RF Transmission Systems Handbook

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RF_Transmission_Systems_Handbook/RF Transmission Systems Handbook/0973_pdf_C01.pdf 2002 by CRC Press LLC

1.1 Introducti

Radio frequency (RF) powaround the world. The widenergy and information bRF energy have many simbroadcasting represent thapplications exist, includin

Induction heating a

Radio communicat

Amateur radio

Radar (ground, air,

Satellite communic

Atomic science rese

Medical research, d

Figure 1.1 illustrates the e

Modulation System

The primary purpose of manother. The message signfrequency to provide for efto a few megahertz for vi

Jerry C. Whitaker

Editor1Applications of RF

Technology

1.1 IntroductionModulation Systems Spread-Spectrum Systems RF Power Amplifiers Frequency Sources Operating Class Operating Efficiency Broadband Amplifier Design Amplifier Compensation Stagger Tuning Matching Circuits Power Combining Output Devices

1.2 Broadcast Applications of RF TechnologyAM Radio Broadcasting Shortwave Broadcasting FM Radio Broadcasting Television Broadcasting

1.3 Nonbroadcast ApplicationsSatellite Transmission Radar Electronic Navigation Microwave Radio Induction Heating

on

er amplifiers are used in countless applications at tens of thousands of facilitiese variety of applications, however, stem from a few basic concepts of conveying

y means of a radio frequency signal. Furthermore, the devices used to produceilarities, regardless of the final application. Although radio and television

e most obvious use of high-power RF generators, numerous other commong:

nd process control systems

ions (two-way mobile radio base stations and cellular base stations)

and shipboard)

ations

arch

iagnosis, and treatment

lectromagnetic spectrum and major applications.

s

ost communications systems is to transfer information from one location toals used in communication and control systems usually must be limited in

ficiency transfer. This frequency may range from a few hertz for control systemsdeo signals. To facilitate efficient and controlled distribution of these signals,

2002 by CRC Press LLC

an

encoder

is generally req

modulate

the signal, produ

the characteristics of a wav

waveform. Frequency tranwhere the modulated wavThere are a number of rea

Frequency translatio

quency translation its assigned frequen

Signal processing

. It to another.

Antenna efficiency

. with the signal wavand beamwidth to permits antenna str

Bandwidth modifica

increased or decreaefficient use of the spermits increased i

Signal multiplexing

several different sigvehicle for such

mu

at the transmission

accomplished using

Modulation of a sig

the addition of noissystem designer.

FIGURE 1.1

The electroma

10 E2210 E2110 E2010 E1910 E1810 E1710 E1610 E1510 E1410 E1310 E1210 E1110 E10

(1 GHz) 10 E910 E810 E7

(1 MHz) 10 E610 E510 E4

(1 kHz) 10 E310 E210 E1

0uired between the source and the transmission channel. The encoder acts tocing at its output the modulated waveform. Modulation is a process wherebye (the carrier) are varied in accordance with a message signal, the modulating

slation is usually a by-product of this process. Modulation can be continuous,e is always present, or pulsed, where no signal is present between pulses.sons for producing modulated waves, including:

n. The modulation process provides a vehicle to perform the necessary fre-required for distribution of information. An input signal can be translated tocy band for transmission or radiation.

is often easier to amplify or process a signal in one frequency range as opposed

Generally speaking, for an antenna to be efficient, it must be large comparedelength. Frequency translation provided by modulation allows antenna gainbecome part of the system design considerations. Use of higher frequenciesuctures of reasonable size and cost.

tion. The modulation process permits the bandwidth of the input signal to besed as required by the application. Bandwidth reduction can permit morepectrum, at the cost of signal fidelity. Increased bandwidth, on the other hand,mmunity to transmission channel disturbances.

. In a given transmission system, it may be necessary or desirable to combinenals into one baseband waveform for distribution. Modulation provides theltiplexing. Various modulation schemes allow separate signals to be combined end, and separated (demultiplexed) at the receiving end. Multiplexing can be frequency-domain multiplexing (FDM) or time-domain multiplexing (TDM).

nal does not come without undesirable characteristics. Bandwidth restriction ore or other disturbances are the two primary problems faced by the transmission

gnetic spectrum.

Cosmic rays

Gamma rays

X rays

Ultraviolet light

Infrared light

Radar

Television and FM radioShortwave radioAM radio

Sonic

Subsonic

550 nm600 nm650 nm700 nm750 nm800 nm

500 nm450 nm400 nm Ultraviolet

Violet

BlueGreenYellowOrange

Red

Infrared

Visible light

Radio frequencies

Wavelength = Speed of light Frequency


Spread-Spectrum Sy

The specialized requireme

systems. As the name implinformation-bearing signa

Low interference to

Ability to reject hig

Immunity to jamm

Provides for secure

Operates over mult

Spread-spectrum systemsystems discussed previoupossible reception and dsystems, on the other haunauthorized persons. Thcommunications include:

Frequency hopping,

the carrier frequen

hopping

, where the

the hopping rate is of a single piece ofmultiple-access caponly a fraction of t

Time hopping,

whewithin a series of fr

Message corruption,

Chirp spread spectr

the transmitted speto communications

In a spread-spectrum systhas a small average powershare a given frequency ba

RF Power Amplifie

The process of generatingAdvancements in devices aefficiency and maximum othe RF design engineer, the

Frequency Sources

Every RF amplifier requireQuartz acts as a stable hi

frequencies ranging from The operating character

crystal. The behavior of thof the cut. To provide formore selected modes, are stems

nts of the military led to the development of spread-spectrum communicationsies, such systems require a frequency range substantially greater than the basicl. Spread-spectrum systems have some or all of the following properties:

other communications systems

h levels of external interference

ing by hostile forces

communications paths

iple RF paths

s operate with an entirely different set of requirements than the transmissionsly. Conventional modulation methods are designed to provide for the easiestemodulation of the transmitted intelligence. The goals of spread-spectrumnd, are secure and reliable communications that cannot be intercepted bye most common modulation and encoding techniques in spread-spectrum

where a random or pseudorandom number (PN) sequence is used to changecy of the transmitter. This approach has two basic variations: slow frequency hopping rate is smaller than the data rate; and fast frequency hopping, wherelarger than the data rate. In a fast frequency hopping system, the transmission data occupies more than one frequency. Frequency hopping systems permitability to a given band of frequencies because each transmitted signal occupieshe total transmitted bandwidth.

re a PN sequence is used to switch the position of a message-carrying pulseames.

where a PN sequence is added to the message before modulation.

um, where linear frequency modulation of the main carrier is used to spreadctrum. This technique is commonly used in radar and has also been applied systems.

em, the signal power is divided over a large bandwidth. The signal, therefore, in any single narrowband slot. This means that a spread-spectrum system cannd with one or more narrowband systems.

rs

high-power RF signals has been refined over the years to an exact science.nd circuit design continue to be made each year, pushing ahead the barriers ofperating frequency. Although different applications place unique demands on

fundamental concepts of RF amplification are applicable to virtually any system.

s a stable frequency reference. At the heart of most systems is a quartz crystal.gh Q mechanical resonator. Crystal resonators are available for operation at1 kHz to 300 MHz and beyond.istics of a crystal are determined by the cut of the device from a bulk mothere device strongly depends on the size and shape of the crystal and the angle operation at a wide range of frequencies, different cuts, vibrating in one orused.


Crystals are temperaturchanges in temperature is with time. Such

aging

is c

Mass transfer to or

Stress relief within

Crystal aging is most prorelieved, the aging process

The stability of a quartcommon methods are use

Oven-controlled cry

controlled box. Beccrystal remains on

temperature

of the

Temperature-compe

ature changes of this typically used to desired on-frequen

Operating Class

Power amplifier (PA) stagsystem. As the power levemore important. Increasedof the system. The operatthe maximum possible effi

All electron amplifying

divisions apply to RF gene

Class A:

a mode whteristic. This modebasic operating efficdistortion, making

FIGURE 1.2

The effects of e sensitive, as shown in Fig. 1.2. The extent to which a device is affected bydetermined by its cut and packaging. Crystals also exhibit changes in frequencyaused by one or both of the following:

from the resonator surface

the device itself

nounced when the device is new. As stress within the internal structure is slows.z crystal is inadequate for most commercial and industrial applications. Twod to provide the required long-term frequency stability:

stal oscillator: a technique in which the crystal is installed in a temperature-ause the temperature is constant in the box, controlled by a thermostat, the-frequency. The temperature of the enclosure is usually set to the turnovercrystal. (The turnover point is illustrated in Fig. 1.2.)

nsated crystal oscillator (TCXO): a technique where the frequency-vs.-temper-e crystal are compensated by varying a load capacitor. A thermistor networkgenerate a correction voltage that feeds a varactor to re-tune the crystal to thecy value.

e operating efficiency is a key element in the design and application of an RFl of an RF generator increases, the overall efficiency of the system becomes efficiency translates into lower operating costs and usually improved reliabilitying mode of the final stage, or stages, is the primary determining element inciency of the system.

devices are classified by their individual class of operation. Four primary classrators:

erein the power amplifying device is operated over its linear transfer charac- provides the lowest waveform distortion, but also the lowest efficiency. Theiency of a class A stage is 50%. Class A amplifiers exhibit low intermodulation

temperature on two types of AT-cut crystals.

