Radio Transmission Systems

Radio Transmission SystemsSection 2Standard broadcastingprimary service areais defined as the area in which the ground or surface-wave signal is not subject to objectionable interference or objectionable fading.

Secondary service arearefers to an area serviced by skywaves and not subject to objectionable interference

Intermittent servicerefers to an area receiving service from either a surface wave or a skywave but beyond the primary service area and subject to some interference and fading

Standard Broadcasting (AM)The carrier frequencies for standard broadcasting in the U.S.are designated in the Federal Communications Commission (FCC) Rules and Regulations. A total of 117 carrier frequencies are allocated from 540 to 1700 kHz in 10-kHz intervals. Each carrier frequency is allowed to deviate no more than plus or minus 20Hz

a daytime signal strength of 2 mV/m is required for reception in populated towns

a signal of 0.5 mV/m is generally acceptable in rural areas

Modulation TechniquesDouble-sideband full-carrier modulation, commonly called amplitude modulation (AM), is used in standard broadcasting for sound transmission

Modulation TechniquesAlthough full fidelity is possible with amplitude modulation, the FCC requires standard broadcast stations to limit the fidelity, thus restricting occupied bandwidth of the transmitted signal

Typical modulation frequencies for voice and music range from 50 Hz to 10 kHz

Each channel is generally thought of as 10 kHz in width

when the modulation frequency exceeds 5 kHz, the radio frequency bandwidth of the channel exceeds 10 kHz and adjacent channel interference may occur

Channel and Station Classifications (AM)AM stations are classified by FCC according to their operating power, protection from interference, and hours of operation.Class AClass BClass CClass DClass A Stations10 to 50 kW of powerservicing a large area with primary, secondary, and intermittent coverageprotected from interference both day and night.called clear channel stations(approximately a 1000 km radius),Class B Stations0.25 to 50 kWrender primary service only over a principal center of population and contiguous rural areamost Class B stations must restrict their power to 5 kW or less to avoid interfering with other stations.Class B stations operating in the 1605 to 1705 kHz band are restricted to a power level of 10 kW daytime and 1 kW nighttimeClass C Stationsoperate on six designated channels (1230, 1240, 1340, 1400, 1450, and 1490)with a maximum power of 1 kWrender primarily local service to smaller communities

Class D stationsoperate on Class A or B frequencieswith Class B transmitter powers during daytime, but nighttime operationif permitted at allmust be at low power (less than 0.25 kW) with no protection from interference.

Propagationsurface-wave propagation occurs over shorter ranges both during day and night periodsSkywave propagation in the AM broadcast band permits longer ranges and occurs during night periods, and thus some stations must either reduce power or cease to operate at night to avoid causing interferenceTransmittersStandard AM broadcast transmitters range in power output from 5 W up to 50 kW unitsModern transmitters utilize low-voltage, high-current metal-oxide-semiconductor field-effect transistor (MOSFET) devices to generate the RF powerThese transistors are combined into modules, the outputs of which are combined to produce the output signalTransmittersHigh-Level AM Modulationthe modulating signal is amplified and combined with the de supply source to the anode of the final RF amplifier stageThe RF amplifier is normally operated class CThe final stage of the modulator usually consists of a pair of tubes / transistor operating class B

TransmittersPulse-Width Modulationis one of the most popular systems developed for modern AM transmittersworks by utilizing a square wave switching systemThis PDM signal becomes the power supply to the final RF amplifier tubecauses the final amplifier to operate in a highly efficient class D switching modemakes it possible to completely eliminate audio frequency transformers in the transmitter which results to result is wide frequency response and low distortionTransmittersPWM

75kHzAfter integrationPWMTransmittersPWM

FM BroadcastingFrequency modulationutilizes the audio modulating signal to vary the frequency of the RF carrierThe greater the amplitude of the modulating frequency, the greater the frequency deviation from the center carrier frequencyThe rate of the frequency variation is a direct function of the frequency of the audio modulating signalmultiple pairs of sidebands are produced, determined by the modulation index (MI) of the system As the MI increases there are more sidebands producedFM broadcast stations are required to restrict frequency deviation to 75 kHz from the main carrier (100 percent modulation)FM BroadcastingFrequency modulationThe power emitted by an FM transmitter is virtually constant, regardless of the modulating signalFM transmitters may utilize Class C type amplifiersaudio frequencies from 50 to 15,000 Hz75-kHz RF bandwidth88 to 108 MHz frequency bandPre-emphasis is employed in an FM broadcast transmitter to improve the received signal-to-noise ratiotime constant of 75 sde-emphasis circuit

