fm multichannel

8/17/2019 FM Multichannel

1/27

Multichannel Terrestrial FM Transmitters

ACKNOWLEDGMENT

I would like to thanks God for all his helps in my life and my staff member who are working on Terrestr

transmitter of ORTO for their cooperation when I was visiting the site.

And I would like to thanks my adviser Dr. ultan !eisso.

"


2/27


ABSTRACT:

In #thiopia as all we know$ there different !% &roadcasters that broadcasts in different

region of #thiopia. !or #'ample in Oromia region #thiopian &roadcasting (orporation !%

!ana &roadcasting corporation !%$ )ami !%$ heger !%$ Oromia !% Radio etc all

broadcast in almost all coverage of Oromia region. All these broadcasters *se their own

Terrestrial Transmitter Tower at different places depending on the coverage area they need

in Oromia almost the side by side. If there are ten broadcasters broadcasting in Oromia$ al

ten should put their Transmitters Tower side by side to cover area coverage they need. &y

this work I want propose one %ultichannel !% Transmitters that all broadcasters can

broadcast with it instead of all putting the same purpose Transmitter.

Table of ContentsA(+,O-#DG%#,T................................................................................................................................

/


3/27


A&TRA(T0..................................................................................................................................................

(1A2T#R O,#...........................................................................................................................................

I,TROD*(TIO,.........................................................................................................................................

&ackground................................................................................................................................................

Ob4ective of the 2ro4ect.............................................................................................................................

(O2#.......................................................................................................................................................

R#2ORT O6#R6I#-..............................................................................................................................

(1A2T#R T-O...........................................................................................................................................

!% T1#OR8................................................................................................................................................

Introduction................................................................................................................................................

(1A2T#R 9..................................................................................................................................................

!% TRA,%ITT#R....................................................................................................................................

&uilding &locks.........................................................................................................................................

!% #:(IT#R.....................................................................................................................................

R! 2O-#R A%2I!I#R...................................................................................................................

I,T#R%#DIAT# 2O-#R A%2I!I#R..........................................................................................

2ower upplies......................................................................................................................................

(1A2T#R


4/27


References......................................................................................................................................................

CHAPTER ONE

INTRODUCTION

Background

#dwin 1. Armstrong$ known as one of the founding fathers of radio technology$ invented the radio receiv

in "="; and fre?uency modulation @!% in "=99 B"C. These two concepts$ along with his regenerativecircuit techni?ue developed in "="/$ formed the basis of radio fre?uency electronics as we know it today

In the *nited tates$ !% radio stations broadcast between radio fre?uencies of ;; %1 to ">; %1 with

channel bandwidth of />> k1.


5/27


-ith a bandwidth of />>+h for one station$ up to ">> stations can be fitted between ;; E ">;%h. !rom

;;%h to ="./%h are for nonFcommercial stations @educational which could be a good area to transmit

in$ but in recent years the band from ;;%1 to ">9%h has been filled by a lot of commercial channels.

Ob!c"#$! o% "&! Pro!c"

The primary purpose of the pro4ect is to propose one %ultichannel !% Transmitters that all broadcaster

in #thiopia so that they can broadcast using this instead of all putting the same purpose their own

Transmitters side by side. This work will save the countrys economy wasting on building Transmitter

towers for all !% broadcasters.

SCOPE

This work covers the design of %ultichannel !% transmitters for ?uality transmission and e'plains some

optional %ultichannel !% Transmitters$ highlighting their coverage prospects. It also covers the

advantages these technologies offer over traditional radio Transmitter and brings to light various

distinguishing features possessed by these technologies.

REPORT O'ER'IEW

(hapter one provides an overview of the work by giving description of the topic and Introduction. (hapt

Two describes basic !% Theory and in chapter three each block diagram of the !% Transmitter is

described and in the final chapter the proposed multichannel Transmitters and conclusions are described.

3


6/27


CHAPTER TWO

(M THEOR)

In"roduc"#on *+,

6oice or information that is going to be transferred is termed as information signal. If the distance

between communication parties is too large$ direct voice communication is impossible. The method

message sender is needed. The message sender could be a dove$ servant or an arrow. The function of

message sender is 4ust to carry the information to the desired destination.