1000 10 20 30 40 50 60 70 80 90Temperature (C)

-20

-15

-10

-5

0

5

10

15

20

D f/f

(ppm

)

Curve 1

Curve 2T ithem well suited for linear RF amplifier applications.


Class B:

a mode whcharacteristic. This tion. Class AB is a device operating in

Class C:

a mode whtransfer characterisrealized with class CC is used extensive

Class D:

a mode thaeither

on

or

off

. Ththe greatest wavefo

The angle of current flow conduction angle for classthan 180. Subscripts can device. The subscript 1 current flow. Figure 1.3 cha

The class of operation ibe grid- or cathode-drivenconfigured for grounded eclass of operation.

Operating Efficienc

The design goal of all RF efficiency. DC input poweto heat. This heat represenof heat is a problem comm

Natural convection

Radiation

Forced convection

Liquid

Conduction

Evaporation

FIGURE 1.3

Plate efficienc

050

100

75

Effic

ienc

y (%

)erein the power amplifying device is operated just outside its linear transfermode provides improved efficiency at the expense of some waveform distor-variation on class B operation. The transfer characteristic for an amplifying this mode is, predictably, between class A and class B.

erein the power amplifying device is operated significantly outside its lineartic, resulting is a pulsed output waveform. High efficiency (up to 90%) can be

operation; however, significant distortion of the waveform will occur. Classly as an efficient RF power generator.

t essentially results in a switched device state. The power amplifying device isis is the most efficient mode of operation. It is also the mode that producesrm distortion.

determines the class of operation for a power amplifying device. Typically, the A is 360; class AB is between 180 and 360; class B is 180; and class C is lessalso be used to denote grid current flow in the case of a power vacuum tubemeans that no grid current flows in the stage; the subscript 2 denotes gridrts operating efficiency as a function of the conduction angle of an RF amplifier.

s not directly related to the type of amplifying circuit. Vacuum tube stages may without regard to the operating class. Similarly, solid-state amplifiers may bemitter, grounded base, or grounded collector operation without regard to the

y

amplifiers is to convert input power into an RF signal at the greatest possibler that is not converted to a useful output signal is, for the most part, convertedts wasted energy, which must be removed from the amplifying device. Removal

on to all high-power RF amplifiers. Cooling methods include:

y as a function of conduction angle for an amplifier with a tuned load.

360180 27090Total conduction angle of tube (deg)

Class A

Class AB

Class BClass C


The type of cooling mepower level involved. For vacuum tubes; conduction

Broadband Amplifie

RF design engineers face transmitted, while preserveters, while not mutually

An ideal RF amplifier wpower, phase, distortion, atype of active device usedbandwidth of 20% or gre

MHz, broadband amplifieMost development in n

MOSFET (metal oxide sem

amplification

, where multibased designs offer benefitcan often be improved beinto reduced lead and com

Amplifier Compens

A variety of methods can Two of the most commontwo basic techniques can requirements, such as pha

Stagger Tuning

Several stages with narrowthrough the use of

stagger

it has been used for yearsfrequencies (and, thereforeis flat and broad.

For example, the first operating frequency of thestage is adjusted below ceefficiency penalty for thisstagger tuning required to

Matching Circuits

The individual stages of anpower level of one stage pThere is a requirement, thaccomplished with several

Quarter-wave trans

wave long, with a cthod chosen is dictated in large part by the type of active device used and theexample, liquid cooling is used almost exclusively for high-power (100 kW) is used most often for low-power (20 W) transistors.

r Design

a continuing challenge to provide adequate bandwidth for the signals to being as much efficiency as possible from the overall system. These two param-exclusive, often involve trade-offs for both designers and operators.ill operate over a wide band of frequencies with minimum variations in outputnd efficiency. The bandwidth of the amplifier depends to a great extent on the, the frequency range required, and the operating power. As a general rule,

ater at frequencies above 100 MHz can be considered broadband. Below 100rs typically have a bandwidth of one octave or more.ew broadband designs focuses on semiconductor technology. Transistor andiconductor field effect transistor) devices have ushered in the era of distributed

ple devices are used to achieve the required RF output power. Semiconductor-s beyond active device redundancy. Bandwidth at frequencies above 100 MHzcause of the smaller physical size of semiconductor devices, which translatesponent inductance and capacitance.

ation

be used to extend the operating bandwidth of a transistorized amplifier stage. methods are series- and shunt-compensation circuits, shown in Fig. 1.4. Thesebe combined, as shown. Other circuit configurations can be used for specificse compensation.

band response (relative to the desired system bandwidth) can be cascaded and,tuning, made broadband. While there is an efficiency penalty for this approach, in all types of equipment. The concept is simple: offset the center operating, peak amplitude response) of the cascaded amplifiers so the resulting passband

stage in a three-stage amplifier is adjusted for peak response at the center system. The second stage is adjusted above the center frequency, and the thirdnter. The resulting composite response curve yields a broadband trace. The scheme varies, depending on the power level of each stage, the amount of achieve the desired bandwidth, and the number of individual stages.

RF generator must be coupled together. Rarely do the output impedance andrecisely match the input impedance and signal-handing level of the next stage.erefore, for broadband matching circuits. Matching at RF frequencies can be different techniques, including:

former: a matching technique using simply a length of transmission line 1/4-haracteristic impedance of:(1.1)Zline Zin Zout=


where

Z

in

and

Z

out

to achieve more favratios for each indi

Balun transformer:

include the interwtransmission line. resonances. Balun Ferrite toroids can

Other types of lum

Short sections of tr

Power Combining

The two most common m

paralleling

of components

and semiconductor designoperating parameters. Twcurrent (supply the same be difficult because of the

The preferred approach

The coupler provides a conto a

reject port

for dissipat

FIGURE 1.4

High-frequenc(After Fink, D. and Christian

are the terminating impedances. Quarter-wave transformers can be cascadedorable matching characteristics. Cascaded transformers permit small matchingvidual section.

a transmission-line transformer in which the turns are physically arranged toinding capacitance as a component of the characteristic impedance of theThis technique permits wide bandwidths to be achieved without unwantedtransformers are usually made of twisted wire pairs or twisted coaxial lines.be used as the core material.

ped reactances.

ansmission line.

ethods of extending the operating power of semiconductor devices are direct and hybrid splitting/combining. Direct paralleling has been used for both tubes; however, application of this simple approach is limited by variations in deviceo identical devices in parallel do not necessarily draw the same amount ofamount of power to the load). Paralleling at UHF frequencies and above can restrictions of operating wavelength. involves the use of identical stages driven in parallel from a hybrid coupler.

y compensation techniques: (a) shunt, (b) series, (c) combination of shunt and series.sen, D., Eds., Electronics Engineers Handbook, 3rd ed., McGraw-Hill, New York, 1989.)

Rg

C

CcL

RLCsTransistor

(a)

RgCc

L

RiCsTransistor

(b)

Rg

CCg

Li

RLCsTransistor

(c)

C

Cg Rg

L2stant-source impedance and directs any reflected energy from the driven stagesion. A hybrid coupler offers a voltage standing wave ratio or VSWR-canceling


effect that improves systemdevices in a system.

Output Devices

Significant changes have oVacuum tubes were the mnents became available attubes can now be met withsilicon field effect transistand television broadcast trcountless other applicatio

Most solid-state designThey are designed to maxdistributed amplification

The principal drawbackcircuit complexity that greduced significantly, and in almost all semiconductsystem. Increased parts trincreased vulnerability to

Efficiency comparisons contrasts expected. Whilesuperior to class B or C osplitting and combining n

It is evident, then, thatBoth technologies will remmoving to higher power l

1.2 Broadcast A

Broadcasting has been arosystem that permitted voicinant throughout the 1920during the 1940s. Televisithe signal and frequency min the mid-1940s. More reand elsewhere using the csystem, but with digital m