FM Broadcast Modulation TechniquesFM stereo was developed in 1961transmission capability for a left- and right-stereo audio signalStereophonic transmission is accomplished by adding the left- and right-channel stereo information together in the baseband signala left-minus-right channel is added and frequency multiplexed on a subcarrier of 38 kHz using double sideband suppressed carrier (An unmodulated 19-kHz subcarrier is derived from the 38-kHz subcarrier to provide a synchronous demodulation reference for the stereophonic receiverDSSC) modulationFM Broadcast Modulation Techniques

Subsidiary Communications Authorization (SCA)used in a variety of ways, such as paging, data transmission, specialized foreign language programs, radio reading services, utility load management, and background musicAn FM stereo station may utilize multiplexed subcarriers within the range of 5399 kHz with up to 20 percent total SCA modulationdigital audio broadcasting (DAB) systems utilize in-band on channel technique to provide high-quality digital audio without interfering with the current analog FM broadcast signalsFrequency Allocationsbroadcast range from 88.1 to 107.9 MHz100 carrier frequencies200-kHz bandwidthThe channels from 88.1 to 91.9 MHz are reserved for educational and noncommercial broadcasting92.1 to 107.9 MHz for commercial broadcastingmaximum frequency swing 75 kHz With SCA 82.5 kHzThe carrier center frequency is required to be maintained within 2000 Hz

Frequency AllocationsThe frequencies used for FM broadcasting limit the coverage to essentially line-of-sight distancesincreasing the power or raising the antenna will increase the coverage areaFM Broadcast Stations Classificationstations are classified by the FCC according to their maximum allowable ERP and the transmitting antenna antenna height above average terrain (HAAT) in their service areaFM broadcast transmitters typically range in power output from 10 W to 50 kWFM Broadcast Stations ClassificationClass A stations radius of about 28 km with 6000 W of ERP at a maximum HAAT of 100 mClass Cmost powerful class operates with maximums of 100,000 W of ERP andheights up to 600 m with a primary coverage radius of over 92 kmFM Broadcast Stations ClassificationAll classes may operate at antenna heights above those specified but must reduce the ERP accordinglyStations may not exceed the maximum power specified, even if the antenna height is reduced The classification of the station determines the allowable distance to other cochannel and adjacent channel stationsTransmitter Performance and MaintenanceTransmitter Performance and MaintenanceMost transmitters can be checked by measuring the audio performance of the overall system

Key System MeasurementsTypical performance targets for an FM station are 1 dB, 50 Hz to 15 kHz.

Typical targets for an AM station are1 1dB, 50 Hz to 10 kHz

Total harmonic distortion (THD)is the creation by a nonlinear device of spurious signals harmonically related to the applied audio waveformdistortion targets of 1 percent or less

Intermodulation distortion (IMD)These distortion components are sum-and-difference (beat notes) mixing productsIMD performance targets for AM and FM transmitters are the same as the TH D targetsKey System MeasurementsSignal-to-noise ratio (S/N)is the amplitude difference, expressed in decibels, between a reference level audio signal and the system's residual noise and humAn FM performance target of 70 dB per stereo channel reflects reasonable exciter-transmitter performance. Most AM transmitters are capable of 60 dB S/N or betterKey System MeasurementsSeparationis a specialized definition for signal crosstalk between the left and right channels of a stereo system. The separation test is performed by feeding a test tone into one channel while measuring leakage into the other channel (whose input is terminated with a 600- wire wound resistor, or other appropriate value). Typical performance targets for an FM station are 40 dB or better from 50 Hz to 15 kHz.Radio Studio to Transmitter Link (STL) Systems 2.2Radio STL Systems

Radio STL SystemsOne of the major concerns in the design and operation of a radio broadcasting facility is the means by which the program audio from the studio is conveyed to the transmitter site

An inferior link will impose an unacceptable limit on overall audio quality

The requirements for reliability and transparent program relay have led to the development of new STL systems based on digital technology