Thus the message sender can be said to be a carrier. The carrier merely sends the information and need

not to be intelligent. The information signal is sometimes called the intelligence signal.

In telecommunications$ the mechanism of putting the information signal into a carrier for it to be

transmitted farther is called modulation. ince the characteristic of the carrier signal is being altered by

the information signal$ the carrier is also a modulated signal. Therefore$ the information signal$ intelligen

signal and modulating signal representing the same thing.

!or the carrier to carry information$ at least one of the carrier signalHs characteristics @amplitude$ phase o

fre?uency must be modified. !re?uency %odulation @!% is a method of modifying fre?uency of carr

signal in order that the receiver can obtain the desired transmitted information.

5


7/27


Modu-a"#on Ind!./ D!$#a"#on Ra"#o and B!00!- (unc"#on

!re?uency modulation is a form of analog angle modulation in which the baseband information carrying

signal$ typically called the message or information signal m@t$ varies the fre?uency of a carrier wave.

Audio signals transmitted by !% radio communications are the most common.The simplest approach to generating !% signals is to apply the message signal directly to a voltageF

controlled oscillator @6(O as shown in !igure below.

!igure /.".!% Generation with a 6(O

A voltage message signal$ m@t$ is applied to the control voltage of the 6(O$ and the output signal$ X !%@t

is a constant amplitude sinusoidal carrier wave whose fre?uency is ideally a linear function of its control

voltage. -hen there is no message or the message signal is ero$ the carrier wave is at its center fre?uenc

f c. -hen a message signal e'ists$ the instantaneous fre?uency of the output signal varies above and below

the center fre?uency and is e'pressed by

where K 6(O is the voltageFtoFfre?uency gain of the 6(O e'pressed in units of 16$ and the ?uantity$

K 6(OJm@t$ is the instantaneous fre?uency deviation. The instantaneous phase of the output signal is e?ua

to /K multiplied by the integral of the instantaneous fre?uency as shown below

7


8/27


-here the initial condition of the phase is assumed to be ero for simplicity. 1ence$ the !% output signa

X !%@t$ is given by the following e?uation

A few observations can be made from the !% output signal. !irst$ the amplitude of an !% signal is

constant regardless of the message signal$ giving it a constant envelope property with an output power

e?ual to Ac// into a " L resistor. econd$ the fre?uencyFmodulated output$ X !%@t$ has a nonlinear

dependence to the message signal$ m@t$ making it difficult to analye the properties of an !% signal. To

estimate the bandwidth of an !% signal$ a single tone message signal is used as shown below

where Am is the amplitude of the message signal and f m is the fre?uency of the message signal.

ubstituting this message signal into the above formulas$ we find

The ?uantity M f N K VCO Am represents the peak fre?uency deviation of the !% signal from the center

fre?uency and is directly proportional to the amplitude of the message signal and the gain of the 6(O. T

?uantity$ M f $ is called the ma'imum instantaneous fre?uency deviation. The ratio of the fre?uency

deviation$ M f, to the message signal fre?uency$ f m$ is called the modulation inde'$ .

;


9/27


!or a single tone message signal$ the number of significant sidebands in the output spectrum is a function

of the modulation inde'. This can be seen by first writing the !% output signal in terms of n th order &es

functions of the first kind B/$ 9C.

&y taking the !ourier transform$ we see a discrete !% output spectrum with magnitude coefficients as a

function of as shown in the e?uation below.

The number of sidebands of an !% signal and its associated magnitude coefficient can be found with the

help of &essel function tables such as the one shown in Table below.

Table ".&essel !unctions of the !irst +ind Rounded to Two Decimal 2laces

A key point of modulation inde'$ $ is that it determines the bandwidth of the signal by determining the

number of effective sidebands of an !% signal. !or instance$ if 1>./3$ only one sideband is neededP wh

if 13$ eight sidebands are re?uired. Another important point about the modulation inde'0 it can change a

lot even for a fi'ed fre?uency deviation because the message signal fre?uency can vary. In general$ as th

=


10/27


modulation inde' increases$ the number of sidebands increases and the bandwidth goes up. 1owever$ the

increase in modulation inde' due to decreasing message fre?uency @recall N M f /f m may not necessarily

increase the !% bandwidth. The bandwidth is e?ual to the number of discrete spectral tones multiplied b

the fre?uency spacing set by the message signal fre?uency f m.!or more complicated message signals$ the bandwidth of an !% signal can also be appro'imated with

(arsons rule$ FM BW Q /@ " f m B/C. The empirical relation states that the number of significant spect

tones in an !% spectrum is Q /@ " $ not including the carrier.