AM Radio Broadcas

AM radio stations operatestations have been establiscountries to allocate the abasic classes are

clear

,

regio

class A, B, and C, respectias little as 250 W for a loc

High-Level AM Modul

High-level anode modulathis system, the modulatin performance. Hybrids also provide a high degree of isolation between active

ccurred within the past 10 years or so with regard to power amplifying devices.ainstay of RF transmission equipment until advanced semiconductor compo- competitive prices. Many high-power applications that demanded vacuum solid-state devices arranged in a distributed amplification system. Metal oxideor (MOSFET) and bipolar components have been used successfully in radioansmitters, shortwave transmitters, sonar transmitters, induction heaters, and

ns.s used today are not simply silicon versions of classic vacuum tube circuits.imize efficiency through class D switching and maximize reliability through

and redundancy. to a solid-state system over a vacuum tube design of comparable power is theoes with most semiconductor-based hardware. Preventive maintenance is in theory repair is simpler as well in a solid-state system. The parts countor-based hardware, however, is significantly greater than in a comparable tubeanslate (usually) into a higher initial purchase price for the equipment anddevice failure of some sort.between vacuum tube and solid-state systems do not always yield the dramatic most semiconductor amplifiers incorporate switching technology that is farperation (not to mention class A), power losses are experienced in the signaletworks necessary to make distributed amplification work. vacuum tubes and semiconductors each have their benefits and drawbacks.

ain viable for many years to come. Vacuum tubes will not go away, but areevels and higher operating frequencies.

pplications of RF Technology

und for a long time. Amplitude modulation (AM) was the first modulatione communications to take place. This simple modulation system was predom-s and 1930s. Frequency modulation (FM) came into regular broadcast service

on broadcasting, which uses amplitude modulation for the visual portion ofodulation for the aural portion of the signal, became available to the public

cently, digital television (DTV) service has been launched in the United Statesonventional television frequency bands and 6-MHz bandwidth of the analogodulation.

ting

on 10-kHz channels spaced evenly from 540 to 1600 kHz. Various classes ofhed by the Federal Communications Commission (FCC) and agencies in othervailable spectrum to given regions and communities. In the United States, thenal, and local. Current practice uses the CCIR (international) designations as

vely. Operating power levels range from 50 kW for a clear channel station toal station.

ationtion is the oldest and simplest way of generating a high power AM signal. Ing signal is amplified and combined with the dc supply source to the anode of


the final RF amplifier stageusually consists of a pairmodulator is shown in Fig

The RF signal is normaor more solid-state or vacgrid of the final class C amsolid state) and used to dprovide the necessary molating power is equal to 50

The modulation transfofinal RF amplifier. The mosignal voltage from the mothe necessity of having dcwould result in magnetic loreactor has been eliminate

The RF amplifier normTypical stage efficiency is the operating frequency ancaused by class C operatio

This type of system wasThe primary drawback is lgreater than 50% efficiencproblem. As energy costs inwere developed. Increased

Pulse-Width Modulati

Pulse-width modulation (popular systems developedscheme. The PDM system

FIGURE 1.5

Simplified dia

RF inputsignal

Bias

Modulatioinput signa. The RF amplifier is normally operated class C. The final stage of the modulator of tubes operating class B in a pushpull configuration. A basic high-level. 1.5.

lly generated in a low-level transistorized oscillator. It is then amplified by oneuum tube stages to provide final RF drive at the appropriate frequency to theplifier. The audio input is applied to an intermediate power amplifier (usuallyrive two class B (or class AB) pushpull output devices. The final amplifiersdulating power to drive the final RF stage. For 100% modulation, this modu-% of the actual carrier power.rmer shown in Fig. 1.5 does not usually carry the dc supply current for thedulation reactor and capacitor shown provide a means to combine the audio

dulator with the dc supply to the final RF amplifier. This arrangement eliminates current flow through the secondary of the modulation transformer, whichsses and saturation effects. In some newer transmitter designs, the modulation

d from the system, thanks to improvements in transformer technology.ally operates class C with grid current drawn during positive peaks of the cycle.75 to 83%. An RF tank following the amplifier resonates the output signal atd, with the assistance of a low-pass filter, eliminates harmonics of the amplifiern.

popular in AM broadcasting for many years, primarily because of its simplicity.ow overall system efficiency. The class B modulator tubes cannot operate withy. Still, with inexpensive electricity, this was not considered to be a significantcreased, however, more efficient methods of generating high-power AM signals efficiency normally came at the expense of added technical complexity.

on

PWM), also known as pulse-duration modulation (PDM), is one of the most for modern vacuum tube AM transmitters. Figure 1.6 shows the basic PDM

gram of a high-level, amplitude-modulated amplifier.

nl

Vcc

Modulated RFoutput works by utilizing a square-wave switching system, illustrated in Fig. 1.7.


The PDM process beginoscillator and used to drisquare wave is then integra summing circuit. The re

FIGURE 1.6

The pulse-dur

FIGURE 1.7

The principles

RF drive

PDM signal

Si

Sq

Tr

Inp+0-

Su

Ws with a signal generator (see Fig. 1.8). A 75-kHz sine wave is produced by anve a square-wave generator, resulting in a simple 75-kHz square wave. Theated, resulting in a triangular waveform that is mixed with the input audio insulting signal is a triangular waveform that rides on the incoming audio. This

ation modulation (PDM) method of pulse-width modulation.

waveforms of the PDM system.

PDMfilter

Damping diode

DCsupply

Tuning andmatching circuits

ne wave

uare wave

iangle wave

ut audio waveform

m of audio + triangle wave

Threshold

idth-modulated pulses


composite signal is then awhenever the value of thethe width of the pulse is prThe pulse output is appliefilter eliminates whatever

The PDM scheme is, inat a 75-kHz rate. The widtsignal is applied to a

switc

or

off

in accordance with thtube is turned on and theminimum value, the mod

This PDM signal becomswitched on, the final ampor modulator tube goes oto operate in a highly efficused to set the carrier (no

A high degree of third-switching-mode operationamplifier that normally opits driver is usually a fract

The damping diode shoovervoltages during the swa period when the final amPDM filters could cause a lto the power supply by thby arcing through the mo

The PWM system maktransmitter. The result is won this amplifier and mobroadcast and shortwave s

Digital Modulation

Current transmitter designnology. High-power MOSgeneration of energy-effici

Most solid-state AM sydriver boards are combine

FIGURE 1.8

Block diagram

75 kHzoscillatorpplied to a threshold amplifier, which functions as a switch that is turned on input signal exceeds a certain limit. The result is a string of pulses in whichoportional to the period of time the triangular waveform exceeds the threshold.d to an amplifier to obtain the necessary power to drive subsequent stages. Atransients may exist after the switching process is complete. effect, a digital modulation system with the audio information being sampledh of the pulses contains all the audio information. The pulse-width-modulatedh or modulator tube. The tube is simply turned on, to a fully saturated state,e instantaneous value of the pulse. When the pulse goes positive, the modulator voltage across the tube drops to a minimum. When the pulse returns to itsulator tube turns off.

es the power supply to the final RF amplifier tube. When the modulator islifier will experience current flow and RF will be generated. When the switch

ff, the final amplifier current will cease. This system causes the final amplifierient class D switching mode. A dc offset voltage to the summing amplifier is modulation) level of the transmitter.harmonic energy will exist at the output of the final amplifier because of the. This energy is eliminated by a third-harmonic trap. The result is a stableerates in excess of 90% efficiency. The power consumed by the modulator andion of a full class B amplifier stage.wn in the previous figure is included to prevent potentially damaging transientitching process. When the switching tube turns off the supply current duringplifier is conducting, the high current through the inductors contained in the

arge transient voltage to be generated. The energy in the PDM filter is returnede damping diode. If no alternative route is established, the energy will returndulator tube itself.es it possible to completely eliminate audio frequency transformers in theide frequency response and low distortion. It should be noted that variations

dulation scheme have been used by other manufacturers for both standardervice.

work for AM broadcsting has focused almost exclusively on solid-state tech-FET devices and digital modulation techniques have made possible a new

ent systems, with audio performance that easily surpasses vacuum tube designs.stems operate in a highly efficient class D switching mode. Multiple MOSFETd through one of several methods to achieve the required carrier power.

of a PDM waveform generator.

Square-wavegenerator

Integrator Summingcircuit

Threshold amplifier

Pulse amplifierPDM output

Audioinput


Shortwave Broadca

The technologies used inment-sponsored shortwavallied with those used in wave stations usually opepowers than AM stations.

International broadcasranging from 5.95 to 26.1 Mintended for reception by tcountries. Table 1.1 showsthe Federal Communicatiinternational broadcast United States. The minimAssignments are made forat specific frequencies.

Very high-power shortwovercome jamming effortsare not uncommon. RF ci

Most shortwave transmoperating frequencies. A vaset to the desired operatinup at each frequency is peare stored in memory. Auin less than 30 seconds in

Power Amplifier Types

Shortwave technology hasductor devices. High-powmitters operating at 500 kWuse vacuum tubes as the pshortwave applications is translates into higher ope

Older, traditional tube-systems:

Doherty amplifier

Chireix outphasing

Dome modulated a

Terman-Woodyard

FM Radio Broadcas

FM radio stations operateStates, channels below 92established three classificacations for stations west o

power

(ERP) to 3 kW or lpower output (TPO) and and allowing for line loss.