The arguments over which approachradio STL or landline (telco)is the better way to convey

program audio from the studio to the transmitter is as old as radio broadcasting itselfSTL Equipment950 MHz radioequalized analog telephone linesdigital transmission

Advantages of Digital STL SystemsNecessity. The station has no line-of-sight to the transmitter, or suitable frequencies are unavailableSound quality. A digital landline STL can sound better than even the best analog systemsCost. A single leased data line can cost less than multiple leased analog lines.Digital STL Systems

STL System Configurationaural STL systems in the U.S. is 944.5 to 951.5 MHzFrequencies ranging from 200 to 940 MHz are used in other parts of the worldFrequency modulation is used for analog STL systemsThe vast majority of radio STL systems in operation today carry a composite stereo baseband signal from the studio to the transmitterThe monaural STL typically has an audio bandwidth of 15kHz for program signals, and usually can accommodate a single FM subcarrier at approximately 39 kHzcomposite transmitter-receiver system

Dual monaural transmitter-receiver system

Baseband spectrum of STL systems: composite

Baseband spectrum of STL systems: monaural

STL System ConfigurationThe composite STL provides superior stereo performance compared to dual monaural radio links in several respects, including:

Elimination of interchannel phase and amplitude errors that can arise in a dual channel system.Elimination of audio headroom considerations because the STL input signal has already been passed through the station's audio processing system and the stereo generator, which are located at the studio.Digital STL vs. Analog STL LinkAdvantages of DSTLGreater immunity to noise and interference in the transmission pathElimination of transmission-path-dependent distortion mechanisms, such as harmonic distortion, intermodulation distortion, and crosstalkEfficient use of baseband and RF spectrumEfficient and predictable regeneration of the digital signalEasy and effective encryption for security and coding purposesAdvantages of DSTL

CODECcoding and decoding deviceencoder and decoder are formed into a single device, or set of devices (a chip set). At the transmission end, the codec provides the necessary filtering to band-limit the analog signal to avoid aliasing, thereby preventing analog-to-digital (A/D) conversion errors. At the receiver, the codec performs the reciprocal digital-to-analog (D/A) conversion and interpolates (smooths) the resulting analog waveform

Digital STL PerformanceA digital STL typically permits broadcasters to extend the fade margin of an existing analog link by 20 dB or moreaudio signal-to-noise (S/N) improvements of at least 10 dBthe maximum possible path distance of a given composite STL transmitter and receiver can be extended

Coding SystemPulse code modulation (PCM)is a common scheme that meets the requirements for speed and accuracy.

PCM

PCMQuantizationUniform Non-uniformCompanding (compression and expansion)using larger quantization steps for high energy signals and smaller steps for low energy signals,efficient use is made of the data bits, while maintaining a specified signal-to-quantizationnoise level

PCMprovides a high-speed string of discrete digital values that represent the input audio waveform Each value is independent of all previous samplesNo encoder memory is requiredAnalog Composite STL Transmitter and Receiver Characteristicsthe composite aural STL is the workhorse of the radio industrygoal: relay of a baseband signal from the studio site to the transmitter

Analog Composite STL Transmitter and Receiver Characteristics

Analog Composite STL Transmitter and Receiver Characteristics

Components of an STL SystemAny STL installation is only as good as the hardware used to interconnect the link. All components, from the transmitter to the output connectors, must be carefully chosen and properly installed. A well-designed system will provide years of trouble-free service. A poorly-designed system will cause problems on a regular basisComponents of an STL SystemTransmitter and ReceiverTransmission LinesAntenna SystemMounting StructuresHardware ConsiderationsTransmitter and ReceiverSTL transmitterit will be necessary to select a unit that will deliver sufficient power to overcome the losses determined by path gain/loss calculationsthe transmitter power output should be converted to gain in decibels above a 1 mW reference (dBm).

