good condition and &uality.

?. !et and study the given schematic diagram in order

to understand the flow of its circuits and the proper place

of its components to avoid confusion.

5. "race it in a -rinted Circuit 8oard (-C8 the design

circuit made.

D. $emove the unnecessary part or circuit of the -C8,

the copper portion of the -C8 by a -heric Chloride in order

to come up with the design layout. -hoto etching is

recommended.

. +a#e the necessary holes for the proper placement of

its components and smoothly grind it with a sand paper in

order to have good circuit conductivity.

B. @older the components with the copper lead for the

components to get in place their re&uired locations in the

-C8. Fust ma#e it sure that no other shorted connections

occur other than what is set in the -C8.

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PCB %ayout for Transmitter

PCB %ayout for "eceiver

9. -lan the overall layout of the pro=ect in the chosen

enclosure. -repare the needed hoo# up wire as designed in

the component layout guide.

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Transmitter Components

"eceiver Components

1 . +ount into the desired enclosure the pro=ect. "a#e

into consideration the location of each parts of the design

in the enclosure to find no problems regarding

connections of one part to another.

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Testing

8efore switching the pro=ect 3A, chec# first all the

connections made, comparing them directly with the schematic

diagram. In particular, chec# the power supply. If all

connections are connected yet the circuit does not wor#,

then testing of the functionality of suspected electronic

components is re&uired most especially ICEs, transistors and

capacitors.

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#i5ing and Testing Components

"he two units must receive signals (audio coming from

each other when operated. "hen one unit is relocated to

compute for the ma)imum distance where the units can have a

clear reception of signal coming from each other. uring

the communication between the two units, an observation is

done regarding its interference to e)isting ad=acent active

stations.

Complete &nits Set7up

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Chapter I?

Presentation of Data

"his chapter presents the results of the wor# of the

researcher regarding the fabrication of Intercom 6nits. "his

includes the deviceE specifications, its performance, and

its comparison in terms of cost to similar e)isting device.

Device Specifications

@pecifications :evel 6nit1. @upply oltage 9 (dual4. -ower 3utput 45 (appro)imate m/>. 're&uency of 3peration 9?.1 +

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