A transmitting antennaangles, rather than allowi

sting

commercial and govern-e broadcasting are closely

AM radio. However, short-rate at significantly higher

t stations use frequenciesHz. The transmissions are

he general public in foreign the frequencies assigned byons Commission (FCC) forshortwave service in the

um output power is 50 kW. specific hours of operation

ave transmitters have been installed to serve large geographical areas and to by foreign governments. Systems rated for power outputs of 500 kW and morercuits designed specifically for high power operation are utilized.itters have the unique requirement for automatic tuning to one of several presetriety of schemes exist to accomplish this task, including multiple exciters (eachg frequency) and motor-controlled variable inductors and capacitors. Tune-rformed by the transmitter manufacturer. The settings of all tuning controls

tomatic retuning of a high-power shortwave transmitter can be accomplished most cases.

advanced significantly within the last 5 years, thanks to improved semicon-er MOSFETs and other components have made solid-state shortwave trans-

and more practical. The majority of shortwave systems now in use, however,ower-generating element. The efficiency of a power amplifier/modulator forof critical importance. Because of the power levels involved, low efficiency

rating costs.type shortwave transmitters typically utilize one of the following modulation

modulated amplifier

mplifier

modulated amplifier

ting

on 200-kHz channels spaced evenly from 88.1 to 107.9 MHz. In the United.1 MHz are reserved for noncommercial, educational stations. The FCC hastions for FM stations operating east of the Mississippi River and four classifi-f the Mississippi. Power levels range from a high of 100 kW effective radiatedess for lower classifications. The ERP of a station is a function of transmitterantenna gain. ERP is determined by multiplying these two quantities together

TABLE 1.1 Operating Frequency Bands for Shortwave Broadcasting

BandFrequency

(kHz)Meter Band

(m)

A 5,9506,200 49B 9,5009,775 32C 11,70011,975 25D 15,10015,450 19E 17,70017,900 16F 21,45021,750 14G 25,60026,100 11 is said to have gain if, by design, it concentrates useful energy at low radiationng a substantial amount of energy to be radiated above the horizon (and be


lost in space). FM and TVradiating elements vertica

At first examination, it mtransmitter power output make the most obvious santennas include:

Effects of high-gain

Limitations on anteand windloading

Cost of the antenna

Stereo broadcasting is usehas contributed in large pais transmitted as a standa

maximum of about 17 kHat the receiver. The left anthat frequency-modulates circuits in the FM receivechannels. Figure 1.9 illustr

Modulation Circuits

Early FM transmitters us

modulator was then multimonaural FM transmissioriers on the FM broadcast

That is, the frequency moequal to the desired transmapplied to the FM modula

Various techniques havuses a variable-capacity dito the diode, which causemodulating signal. Variatimagnitude of the frequencof frequency shift is equal

The direct-FM modulatFM waveform. A block diatypes (stereo left and righpreemphasized before beoscillator is not normally as possible to the carrier ftained by an automatic fre

crystal oscillator or freque

FIGURE 1.9

Composite ba transmitting antennas are designed to provide gain by stacking individuallly.

ight seem reasonable and economical to achieve licensed ERP using the lowestpossible and highest antenna gain. Other factors, however, come into play thatolution not always the best solution. Factors that limit the use of high-gain

designs on coverage area and signal penetration

nna size because of tower restrictions, such as available vertical space, weight,

d almost universally in FM radio today. Introduced in the mid-1960s, stereort to the success of FM radio. The left and right sum (monophonic) informationrd frequency-modulated signal. Filters restrict this main channel signal to az. A pilot signal is transmitted at low amplitude at 19 kHz to enable decodingd right difference signal is transmitted as an amplitude-modulated subcarrierthe main FM carrier. The center frequency of the subcarrier is 38 kHz. Decoderr matrix the sum and difference signals to reproduce the left and right audioates the baseband signal of a stereo FM station.

ed reactance modulators that operated at low frequency. The output of theplied to reach the desired output frequency. This approach was acceptable forn but not for modern stereo systems or other applications that utilize subcar- signal. Modern FM systems all utilize what is referred to as direct modulation.dulation occurs in a modulated oscillator that operates on a center frequency

itter output frequency. In stereo broadcast systems, a composite FM signal istor.

e been developed to generate the direct-FM signal. One of the most popularode as the reactive element in the oscillator. The modulating signal is applieds the capacitance of the device to vary as a function of the magnitude of theons in the capacitance cause the frequency of the oscillator to vary. Again, they shift is proportional to the amplitude of the modulating signal, and the rate to the frequency of the modulating signal.or is one element of an FM transmitter exciter, which generates the compositegram of a complete FM exciter is shown in Fig. 1.10. Audio inputs of varioust signals, plus subcarrier programming, if used) are buffered, filtered, and

ing summed to feed the modulated oscillator. It should be noted that thecoupled directly to a crystal, but a free-running oscillator adjusted as closelyrequency of the transmitter. The final operating frequency is carefully main-quency control system employing a phase-locked loop (PLL) tied to a referencency synthesizer.seband stereo FM signal.


A solid-state class C amFM signal to 20 to 30 W. to several hundred watts foFM transmitters utilize sofor operating powers of 5systems, each stage in an the frequency-modulated power system.

Auxiliary Services

Modern FM broadcast stamore subsidiary channels

Authorization

(SCA) servicontrol signals, and otherservices do not provide thethey perform a public serv

SCA systems provide efis 67 kHz, although higherare permitted by the FCCcertain conditions. The suessentially within the 200-

FM Power Amplifiers

Most high-power FM tranis the most common. Thedifferent transmitters but tis to simulate a resonant tain the cavity to the transmthe elements of the tank

A typical 1/4-wave cavisection (tube) of the plateways. In at least one designmultiple layers of insulatinblocking capacitor. The caThe dc plate voltage is apchimney and inner tube coplate that is insulated from

FIGURE 1.10

Block diagraplifier follows the modulated oscillator and raises the operating power of theOne or more subsequent amplifiers in the transmitter raise the signal powerr application to the final power amplifier stage. Nearly all current high-power

lid-state amplifiers up to the final RF stage, which is generally a vacuum tube kW and above. All stages operate in the class C mode. In contrast to AM

FM power amplifier can operate class C because no information is lost fromsignal due to amplitude changes. As mentioned previously, FM is a constant-

tions are capable of not only broadcasting stereo programming, but one or as well. These signals, referred to by the FCC as Subsidiary Communicationsces, are used for the transmission of stock market data, background music, information not normally part of the stations main programming. These same range of coverage or audio fidelity as the main stereo program; however,ice and can represent a valuable source of income for the broadcaster.

ficient use of the available spectrum. The most common subcarrier frequency subcarrier frequencies may be utilized. Stations that operate subcarrier systems to exceed (by a small amount) the maximum 75-kHz deviation limit underbcarriers utilize low modulation levels, and the energy produced is maintainedkHz bandwidth limitation of FM channel radiation.

smitters manufactured today employ cavity designs. The 1/4-wavelength cavity design is simple and straightforward. A number of variations can be found inhe underlying theory of operation is the same. The goal of any cavity amplifiernk circuit at the operating frequency and provide a means to couple the energyission line. Because of the operating frequencies involved (88 to 108 MHz),

take on unfamiliar forms.ty is shown in Fig. 1.11. The plate of the tube connects directly to the inner-blocking capacitor. The blocking capacitor can be formed in one of several, it is made by wrapping the outside surface of the inner tube conductor withg film. The exhaust chimney/inner conductor forms the other element of the

vity walls form the outer conductor of the 1/4-wave transmission line circuit.plied to the PA (power amplifier) tube by a cable routed inside the exhaustnductor. In this design, the screen-contact fingerstock ring mounts on a metal

m of an FM exciter. the grounded-cavity deck by a blocking capacitor. This hardware makes up


the screen-blocker assembcavity deck through an in

Some transmitters thatwhich the screen contact cathode then operates at screen voltage for the tube

Coarse tuning of the cacavity shorting deck) is fathe assembly for the partiplate-tuning control built is fastened to the inner cofastened to the cavity boxcontrol. This capacity shuand resonant frequency of

Television Broadcas

Television transmitters in

Low-band VHF: ch

High-band VHF: ch

UHF: channels 14 have been assignedon these frequencie

Because of the wide varietsection focuses primarily frequencies used by TV brtype of service. The maxiVHF, it is 316 kW; and fo

The second major factobroadcast industry as

heig

the geography in the vicin

FIGURE 1.11

Physical layoly. The dc screen voltage feeds to the fingerstock ring from underneath thesulated feedthrough. employ the 1/4-wave cavity design use a grounded-screen configuration infingerstock ring is connected directly to the grounded cavity deck. The PA

below ground potential (i.e., at a negative voltage), establishing the required.vity is accomplished by adjusting the cavity length. The top of the cavity (thestened by screws or clamps and can be raised or lowered to set the length ofcular operating frequency. Fine-tuning is accomplished by a variable-capacityinto the cavity. In the example, one plate of this capacitor, the stationary plate,nductor just above the plate-blocking capacitor. The movable tuning plate is, the outer conductor, and is mechanically linked to the front-panel tuning

nts the inner conductor to the outer conductor and varies the electrical length the cavity.