CHAPTER 2

(M TRANSMITTER

Bu#-d#ng B-ock0 *3,

The purpose of the !% transmitter is to convert one or more audio fre?uency @composite baseband inpu

signals into a fre?uency modulated$ radio fre?uency signal at the desired power output level to feed into t

radiating antenna system. In itHs simplest form$ it can be considered to be an !% modulator and an R!

power amplifier packaged into one unit.

Actually the !% transmitter consists of a series of individual subsystems each having a specific function

". The !% e'citer converts the audio baseband into fre?uency modulated R! and determines the ke

?ualities of the signal.

/. The intermediate power amplifier @I2A is re?uired in some transmitters to boost the R! power le

up to a level sufficient to drive the final stage.

9. The final power amplifier further increases the signal level to the final value re?uired to drive the

antenna system.


11/27


!igure 9.".implified block diagram of an !% broadcast transmitter.

!% #:(IT#R

The heart of an !% broadcast transmitter is its e'citer. The function of the e'citer is to generate and

modulate the carrier wave with one or more inputs in accordance with the !(( standards. The !%

modulated carrier is then amplified by a wideband amplifier to the level re?uired by the transmitterHs

following stage.

ince the e'citer is the origin of the transmitterHs signal$ it determines most of the signalHs technical

characteristicsP including signal FtoF noise Fratio$ distortion$ amplitude response$ phase response$ and

fre?uency stability. -aveform linearity$ amplitude bandwidth$ and phase linearity must be maintained

within acceptable limits throughout the baseband chain from the stereo and subcarrier generators to the !

e'citerHs modulated oscillator. !rom here$ the !% carrier is usually amplified in a series of class S(S non

linear power amplifiers$ where any amplitude variation is removed. The amplitude and phase responses o

all the R! networks which follow the e'citer must also be controlled to minimie degradation of the

baseband.

""

2!Intermediate

power Amplifier

!inal 2ower

Am lifier

!% #'citer


12/27


(M Modu-a"or L#n!ar#"4

,on Flinearities in the !% oscillator can$ by altering the waveform of the baseband signal$ create distorti

in the demodulated output at the receiver. A secondary effect of this distortion may include stereo crossta

into the (A.The composite baseband signal is translated to a fre?uency modulated carrier fre?uency by the modulate

oscillator. !re?uency modulation is produced by applying the composite baseband signal to a voltage

tunable R! oscillator. The modulated oscillator usually operates at the carrier fre?uency and is voltage

tuned by varactor diodes$ operating in a parallel ( circuit. To have perfect modulation linearity$ the R!

output fre?uency @!e must change in direct proportion to the composite modulating voltage @6m applie

to the varactor diodes @(. This re?uirement implies that the capacitance of the varactor diodes must chan

as nearly the s?uare of the modulating voltage as shown in following relationships0 @!e is proportional t

@6m @Desired linear voltage to fre?uency translation

-here0 !c N Instantaneous carrier fre?uency

N Inductance of resonant circuit (t N Total capacitance across @( fi'ed ( varactors

(v N (apacitance of varactor tuning diodes

+ N 6aractor constant

6m N &aseband modulating voltage

R! 2O-#R A%2I!I#R

The remainder of the !% transmitter consists of a chain of power amplifiers$ each having from 5 to /> d&

of power gain. Ideally$ the transmitter should have as wide a bandwidth as practical with a minimum of

tuned stages. &roadband solid state amplifiers are preferred to eliminate tuned networks in the R! path.

1igher powered transmitters in the multi Fkilowatt range may use multiple tube stages each with fairly lo

gain such as in the grounded grid configuration or a single grid driven 2A stage with high gain and

"/


13/27


efficiency. Design improvements in tube type power amplifiers have concentrated on improving bandwid

reliability$ and cost effectiveness.