Transmitter and ReceiverSTL receiverIn receiver design, sensitivity, S/N, selectivity, and the method of demodulation are determining factors of receiver quality

Transmission Lines

Transmission LinesCriteria for the selection of transmission line includes the followingAmount of signal attenuationPhysical parameters (dielectric material and size)larger the diameter of the transmission line, the lower the loss, and the greater the cost of the linemost common types of dielectric are air and foamAir dielectric cable typically requires pressurization and is, therefore, seldom used for 950 MHz installationsPurchase and installation costTransmission LinesOther factorsConnector lossStrain reliefPermits movement without straining cable and chassis connectionsSo-called pigtail or jumper cables are commonly usedpigtails commonly are terminated with N-type male connectors on both ends, ends, the main transmission line must be configured with female N-type connectors on both ends if a pair of pigtails are used

Transmission Lines

Antenna SystemAntenna models differ in a number of respectsGain (directly proportional to size)Operating frequency rangePolarization (most antennas can be set for either horizontal or vertical polarization using universal mounting hardware kits)BeamwidthFront-to-back ratioWindloadingStructural strength

Antenna SystemAntenna gainis specified in decibels referenced to an isotropic antenna (dBi) or decibels referenced to a dipole antenna (dBd)For path analysis calculations of system gains and losses, dBi is usedConversion from dBd to dBi is as follows:

Antenna SystemRadial Plots

Antenna System

System PlanningThe ultimate goal in selecting STL equipment is to choose a combination of STL transmitter, transmission line, antennas, and STL receiver that will give adequate quieting (S/N) when transmitting over the path between the studio and the station transmitter. Allowance in the form of fade margin must also be made for the uncertainties that the path imposes on the received signalFrequency Selectionthe choice of operating frequency is governed by the availability of unused STL channels in the area, and is further dictated by the need to avoid interference with other STL usersSTL engineering can be divided into three broad categoriesFrequency selectionPath layoutensure a correctly-oriented and unobstructed route for unhampered propagation of the radio wavePath gain/loss calculationsinvolve analyzing RF power levels from the transmitter output to the receiver input so that an adequate receive level is providedSpectrum Considerationsan STL system should be designed to be as spectrum-efficient as possiblefirst rule of spectrum-efficiency is to use only the effective radiated power (ERP) necessary to do the jobimmune to undesired transmissions as possible

Path LayoutCareful path engineering should be performed prior to any licensing work to determine if the proposed locations of the STL transmitter and receiver will be able to achieve the desired resultsPath LayoutLine of Sightmicrowave frequencies are used for STL systemsthe signal path is theoretically limited to the line-of-sight between the studio and transmitter locations

ReviewRadio horizonsituated beyond the visual horizonThis is the result of the gradual decrease in the refractive index of the atmosphere with increasing altitude above the earthThe degree of bending is characterized by the K factorwhich is the ratio of the effective earth radius to the true earth radiusA typical value for K is 4/3, or 1.33, valid over 90 percent of the time in most parts of the world

Path LayoutSTL Site Selection

Path LayoutSTL Site Selection

Path LayoutTerrain ConsiderationsThe radius of the first Fresnel zone, which defines the boundary of the elliptical volume

Path LayoutPath Layoutperformance is substantially the same as long as H is greater than 0.6 F1.

Path LayoutPath analysis

Path Reliabilitymost important factors are free space loss and allowance for fade margingain and loss balance sheet should be computed to determine the fade margin of the planned STL systemfade margin is vital to reliable performancePath ReliabilitySTL fade margin can be computed using the following equations

Path ReliabilityThe total system losses are then computed per

Path Reliability

Path Reliabilityfade margin can be calculated per

Path ReliabilityReceiver Sensitivity

Causes of Signal Fadechanges in the refractive indexes of the atmosphere along the signal pathearth bulge (or inverse beam) fadingPrecipitation

Dealing with Problem PathsThe distance is too great to be reasonably covered by a single-hop system.A direct path cannot be used because of obstructions of some type.An unusually large fade margin is required for the application.Dealing with Problem Paths

Hot Standby

Pre-Installation CheckoutThe STL transmittercan be operated into a dummy load to confirm proper operationWith an ideal load, the transmitter front panel readings should correspond closely with the final test sheet supplied by the manufactureSTL receiveroperating the transmitter into a dummy load and attaching a short wire to the receiver antenna input. Be sure to use the proper connector on the receiver to avoid possible damage to the center pin. Maintain sufficient separation of the transmitter and receiver to prevent overloading the receiver front-endPre-Installation CheckoutAntennas should be given a close visual inspectiontransmission line and connectors require no pre-installation quality controlConsider ordering a couple extra connectors just in case a part is lost or damaged during constructiontest equipment required for pre-installation checkoutA high-quality 50 dummy load capable of dissipating approximately 25 W.An in-line RF power output meter capable of reading forward and reverse power at 1.0 GHz.Audio frequency signal generator.Audio frequency distortion analyzer.Frequency counter accurate to 1.0 GHz.