ting

the United States operate in three frequency bands:

annels 2 through 6 (5472 MHz and 7688 MHz)

annels 7 through 13 (174216 MHz)

through 69 (470806 MHz). UHF channels 70 through 83 (806890 MHz) to land mobile radio services. Certain TV translators may continue to operates on a secondary basis.

y of operating parameters for television stations outside the United States, thison TV transmission as it relates to the United States (Table 1.2 shows theoadcasting). Maximum power output limits are specified by the FCC for eachmum effective radiated power for low-band VHF is 100 kW; for high-bandr UHF, it is 5 MW.r that affects the coverage area of a TV station is antenna height, known in theht above average terrain (HAAT). HAAT takes into consideration the effects of

ut of a common type of 1/4-wave PA cavity for FM broadcast service.ity of the transmitting tower. The maximum HAAT permitted by the FCC for


a low- or high-band VHFwest of the Mississippi. Um) anywhere in the Unite

The ratio of visual outpthe aural is typically operof the reception charactersumers receiver to recoveroutput is intended to be subeyond. It is of no use for

In addition to the full-phave been established by t

Translators:

low-powTranslators are desicommunity becausW power output (E

Low-power televisio

particular communand UHF stations abe assigned by the power station is aff

TABLE 1.2

Channel Desi

Channel Designation

Frequen(MH

2 543 604 665 766 827 1748 1809 186

10 19211 19812 20413 21014 47015 47616 48217 48818 49419 50020 50621 51222 51823 52424 53025 53626 54227 54828 55429 560 station is 1000 ft (305 m) east of the Mississippi River, and 2000 ft (610 m)HF stations are permitted to operate with a maximum HAAT of 2000 ft (610d States (including Alaska and Hawaii).ut power to aural power can vary from one installation to another; however,

ated at between 10 and 20% of the visual power. This difference is the resultistics of the two signals. Much greater signal strength is required at the con- the visual portion of the transmission than the aural portion. The aural powerfficient for good reception at the fringe of the stations coverage area, but not

a consumer to be able to receive a TV stations audio signal, but not the video.ower stations discussed previously, two classifications of low-power TV stationshe FCC to meet certain community needs. They are:

er systems that rebroadcast the signal of another station on a different channel.gned to provide fill-in coverage for a station that cannot reach a particulare of the local terrain. Translators operating in the VHF band are limited to 100RP), and UHF translators are limited to 1 kW.

n (LPTV): a service established by the FCC to meet the special needs ofities. LPTV stations operating on VHF frequencies are limited to 100 W ERPre limited to 1 kW. LPTV stations originate their own programming and canFCC to any channel, as long as full protection against interference to a full-

gnations for VHF and UHF Television Stations in the U.S.

cy Band z)

Channel Designation

Frequency Band (MHz)

Channel Designation

Frequency Band (MHz)

60 30 566572 57 72873466 31 572578 58 73474072 32 578584 59 74074682 33 584590 60 74675288 34 590596 61 752758180 35 596602 62 758764186 36 602608 63 764770192 37 608614 64 770776198 38 614620 65 776782204 39 620626 66 782788210 40 626632 67 788794216 41 632638 68 794800476 42 638644 69 800806482 43 644650 70 806812488 44 650656 71 812818494 45 656662 72 818824500 46 662668 73 824830506 47 668674 74 830836512 48 674680 75 836842518 49 680686 76 842848524 50 686692 77 848854530 51 692698 78 854860536 52 698704 79 860866542 53 704710 80 866872548 54 710716 81 872878554 55 716722 82 878884560 56 722728 83 884890566orded.


Television Transmissi

Analog television signals t

All systems use two

All contain lumina

All use amplitude m

Modulation polaritduring the sync int

The sound is transcarrier, using frequ

All systems use a ve

All systems derive aponents.

There the similarities stop in use.

NTSC (National TAmerica, most of Svarious countries oof the NTSC signal

PAL (Phase Alternaby the British ComPAL systems.

SECAM (SEquentiinfluenced by Franand other areas infl

The three standards are in

Channel assignmencountries have VHUHF do not necess

Channel bandwidth6-MHz, 7-MHz, an

Vision bands are diSECAM has 6-MH

The line structure osecond. PAL and Sthe scanning freque

The color subcarrieMHz, while SECAMvalues are derived the luminance sign

The color encoding

The offset between is 5.5 or 6 MHz, de

One form of SECAMoutput of transmitt

Channels transmittand SECAM. Differon Standards

ransmitted throughout the world have the following similarities.

fields interlaced to create a complete frame.

nce, chrominance, syncronization, and sound components.

odulation to put picture information onto the visual carrier.

y, in most cases, is negative (greatest power output from the transmitter occurserval; least power output occurs during peak white).

mitted on an aural carrier that is offset on a higher frequency than the visualency modulation in most cases.

stigial lower sideband approach.

luminance and two-color difference signals from red, green, and blue com-

and the differences begin. There are three primary color transmission standards

elevision Systems Committee): used in the United States, Canada, Centralouth America, and Japan. In addition, NTSC has been accepted for use in

r possessions heavily influenced by the United States. The major components are shown in Fig. 1.12.

tion each Line): used in England, most countries and possessions influencedmonwealth, many western European countries, and China. Variation exists in

al Color with [Avec] Memory): used in France, countries and possessionsce, the U.S.S.R. (generally the Soviet Bloc nations, including East Germany),uenced by Russia.

compatible for the following reasons.

ts are made in different frequency spectra in many parts of the world. SomeF only; some have UHF only; others have both. Assignments with VHF andarily coincide between countries.

s are different. NTSC uses a 6-MHz channel width. Versions of PAL exist withd 8-MHz bandwidths. SECAM channels are 8-MHz wide.

fferent. NTSC uses 4.2 MHz. PAL uses 4.2 MHz, 5 MHz, and 5.5 MHz, whilez video bandwidth.

f the signals varies. NTSC uses 525 lines per frame, 30 frames (60 fields) perECAM use 625 lines per frame, 25 frames (50 fields) per second. As a result,ncies also vary.

r signals are incompatible. NTSC uses 3.579545 MHz, PAL uses 4.43361875 utilizes two subcarriers, 4.40625 MHz and 4.250 MHz. The color subcarrier

from the horizontal frequencies in order to interleave color information intoal without causing undue interference.

system of all three standards differ.

visual and aural carriers varies. In NTSC, is it 4.5 MHz; in PAL, the separationpending on the PAL type; and SECAM uses 6.5-MHz separation.

uses positive polarity visual modulation (peak white produces greatest powerer) with amplitude modulation for sound.ed on UHF frequencies may differ from those on VHF in some forms of PALences include channel bandwidth and video bandwidth.


It is possible to convertof the conversion process disassembled in the input Complex computer algorinew lines required (for conNon-moving objects in thobjectionable artifacts as t

Transmitter Design Co

An analog television transm

the video input, amplitudeand (2) the

aural

section, amplifies the signal to feeda single radiating antennato the design and constructo basic system design. Tra

Output power

Final stage design

Modulation system

Output Power

When the power output oOutput power refers to

FIGURE 1.12

The major conext line. V = time from the from one television standard to another electronically. The most difficult partresults from the differing number of scan lines. In general, the signal must besection of the standards converter, and then placed in a large dynamic memory.thms compare information on pairs of lines to determine how to create theversion to PAL or SECAM) or how to remove lines (for conversion to NTSC).

e picture present no great difficulties, but motion in the picture can producehe result of the sampling system.

nsiderations

itter is divided into two basic subsystems: (1) the visual section, which accepts-modulates an RF carrier, and amplifies the signal to feed the antenna system;

which accepts the audio input, frequency-modulates a separate RF carrier, and the antenna system. The visual and aural signals are usually combined to feed. Different transmitter manufacturers have different philosophies with regardtion of a transmitter. Some generalizations can, however, be made with respectnsmitters can be divided into categories based on the following criteria:

f a TV transmitter is discussed, the visual section is the primary consideration.the peak power of the visual stage of the transmitter (peak of sync). The

mponents of the NTSC television signal. H = time from start of line to the start of thestart of one field to the start of the next field.


FCC-licensed ERP is equagain of the antenna.