TRANSMITTER POWER OUTPUT RE5UIREMENTS

The !(( regulates the power of !% broadcast stations in terms of effective radiated power @#R2. The

authoried #R2 applies only to the horiontally polaried component of radiation.

The transmitter power re?uirement can be reduced by increasing the gain of the antenna. There is$ of

course$ an economic tradeoff between the cost of a higher gain antenna versus the cost of a larger

transmitter and the added primary power costs. !% transmitters are designed to operate over a range of

power outputs so that with a few basic sies any re?uired power output can be furnished. 2opular

ma'imum ratings range from /3> watts to 5> kilowatts.

R( POWER AMPLI(IER PER(ORMANCE RE5UIREMENTS

The basic function of the power amplifier is to amplify the power of the e'citer output to the authoried

transmitter power output level. %ost of the overall transmitter performance characteristics are determine

by the e'citer but a few are established or affected by the power amplifier characteristics0

". The output at harmonics of the carrier fre?uency is almost completely a function of the

attenuation provided by the output tank circuit and output low Fpass notch filters. The limit

decibels is B d& for 3 +- and higher./. The ma4or source of A% noise usually originates in the last power amplifier stage. The !((

limit is 3> d& below ">> percent e?uivalent A% modulation.

9. The R! power output control system which must keep the output within 3 percent and F">

percent of authoried output is usually achieved in the final power amplifier.


14/27


modulation performance. Available bandwidth determines the amplitude response and group delay

response.

There is a tradeFoff involved between the bandwidth$ gain and efficiency in the design of a power amplif

The bandwidth of an amplifier is determined by the load resistance across the tuned circuit and the outpuor input capacitance of the amplifier. !or a single solid tuned circuit$ the bandwidth is proportional to the

ratio of capacitive reactance to resistance0

where0 BW N&andwidth between halfFpower points @ BW 9d&

K N 2roportionality constant

R L N oad resistance @appearing across tuned circuit

C N Total capacitance of tuned circuit @includes stray capacitances plus output or

input capacitances of the tube

X C N (apacitive reactance of C

f c N (arrier fre?uency

The load resistance is directly related to the R! voltage swing on the tube element. !or the same power

and efficiency$ the bandwidth can be increased if the capacitance is reduced.

I,T#R%#DIAT# 2O-#R A%2I!I#R

The intermediate power amplifier @I2A is located between the e'citer and the final amplifier in higher

power transmitters that re?uire more than about 9> watts of drive to the final amplifier.

The I2A may consist of one or more tubes or solid state amplifier modules.

The separate I2A output circuit and the final amplifier input circuit are often coupled together by a coa'ia

transmission line. The interconnecting transmission line between the coupling circuits should be properly

matched to avoid a high voltage standing wave ratio @6-R.

The 6-R is established by the match at the load end of the transmission line. The transmission line

matching problem is eliminated in some transmitters by integrating an I2A stage utiliing a tube@s into th

grid circuit of the final amplifier stage by having the plate of the I2A and the grid of the final tube share

a common tuned circuit.

"


15/27


So-#d 9S"a"! IPA S40"!70

A solid Fstate I2A almost always consists of a system of individual amplifier modules that are combined

provide the desired power output. The advantages of using several lower power modules instead of a sin

high power amplifier are0". Redundancy is provided by isolating the input and output of each module$ permitting

uninterrupted operation at reduced power if one or more of the modules fails.

/. The ability to repair or replace failed modules without having to go SoffF the Fair S.

9. %ore effective cooling of each power device 4unction by splitting the concentration o

heat to be dissipated into several areas instead of one small area.


16/27


-here0 , N efficiency in percent

V N loaded SVS of cavity

Vu N unloaded SVS of cavity

The loaded SVS depends on the plate load impedance and output circuit capacitance. *nloaded SVS

depends on the cavity volume and the R! resistivity of the conductors due to skin effects.

A high unloaded SVS is desirable$ as is a low loaded SV S$ for best efficiency. As the loaded SVS goes up$

the bandwidth decreases.