InstallationSTL transmitter and receiver should be mounted in an equipment rack in a protected location adjacent to the stereo generator at the studio site, and adjacent to the exciter at the broadcast transmitter siteKeep all cable runs as short and direct as possible.Follow good grounding practices at all timesInstallation

Digital Radio SystemsIntroductionIntroductiondigital audio radio (DAR)digital audio broadcasting (DAB)Instead of using analog modulation methods such as AM or FM, DAR transmits audio signals digitallyis designed to eventually replace analog AM and FM broadcastingproviding a signal that is robust against reception problems such as multipath interference,with fidelity comparable to that of the compact disc. supports auxiliary data transmissiontext, graphics, or still video imagesIntroductionTwo principal DAR technologiesEureka 147 DABin-band on channel (IBOC) broadcastingTechnical ConsiderationsThe World Administrative Radio Conference (WARC) allocated 40 MHz at 1500 MHz (L-band) for digital audio broadcasting via satelliteFCC allocated the S-band (23102360 MHz) spectrum to establish satellite-delivered digital audio broadcasting servicesData ReductionDAR must use data reduction to reduce the spectral requirementFor example, instead of a digital signal transmitted at a 2-Mbits/s rate, a data-reduced signal might be transmitted at 256 kbits/sEureka 147/DABwas selected as the European standard in 1995 for broadcasting to mobile, portable, and fixed receiversSuitable for use in terrestrial, satellite, hybrid (satellite and terrestrial), and cable applications

Eureka 147/DABdigitally combines multiple audio channels, and the combined signal is interleaved in both frequency and time across a wide broadcast band

Transmitter

Receiver

DAB standard defines three basic transmission mode optionsMode I with a frame duration of 96 ms, 1536 carriers, and nominal frequency range of less than 375 MHz is suited for a terrestrial VHF network because it allows the greatest transmitter separations.Mode IIwith a frame duration of 24 ms, 384 carriers, and nominal frequency range of less than 1.5 GHz is suited for UHF and local radio applications.Mode III with a frame duration of 24 ms (as in Mode II), 192 carriers, and nominal frequency range of less than 3 GHz is suited for cable, satellite, and hybrid (terrestrial gap filler) applications.DAB standarduses ISO/MPEG-1 Layer II bit rate reductionBit rates may range from 32 to 384 kbits/s in 14 stepsnominally, a rate of 128 kbps per channel is usedNominally, a sampling frequency of 48 kHz is usedSpectrum IssuesThe narrowest Eureka 147 configuration uses 1.5 MHz to transmit six stereo channels

In-Band Digital Radioin-band schemes that convey digital audio signals in existing FM (88 to 108 MHz) and AM (510 to 1710 kHz) bands along with analog radio signals.These systems are hybrids because the analog and digital signals are broadcast simultaneouslypermit broadcasters to simultaneously transmit analog and digital programsdigital receiver is able to reject the analog signalsit is more difficult for an analog receiver to reject the digital signal's interferencein-band on-channel (IBOC) systemDAR signals are superimposed on current FM and AM transmission frequencies

in-band on-channel (IBOC) systemit fits within much of the existing regulatory statutes and commercial interestsNo modifications of existing analog AM and FM receivers are required, and DAR sets receive both analog and digital signalsstart-up costs are lowProvides improved frequency response, and lower noise and distortion within existing coverage areasReceivers can be designed so that if the digital signal is lost, the radio will automatically switch to the analog signaliBiquity Digital Radiowas created by the merger of USA Digital Radio and Lucent Digital Radio, two early proponents of IBOC technologyiBiquity IBOC system provides a method of transmitting compact-disc quality audio signals to radio receivers along with data services, such as station, song and artist identification, stock and news information, and local traffic and weatherThe system allows existing radio stations to use their current AM and FM spectrum to transmit analog signals simultaneously with new higher quality digital signalsNational Radio Systems CommitteeNRSC is an industry standards setting body sponsored by the National Association of Broadcasters (NAB) and the Consumer Electronics Association (CEA)FCC ActionsOrder issued October 10, 2002, the FCC selected in-band, on-channel (IBOC) as the technology to bring the benefits of digital audio broadcasting to AM and FM radio broadcasters efficiently and rapidlyIBOC AM Digital Radio SystemiBiquity AM IBOC Systemsupports transmission of digital audio and auxiliary digital data within an existing AM channel allocation by placing six groups of digitally modulated carrier signals within and adjacent to an analog AM signalthe AM IBOC system is not compatible with analog AM stereo signalsOrthogonal frequency division multiplexing (OFDM) modulation is utilized in the AM IBOC system