A low-band VHF statiotransmitter and antenna cdo the trick, as would a 10a high-band VHF stationantenna with a gain of 8, tassume reasonable feedlinpower output of the trans

UHF stations that wantvery high power (and verkW and an antenna with case, the antenna could ppossible but not advisablHigh-gain antennas have anear the transmitter site. WUHF stations therefore op

Final Stage Design

The amount of output poPower levels usually dictatwater, or vapor cooling mhigh-voltage control and ssignals (rather than separa

Tetrodes are generally u(below 5 kW). As solid-stdesign steadily increase. As

In the realm of UHF trelectron bunching techniqUHF frequencies. They arparticularly efficient. A stschemes have been devisetypes of pulsing are in com

Mod-anode pulsing,color burst and vid

Annular control elecrating the pulsing stage (as with mod

Variations of the basic k

The inductive outputhe tetrode with th

The multi-stage dethrough a redesign the electron stream

Modulation system:carrier. Most systemallows superior diseconomic advantagl to the transmitter power output times feedline efficiency times the power

n can achieve its maximum 100-kW power output through a wide range ofombinations. A 35-kW transmitter coupled with a gain-of-4 antenna would-kW transmitter feeding an antenna with a gain of 12. Reasonable parings for

would range from a transmitter with a power output of 50 kW feeding ano a 30-kW transmitter connected to a gain-of-12 antenna. These combinationse losses. To reach the exact power level, minor adjustments are made to themitter, usually by a front panel power control. to achieve their maximum licensed power output are faced with installing ay expensive) transmitter. Typical pairings include a transmitter rated for 220a gain of 25, or a 110-kW transmitter and a gain-of-50 antenna. In the latterose a significant problem. UHF antennas with gains in the region of 50 aree for most installations because of the coverage problems that can result. narrow vertical radiation pattern that can reduce a stations coverage in areashatever way is chosen, getting 5-MW ERP is an expensive proposition. Most

erate considerably below the maximum permitted ERP.

wer required of a transmitter will have a fundamental effect on system design.e whether the unit will be of solid-state or vacuum tube design; whether air,ust be used; the type of power supply required; the sophistication of the

upervisory circuitry; and whether common amplification of the visual and auralte visual and aural amplifiers) is practical.sed for VHF transmitters above 5 kW and for low-power UHF transmitters

ate technology advances, the power levels possible in a reasonable transmitter of this writing, all-solid-state VHF transmitters of 60 kW have been produced.ansmitters, the klystron and related devices reign supreme. Klystrons use anue to generate high power (55 kW from a single tube is not uncommon) at

e currently the first choice for high-power service. Klystrons, however, are notock klystron with no special circuitry might be only 40% efficient. Variousd to improve klystron efficiency, one of the oldest being beam pulsing. Two

mon use:

a technique designed to reduce power consumption of the device during theeo portion of the signal (and thereby improve overall system efficiency).

trode (ACE) pulsing, which accomplishes basically the same thing by incorpo-signal into a low-voltage stage of the transmitter, rather than a high-voltage-anode pulsing).

lystron intended to improve UHF transmitter efficiency include the following:

t tube (IOT): a device that essentially combines the cathode/grid structure ofe drift tube/collector structure of the klystron.

pressed collector (MSDC) klystron: a device that achieves greater efficiencyof the collector assembly. A multi-stage collector is used to recover energy from inside the klystron and return it to the beam power supply.

a number of approaches may be taken to amplitude modulation of the visuals utilize low-level, intermediate frequency (IF) modulation. This approach

tortion correction, more accurate vestigial sideband shaping, and significant

es to the transmitter manufacturer.


Elements of the Trans

An analog television trans

The exciter

Intermediate power

Power amplifier

High-voltage powe

Figure 1.13 shows the audicircuits:

Video input buffer

Exciter-modulator

RF processor

Depending on the design into the exciter itself. A pothe transmitter.

FIGURE 1.13 Simplified blmitter

mitter can be divided into four major subsystems:

amplifier (IPA)

r supply

o, video, and RF paths for a typical design. The exciter includes of the following

of the transmitter, these sections may be separate units or simply incorporatedwer supply section supplies operating voltages to the various subassemblies of

ock diagram of a television transmitter.


Intermediate Power A

The function of the IPA isthe aural and visual systemdrive, and a high-band, 3high-gain klystron final tutube needs about 80 W. Beof the visual power outpu

Virtually all transmittepreferred because of theirwithout retuning. Presentmost transmitters in a sinvariety of schemes.

A typical building bloc200 W. To meet the requirwould have to be combinsystem to compensate for

Most solid-state IPA circmodules will continue to owill continue to operate wto service at a convenient

Because the output of tto produce the required drthat feeds equal RF drive that provides isolation beonly the modulated carrie

The inherent design of Most drivers are broadbanat the factory (dependingrequire virtually no attenchannels without tuning.

Advances continue to bextend the reach of semicobetter circuit designs, inclfactor in achieving high poitself.

Power Amplifier

The power amplifier (PA)As noted previously, solidhigh-power transmitters. Stubes. The workhorse of Vand good reliability. In U20 kW.

Tetrodes in television smaintaining a linear translinear performance becaube used instead of two inthat the transfer characterso, some nonlinearity is g(anode) circuit of a tetrodand reliable tank.mplifier

to develop the power output necessary to drive the power amplifier stages fors. A low-band, 16- to 20-kW transmitter typically requires about 800 W RF

5- to 50-kW transmitter needs about 1600 W. A UHF transmitter utilizing abe requires about 20 W drive, while a UHF transmitter utilizing a klystrodecause the aural portion of a television transmitter operates at only 10 to 20%t, the RF drive requirements are proportionately lower.rs manufactured today utilize solid-state devices in the IPA. Transistors are inherent stability, reliability, and ability to cover a broad band of frequencies solid-state technology, however, cannot provide the power levels needed bygle device. To achieve the needed RF energy, devices are combined using a

k for a solid-state IPA provides a maximum power output of approximatelyements of a 20-kW low-band VHF transmitter, a minimum of four such unitsed. In actual practice, some amount of headroom is always designed into thecomponent aging, imperfect tuning in the PA stage, and device failure.uits are configured so that in the event of a failure in one module, the remainingperate. If sufficient headroom has been provided in the design, the transmitterithout change. The defective subassembly can then be repaired and returnedtime.he RF up-converter is about 10 W, an intermediate amplifier is generally usedive for the parallel amplifiers. The individual power blocks are fed by a splitterto each unit. The output of each RF power block is fed to a hybrid combinertween the individual units. The combiner feeds a bandpass filter that allowsr and its sidebands to pass.a solid-state RF amplifier permits operation over a wide range of frequencies.d and require no tuning. Certain frequency-determined components are added on the design); however, from the end-user standpoint, solid-state drivers

tion. IPA systems are available that cover the entire low- or high-band VHF

e made in solid-state RF devices. New developments promise to substantiallynductors into medium-power RF operation. Coupled with better devices are

uding parallel devices and new pushpull configurations. Another significantwer from a solid-state device is efficient removal of heat from the component

raises the output energy of the transmitter to the required RF operating level.-state devices are increasingly being used through parallel configurations intill, however, the majority of television transmitters in use today utilize vacuumHF television is the tetrode, which provides high output power, good efficiency,HF service, the klystron is the standard output device for transmitters above

ervice are operated in the class B mode to obtain reasonable efficiency whilefer characteristic. Class B amplifiers, when operated in tuned circuits, providese of the fly-wheel effect of the resonance circuit. This allows a single tube to pushpull fashion. The bias point of the linear amplifier must be chosen soistic at low modulation levels matches that at higher modulation levels. Evenenerated in the final stage, requiring differential gain correction. The plate

e PA is usually built around a coaxial resonant cavity, which provides a stable


UHF transmitters usingalso most inefficient operis approximately 40%. Pulpeak of sync), can improv

Two types of klystrons theory of operation is ideIn the integral cavity klystcavity klystron, the cavitiesis installed in the transmi

A number of factors comdesigns. Primary consider

The PA stage includes Because of the power leveprevent damage to compoRF sample, obtained fromprovide automatic power-

The transmitter systemration is normally used fowhich the aural and visuabut at the cost of addition

PA stages are often conWhile this represents a goprotection in the event of aural and visual signals at

The aural output stagetransmitter. Tetrode outpuaural PAs are used in UHF

Coupling/Filtering Sys

The output of the aural athat is electrically ready tofiltering system of a TV tr

Color notch filter

Aural and visual ha

Diplexer

In a low-power transmitteitself. Normally, however,

Color Notch Filter

The color notch filter is usof the FCC. The color nota coax or waveguide stub tso that energy in the vestiat 3.58 MHz.