!or a given tube output capacitance and power level$ loaded SVS decreases with decreasing plate voltage

with increasing plate current. The increase in bandwidth at reduced plate voltage occurs because the load

resistance is directly related to the R! voltage swing on the tube element. !or the same power and

efficiency$ the bandwidth can also be increased if the output capacitance is reduced. 2ower tube selection

and minimiation of stray capacitance are areas of prime concern when designing for ma'imum bandwid

The methods used to improve the bandwidth of 2A output circuits include minimiing added tuning

capacitance. The ideal case would be to resonate the plate capacitance alone with a SperfectS inductor$ bu

practical ?uarter wave cavities re?uire either the addition of a variable capacitor or a variable inductor

using sliding contacts for tuning. The inherent mechanical and electrical compromises are the re?uiremen

for a

plate dc blocking capacitor and the presence of ma'imum R! current at the grounded end of the line whe

the conductor may be nonFhomogeneous.

2O-#R *22I#

2ower supplies provide the appropriate dc or ac voltages to the various subsystems with the transmitter. I

a typical !% transmitter$ the voltages and currents can range from less than 3 6 at a few milliamperes to

over ">$>>> 6 at several amperes. afety must therefore be a prime consideration when working around

potentially lethal power supplies. 2ower supplies must be designed with ade?uate bleeder resistors and

interlocks to discharge high voltages before an operator can come in contact with these circuits. The degr

"5


17/27


to which the ac components are filtered out of the dc outputs of the power supplies will$ in large part$

determine the WasynchronousX @without !% modulation A% noise of the !% transmitter.

!% transmitters usually contain multiple power supplies for each of the functional blocks within the

system. These power supplies fall into two general categories0 ingleFphase supplies @single input winding on the transformer

2olyphase supplies @three or more input windings on the transformer.

CHAPTER

MULTICHANNEL (M

INTRODUCTION

2rototype One

1aving several !% stations share a single broad band antenna system is becoming more and more popul

A special device called a filterple'er @also known as an R! multiple'er is used to connect several

transmitters on different fre?uencies together onto one antenna system.

The filterple'er provides isolation between the various transmitters while efficiently combining their pow

into a single transmission line. This is usually accomplished by a system of bandFpass filters$ bandFre4ect

filters and hybrid combiners. The isolation is re?uired to prevent power from one transmitter from enterin

another transmitter with resulting spurious emissions as well as to keep the rest of the system running in

event of the failure of one or more transmitters.

"7


18/27


An important consideration in the design of a filterple'ing system is the effect on the phase response

@group delay characteristic in the pass band of each of the signals passing through the system due to

individual bandwidth limitations on each of the inputs.

. Antenna

.

.

!igure


19/27


R! Intermodulation &etween !% &roadcast Transmitters

Interference to other stations within the !% broadcast band$ as well as to other services outside the

broadcast band$ can be caused by R! intermodulation between two or more !% broadcast transmitters.

Transmitter manufacturers have begun to characterie the susceptibility of their e?uipment to R!intermodulation so this information is becoming available to the designers of filterple'ing e?uipment.

The degree of intermodulation interference generated within a given system can be accurately predicted

before the system is built if the actual mi'ing loss of the transmitters is available when the system is

designed. Accurate data on “Mixing Loss” or “Turn-Around-Loss” not only speeds the design of

filterple'ing e?uipment$ but also results in higher performance and more costFeffective designs because t

e'act degree of isolation re?uired is known before the system is designed.

!ilterple'er characteristics$ as well as antenna isolation re?uirements$ can be tailored to the specific

re?uirements of the transmitters being used. The end user is assured in advance of construction that the

system will perform to specification without fear of overdesign or under design of the components within

the system.

%itigation techni?ues

A number of techni?ues have been developed to reduce intermodulation in transmitter power amplifiers$

and some of these are briefly described. The intermodulation factors used to make these calculations are

affected by many parameters and they also depend on electrical resonances in the components which ma

up the mast and antenna arrays.