Test Program IssuesProximity of digital sidebands to first-adjacent channel signalsinterference with a first-adjacent analog AM signal

Test Program IssuesProximity of digital sidebands to second-adjacent channel signalspotentially interfere with (and receive interference from) a second-adjacent AM signals digital sidebands

Test Program IssuesProximity of digital sidebands to third-adjacent channel

Test Program IssuesBlend-to-analogThe point at which the BLER of an AM IBOC receiver falls below some predefined threshold and the digital audio is faded out while the analog audio is simultaneously faded in. This prevents the received audio from simply muting when the digital signal is lost. The receiver audio will also blend to digital upon re-acquisition of the digital signal.

BLER (block error rate) A ratio of the number of data blocks received with at least one erroneous bit to the number of blocks received.

The iBiquity AM IBOC system simulcasts a radio stations main channel audio signal using the analog AM carrier and IBOC digital sidebands, and under certain circumstances, the IBOC receiver will blend back and forth between these two signals

Analog Compatibilitythe AM IBOC system was found to have little effect on the host analog signalthe amount of interference to the host analog signal was receiver-dependentCo-channel compatibilityAM IBOC was not expected to have any impact on the level of co-channel interference due to the design of the AM IBOC systemAnalog CompatibilityFirst adjacent compatibility.the interference caused by the introduction of the IBOC signal was predominantly determined by the D/U ratioD/URatio of desired and undesired signalsFCC allocation rules permit 6 dB D/U ratios at an AM stations daytime protected contourIBOC FM Digital Radio SystemiBiquity FM IBOC Systemsupports transmission of digital audio and auxiliary digital data within an existing FM channel allocation by placing two groups of digitally modulated carrier signals adjacent to an analog FM signal

NRSC Test IssuesProximity of digital sidebands to first-adjacent channel signals

NRSC Test IssuesProximity of digital sidebands to second-adjacent channel signalsThe FM IBOC system design allows for approximately 4 kHz of guard band between second-adjacent IBOC digital sidebands

NRSC Test IssuesBlend-to-analogThe iBiquity FM IBOC system simulcasts a radio stations main channel audio signal using the analog FM carrier and IBOC digital sidebands, and under certain circumstances, the IBOC receiver will blend back and forth between these two signalsSummaryAM Broadcast StandardsParametersPhilippine StandardsInternational StandardsFrequency Band535-1605kHz535-1605kHzNo. Of channels118107Bandwidth/Ch9kHz10kHzPermitted bandwidth30kHz30kHzChannel Spacing36kHz30kHzCenter Freq Stability20Hz20HzBaseband Freq50-15000Hz50-15000HzType of ModulationAMAMType of EmissionA3EA3EGuardband36kHz30kHzAntenna PolarizationVerticalVerticalType of receiverSuperhetSuperhetIntermediate Freq455kHz455kHzFM Broadcast StandardsParametersPhilippine StandardsFrequency Band88-108 MHzNo. Of channels25Bandwidth/Ch200kHz Permitted bandwidth240kHz (mono)Channel Spacing800kHzCenter Freq Stability2kHzBaseband Freq50-15000HzType of ModulationFMType of EmissionA3EGuardband25kHzAntenna PolarizationHorizontal / circularFrequency Deviation75kHzType of receiverSuperhetIntermediate Freq10MHzPilot Carrier19kHzSubcarrier38kHzFM Broadcast Frequency allocation

FM = Channel Frequency in MHz FM1 = 1st FM Channel 88.1MHz n = channel number BW = channel Bandwidth (200kHz)