Harmonic Filters

Harmonic filters are usedcompliance with FCC requare of coaxial constructionalso used, typically adjustefrequency of the visual ca a klystron in the final output stage must operate class A, the most linear butating mode for a vacuum tube. The basic efficiency of a non-pulsed klystronsing, which provides full available beam current only when it is needed (duringe device efficiency by as much as 25%, depending on the type of pulsing used.are presently in service: integral cavity and external cavity devices. The basicntical for each tube; however, the mechanical approach is radically different.ron, the cavities are built into the klystron to form a single unit. In the external are outside the vacuum envelope and bolted around the tube when the klystrontter.

e into play in a discussion of the relative merits of integral-vs.-external cavityations include operating efficiency, purchase price, and life expectancy.a number of sensors that provide input to supervisory and control circuits.ls present in the PA stage, sophisticated fault-detection circuits are required tonents in the event of a problem either external to or inside the transmitter. An a directional coupler installed at the output of the transmitter, is used to

level control. discussed thus far assumes separate visual and aural PA stages. This configu-r high-power transmitters. Low-power designs often use a combined mode inl signals are added prior to the PA. This approach offers a simplified system,al pre-correction of the input video signal.figured so that the circuitries of the visual and aural amplifiers are identical.od deal of overkill insofar as the aural PA is concerned, it provides backupa visual PA failure. The aural PA can then be reconfigured to amplify both the reduced power. of a television transmitter is similar in basic design to an FM broadcastt devices generally operate class C, providing good efficiency. Klystron-based transmitters.

tem

nd visual power amplifiers must be combined and filtered to provide a signal be applied to the antenna system. The primary elements of the coupling andansmitter are:

rmonic filters

r (below 5 kW), this hardware may be included within the transmitter cabinetit is located external to the transmitter.

ed to attenuate the color subcarrier lower sideband to the 42 dB requirementsch filter is placed across the transmitter output feedline. The filter consists ofuned to 3.58 MHz below the picture carrier. The Q of the filter is high enoughgial sideband is not materially affected, while still providing high attenuation

to attenuate out-of-band radiation of the aural and visual signals to ensureirements. Filter designs vary, depending on the manufacturer; however, most utilizing components housed within a prepackaged assembly. Stub filters are

d to provide maximum attenuation at the second harmonic of the operating

rrier and the aural carrier.


Diplexer/Combiner

The filtered visual and authe antenna (see Fig. 1.14)quadrature-phased outputaural and visual harmonic

A hybrid combiner servvisual RF signals to feed asection of the transmitter.

The notch diplexer conconfigured so that all of tof the energy from the authat the input signals canvisual signals through thechange from the input pothus preserving signal pur

1.3 Nonbroadca

Radio and television broadof installations, however, nrange from microwave comto a million watts or more

Satellite transmissio

Radar

Electronic navigatio

Induction heating

Satellite Transmiss

Commercial satellite commacross the Atlantic Oceangeostationary relay satellitenications industry. In the an elliptical orbit to betterinclined about 64 relative

All commercial satellitethat maintains a fixed po

FIGURE 1.14 Functional dtransmitter for application to

LoadB

AVisual inral outputs are fed to a diplexer where the two signals are combined to feed. For installations that require dual-antenna feedlines, a hybrid combiner withs is used. Depending on the design and operating power, the color notch filter, filters, and diplexer can be combined into a single mechanical unit.es as the building block of the notch diplexer, which combines the aural and single-line antenna system and provide a constant impedance load to each

sists of two hybrid combiners and two sets of reject cavities. The system ishe energy from the visual transmitter passes to the antenna (port D), and allral transmitter passes to the antenna. The phase relationships are arranged socel at the resistive load (port B). Because of the paths taken by the aural and notch diplexer, the amplitude and phase characteristics of each input do notrts (port A for the visual and port C for the aural) and the antenna (port D),ity.

st Applications

casting are the most obvious applications of RF technology. In total numbersonbroadcast uses for RF far outdistance radio and TV stations. Applicationsmunications to induction heating. Power levels range from a few tens of watts

. The areas of nonbroadcast RF technology covered in this section include:

n

n

ion

unication began on July 10, 1962, when television pictures were first beamed through the Telstar 1 satellite. Three years later, the INTELSAT system ofs saw its initial craft, Early Bird 1, launched into a rapidly growing commu-

same year, the U.S.S.R. inaugurated the Molnya series of satellites traveling in meet the needs of that nation. The Molnya satellites were placed in an orbit to the equator, with an orbital period half that of the Earth.

iagram of a notch diplexer, used to combine the aural and visual outputs of a television the antenna.

A1

A2

C1

C2

AntennaD

CAural in

Hybrid-2

Reflectingcavities (1/4) Aural}

P

Lowercolorsideband

Hybrid -1

Lowercolorsideband

Reflectingcavities (1/4)Aural}s in use today operate in a geostationary orbit. A geostationary satellite is onesition in space relative to Earth because of its altitude, roughly 22,300 miles


above the Earth. Two primGHz). Any satellite relay s

An uplink transmittionary orbit

The satellite (the ssignals back to Eart

The downlink recei

Because of the frequencmunication is designated vice. As such, certain requsystem. Like terrestrial mictransmitter and receiver morological conditions, suchdetrimental attenuation ofmust be made to shield from terrestrial interferencstrength is based on the directional transmit and rare used, which in turn aiming accuracy. To countand thermal noise sourcamplifiers are designed focharacteristics. Figure 1.1ments of a satellite relay s

Satellite Communicat

The first satellites launchedusers contained only one(transponders). Pressure fservices has driven engineGenerally, C-band satellitbandwidths. Ku-band syst

Users of satellite commuit may be possible to purchaboard each craft, allowin

By assigning transpondfacilities. The Earth stationexample, a corporate videsatellite. To do so, the opeinterest. The receiver handon the antenna.

Each transponder has a to one frequency plan, all bered ones use vertical podoes not cause interferenccross-polarization. This corbit. Center frequencies the first craft. In addition,assignments are still offsepolarization offset must bary frequency bands are used: the C-band (46 GHz) and the Ku-band (1114ystem involves three basic sections:

ting station, which beams signals toward the satellite in its equatorial geosta-

pace segment of the system), which receives, amplifies, and retransmits theh

ving station, which completes the relay path

ies involved, satellite com-as a microwave radio ser-irements are placed on therowave, the path betweenust be line-of-sight. Mete- as rain and fog, result in the signal. Arrangementssatellite receive antennase. Because received signal

inverse square law, highlyeceive parabolic antennasrequires a high degree oferact the effects of galactices on low-level signals,r exceptionally low noise5 shows the primary ele-ystem.

ions

for INTELSAT and other or two radio relay unitsor increased satellite linkers to develop more economical systems with multiple transponder designs.es placed in orbit now typically have 24 transponders, each with 36-MHzems often use fewer transponders with wider bandwidths.nication links are assigned to transponders generally on a lease basis, althoughase a transponder. Assignments usually leave one or more spare transponders

g for standby capabilities in the event a transponder should fail.ers to users, the satellite operator simplifies the design of uplink and downlink controller can be programmed according to the transponder of interest. Foro facility may need to access four or five different transponders from onerator needs only to enter the transponder number (or carrier frequency) ofles retuning and automatic switching of signals from a dual-polarity feedhorn

fixed center frequency and a specific signal polarization. For example, accordingodd-numbered transponders use horizontal polarization while the even-num-larization. Excessive deviation from the center carrier frequency by one signale between two transponders and signals because of the isolation provided by

oncept is extended to satellites in adjacent parking spaces in geosynchronousfor transponders on adjacent satellites are offset in frequency from those on an angular offset of polarization is employed. The even and odd transponder

FIGURE 1.15 A satellite communications linkconsists of an uplink, the satellite (as the spacesegment), and a downlink.t by 90 from one another. As spacing is decreased between satellites, thee increased to reduce the potential for interference.


Satellite Uplink

The ground-based transmintermediate frequency (I

The baseband section inbeing used. Signals providlation characteristics (digbrings the signals to the uvideo), some degree of pcombined into a single co

When the incoming sigto an FM modulator, whicThe use of an IF section h

A direct conversioning frequency stabi

Any mixing or modat the IF may be us

Many terrestrial muplink site by terresthe IF section of th

From the 70-MHz IF, th(for C-band) or 14 GHz (fEarth station transmittersTransmitters designed for

Several amplifying devirequirements. For the highwith pulsed outputs rangithat a separate klystron is

The traveling wave tubeWhile similar in some areaat least ten times wider thato several transponders ocapability of the TWT off

Solid-state amplifiers bHPA systems. The power 1 to 6 W for Ku-band. Suc

Uplink Antennas

The output of the HPA, wwhen referenced to an idea10 m in diameter offer gainto an effective radiated pothe observation of certainNot surprisingly, smaller international satellite comparabolic antenna designsFig. 1.16):

Prime focus, single pthe reflector precisemploy a feedhornitting equipment of the satellite system consists of three sections: baseband,F), and radio frequency (RF).terfaces various incoming signals with the transmission format of the satelliteed to the baseband section may already be in a modulated form with modu-

ital, analog, or some other format) determined by the terrestrial media thatplink site. Depending on the nature of the incoming signal (voice, data, or

rocessing will be applied. In many cases, multiple incoming signals will bemposite uplink signal through multiplexing.nals are in the correct format for transmission to the satellite, they are appliedh converts the composite signal upward to a 70-MHz intermediate frequency.as several advantages:

between baseband and the output frequency presents difficulties in maintain-lity of the output signal.

ulation step has the potential of introducing unwanted by-products. Filteringed to remove spurious signals resulting from the mixing process.