The overall loss$ AC $ between a transmitter providing the unwanted emissions giving rise to the

intermodulation product is given by the sum of0

A A AC C

= +

where AC is the coupling loss defined as the ratio of the power emitted from one transmitter to the power

level of that emission at the output of another transmitter which may produce the unwanted

intermodulation product. The intermodulation conversion loss$ A $ is the ratio of power levels of the

interfering signal from an e'ternal source and the intermodulation product$ both measured at the output othe transmitter. B3C

*sing this definition$ mitigation of intermodulation products means increasing the overall loss AC . It is

obvious that a reduction of the nonFlinearity$ particularly of the oddFnumbered orders$ will improve the

overall performance and increase the value of intermodulation conversion loss

"=


20/27

Input (narrow-band)

Input (wideband)

Output to antenna

Block diagram of a directional multiplexer


E.a78-!0 o% #n"!r7odu-a"#on 8roduc"0 g!n!ra"!d on a rad#o 0#"! 6#"& (M

!%0 !%"0 f " N ;; %1$ !%/0 f / N =/.7 %1 and !%90 f 9 N =3.7 %1

The three !% transmitters are combined via directional multiple'ers @%u' made of 9 d& couplers and

filters @see !igure below0

!igure


21/27

f 1

f 2

2 f 1 – f 3

f 3

Mu1 Mu2

IM3 at2 f 1 – f 3! "#$3 M%&

FI'* +,

f 2

2 f 1 – f 2

f 3

2 f 3 – f 2

f 1 Mu1 Mu2

FI'* +

IM3 at2 f 1 – f 2! "3$3 M%&and at2 f 3 – f 2! ."$/ M%&


/"


22/27

f 2

2 f 1– (2 f 3– f 2)

f 3

2 f 3 – f 2

f 1 Mu1 Mu2

FI'* +0

IM at2 f 1– 2 f 3 f 2! //$3 M%&

owerampli4er

F output

FI'*

Vector modulator transmitter architecture

5ectormodulator

aseband

inputs

I

6

.#7


!igure


23/27


B; (#-"!r#ng

!iltering @generally bandpass filtering of the transmitter output can be used in con4unction with the other

techni?ues discussed in this Report to reduce the residual spurious output levels. The choice of the type o

filter to be used is$ as usual$ a compromise between a number of interacting$ usually conflicting$re?uirements such as outFofFband re4ection$ bandpass attenuation$ time domain response$ sie$ weight$ co

etc.

C; L#n!ar#


24/27


The Another proposed system has been designed$ with main !re?uency %odulation @!% channel at its

ma'imum power and another !% channel with lower power$ simultaneously. The system has been

designed E developed to utilie the capacity of an e'isting transmitter to its ma'imum e'tent$ without

affecting the e'isting main carrier R! power. The capacity of the e'isting single channel !% transmitterscan be increased by using wide band capability and by applying two or multi tone e'citation. This way tw

or more carriers can be produced at the R! output$ for broadcasting more than one audio channel@s.

2rototype of this design has been successfully tested for one additional channel in normal broadcasting

environment without any performance degradation to any of the channels.

The additional channel@s are meant for small coverage area than the main channel as the additional

channel@s will have to be broadcasted at lower power level. The techni?ue can be appropriately named a

FM Multicasting ; B


25/27


". (overt the e'isting wide band filter at the O2 of intermediate stage @"> - and 9> - to a narrow

band. &2! by tuning the variable capacitors.

/. #'tra sharpFcut off filter may be introduced at the output of intermediate stage and at final outpu

stage. 1owever the filter at final out will be at the cost of reduction in radiated power of the order

>.3 d& or more.

!igure ".9 %1 is shown in !igure below

/3

To Antenna


26/27


!igure


27/27


RE(ERENCES

B"C #. 1. Armstrong -eb ite$ http0users.erols.comoldradio

B/C . 1aykin$ (ommunication ystems$ 9rd #dition$ -iley$ "==<

B9C R. #. )iemer$ -. 1. Tranter$ 2rinciples of (ommunications$ ystems$ %odulation$ and

,oise$ !ourth #dition$ -iley$ "==3

B/" @2ROD*(TIO, A,D %ITIGATIO, O! I,T#R%OD*ATIO,

2ROD*(T I, T1# TRA,%ITT#R

B5C !% &roadcast Transmitters @by Geoffrey ,. %endenhall$ 2.#. 6ice 2resident of #ngineering The

#ngineering taff of &roadcast #lectronics Inc. Vuincy$ Illinois

fm multichannel

Documents