icrowave systems include a 70 MHz IF section. If a signal is brought into thetrial microwave, it becomes a simple matter to connect the signal directly intoe uplink system.

e signal is converted upward again, this time to the output frequency of 6 GHzor Ku-band) before application to a high-power amplifier (HPA). Conventional operate over a wide power range, from a few tens of watts to 12 kW or more. deep space research can operate at up to 400 kW.ces are used in HPA designs, depending on the power output and frequencyest power level in C- or Ku-band, klystrons are employed. Devices are availableng from 500 W to 5 kW, and a bandwidth capability of 40-MHz. This meansrequired for each 40 MHz wide signal to be beamed upward to a transponder. (TWT) is another type of vacuum power device used for HPA transmitters.s of operation to klystrons, the TWT is capable of amplifying a band of signalsn the klystron. Thus, one TWT system can be used to amplify the signals sent

n the satellite. With output powers from 100 W to 2.5 kW, the bandwidthsets its much higher price than the klystron in some applications.ased on MOSFET technology can be used for both C- and Ku-band uplinkcapabilities of solid-state units are limited, 5 to 50 W or so for C-band andh systems, however, offer wideband performance and good reliability.

hen applied to a parabolic reflector antenna, experiences a high degree of gainl isotropic antenna (dBi). For example, large reflector antennas approximatelys as high as 55 dB, increasing the output of a 3-kW klystron or TWT amplifier

wer of 57 to 86 dBW. Smaller reflector sizes (6 to 8 m) can also be used, with restrictions in regard to interference with other satellites and other services.antennas provide lower gain. For a 30-m reflector, such as those used formunications, approximately 58 dB gain can be achieved. Several variations of are used for satellite communications services, including the following (see

arabolic reflector: places the source of the signal to be transmitted in front of

ely at the focal point of the parabola. Large antennas of this type commonly supported with a tripod of struts. Because the struts, the waveguide to the


feedhorn, and the effort is made to denough to withstan

Offset reflector: remreflector maintains The feedhorn, whilof the parabola sha

Double reflector: thein front of the focareflector is located defines the positionflector are spread atoward the satellite.type: (1) the overallmounting and decreflector generates main reflector; andreflector type of str

The antenna used for ssatellite. The major changcoupling to prevent enerdevices or circulators use wto the antenna feedhorn, the receiver input.

Signal Formats

The signal transmitted frfrequency modulation. Liming from excessive bandwi

FIGURE 1.16 Satellite tranreflector; (c) double reflectorhorn assembly itself are located directly within the transmitted beam, everyesign these components with as little bulk as possible, yet physically strongd adverse weather conditions.

oves the feedhorn and its support from the radiated beam. Although thethe shape of a section of a parabola, the closed end of the curve is not included.e still located at the focal point of the curve, points at an angle from the vertexpe.

primary reflector is parabolic in shape while the subreflector surface, mountedl point of the parabola, is hyperbolic in shape. One focus of the hyperbolicat the parabolic focal point, while the second focal point of the subreflector for the feedhorn signal source. Signals reflected from the hyperbolic subre-

cross the parabolic prime reflector, which then directs them as a parallel beam This two-reflector antenna provides several advantages over a single-reflector front-to-back dimension of the two-reflector system is shorter, which simplifiesreases wind-loading; (2) placement of the subreflector closer to the mainless spillover signal because energy is not directed as closely to the edge of the (3) the accuracy of the reflector surfaces is not as stringent as with a single-ucture.

ignal transmission to the satellite can also be used to receive signals from thee needed to provide this capability is the addition of directional switching orgy from the transmitter HPA from entering the receiver system. Switching

aveguide characteristics to create a signal path linking the transmitter signalwhile simultaneously providing a received signal path from the feedhorn to

om the uplink site (or from the satellite, for that matter) is in the form ofitations are placed on uplinked signals to avoid interference problems result-

dth. For example, a satellite relay channel for television use typically containssmitting/receiving antennas: (a) prime focus, single reflector; (b) offset feed, single.


only a single video signal stacked onto the video sigonto its subcarrier frequeas modulation to the uplovermodulation.

In the case of telephonvoice circuits are combinedmeans. The result is that thsatellite.

Satellite Link

Like other relay stations, thsion. From the antenna, stransmit band. A high-pothe information to a pred

Power to operate the batteries, kept recharged bis eclipsed by the Earth. F

Power to the electronicsMost of the equipment ophigh-power amplifier. Thestate power amplifier (SSrequire multiple voltages collection electrodes requof voltages, the satellite po

From these potentials, tsystems are operated at ththe lifetime of the spacecr

A guidance system is inrocket engines are providFig. 1.19). This work is kn

Satellite Antennas

The antenna system for aassembly. One is for recei

FIGURE 1.17 Block diagra

Antennafeed horn

Receiv

Dand its associated audio. Audio is carried on one or more subcarriers that arenal. To develop the composite signal, each audio channel is first modulated

ncy. Then, each of the subcarriers and the main channel of video are appliedink carrier. The maximum level of each component is controlled to avoid

e relay circuits, the same subcarrier concept is used. A number of individual into groups, which are then multiplexed to subcarriers through various digitalousands of telephone conversations can occur simultaneously through a single

e communications spacecraft contains antennas for receiving and retransmis-ignals pass through a low-noise amplifier before frequency conversion to thewer amplifier feeds the received signal to a directional antenna, which beamsetermined area of the Earth to be served by the satellite (see Fig. 1.17).electronics hardware is generated by solar cells. Inside the satellite, storagey the solar cell arrays, carry the electronic load, particularly when the satelliteigure 1.18 shows the two most common solar cell configurations. on the craft requires protective regulation to maintain consistent signal levels.erates at low voltages, but the final stage of each transponder chain ends in a

HPA of C-band satellite channels may include a traveling wave tube or a solid-PA). Ku-band systems typically rely on TWT devices. Klystrons and TWTslevels. The filaments operate at low voltages but beam focus and electron

ire voltages in the hundreds and thousands of volts. To develop such a rangewer supply includes voltage converters.he klystron or TWT produces output powers in the range of 8.5 to 20 W. Moste lower end of the range to increase reliability and life expectancy. In general,aft is assumed to be 7 years.cluded to stabilize the attitude of the craft as it rotates around the earth. Smalled for maintaining an exact position in the assigned geostationary arc (seeown as station-keeping.

communications satellite is really several antennas combined into a singleving signals from Earth. Another, obviously, is for transmitting those signals

e

Transmit

iplexer

Bandpassfilter

Frequency conversionand gain

Localoscillator

Limiter orattenuator

Power amplifier(traveling

wave tube orGaAs FET)m of a satellite transponder channel.


back to Earth. The transmmultiple signal beams. Figround-based satellite con

At the receiving end of through a channelizing neamplifier, and HPA. At thfed to the transmitting an

The approach to designhorizontal and vertical polseparated. Multilayer, dichpurposes. Also, multiple fefor different requirementsreflector. The parabolic dstructure that are parallel from the focal point are ruse of spherical and ellipt

Satellite Downlink

Satellite receiving stationsequipment to satellite tra(LNA), 4-GHz (C-band) o

Downlink Antennas

Antenna type and size fooperation, location of th

FIGURE 1.18 The two mtypes of solar cell arrays usenications satellites.

FIGURE 1.19 Attitude of thitting antenna can be made of more than one section to handle the needs ofnally, a receive-transmit beacon antenna provides communication with thetrol station.

the transponder, signals coming from the antenna are split into separate bandstwork, allowing each input signal to be directed to its own receiver, processinge output, a combiner brings all channels together again into one signal to betenna.ing the complex antenna system for a relay satellite depends a good deal onarization of the signals as a means to keep incoming and outgoing informationroic reflectors that are sensitive to the polarizations can be used for such

edhorns may be needed to develop one or more beams back to Earth. Antennas may combine several antenna designs, but nearly all are based on the parabolicesign offers a number of unique properties. First, rays received by such ato the feed axis are reflected and converged at the focus. Second, rays emittedeflected and emerge parallel to the feed axis. Special cases may involve someical reflector shapes, but the parabolic is of most importance.

, like uplink equipment, perform the function of interfacing ground-basednsponders. Earth stations consist of a receiving antenna, low noise amplifierr 11-GHz (Ku-band) tuner, 70-MHz IF section, and baseband output stage.

r any application are determined by the mode of transmission, band ofe receiving station, typical weather in the receiving station locale, and the

ost commond for commu-

e spacecraft is determined by pitch, roll, and yaw rotations around three reference axes.


required quality of the oubecause the decoding eqperiodically includes infocorrections. Sophisticatedextent. Greater emphasis or life-critical data, such aming, such as TV broadcprimarily to concealment

Receiving antennas for special services or teleconfKu-band units can be smdepending on the type odownlink. The nature of tif one of the spherical typ

In general, the gain andof the reflector re

RF Transmission Systems Handbook

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