upadhyay jay

8/7/2019 Upadhyay Jay

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Advantages of Digital BroadcastingAdvantages of Digital Broadcasting

Better signal to noise ratioBetter signal to noise ratio

Reduced interferenceReduced interference

Possible single frequency networksPossible single frequency networks

Less multipath distortionLess multipath distortion

Automatic tuningAutomatic tuning

Auxiliary data servicesAuxiliary data services


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DRMDRM -- Digital Radio MondialeDigital Radio Mondiale

Developed by a group including broadcasters, transmitterDeveloped by a group including broadcasters, transmitterand receiver manufacturers, research labs, universities,and receiver manufacturers, research labs, universities,and government agencies.and government agencies.

Designed to eventually replace AM in the present AMDesigned to eventually replace AM in the present AM

bands (0.15 to 30 MHz).bands (0.15 to 30 MHz).

Transmitted bandwidth typically 9 kHz. Standard coversTransmitted bandwidth typically 9 kHz. Standard covers4.5 to 20 kHz.4.5 to 20 kHz.

Complete specifications documented with ETSI and ISO.Complete specifications documented with ETSI and ISO.Consumer receivers just starting production.Consumer receivers just starting production.

Different configurations for local and shortwave conditions.Different configurations for local and shortwave conditions.


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Processing of SignalsProcessing of Signals A/D conversionA/D conversion

Audio processingAudio processing

Source codingSource coding -- MPEGMPEGAACAAC

Forward error correctForward error correct--ion (FEC) codingion (FEC) coding

OFDM (OrthogonalOFDM (Orthogonal

frequency divisionfrequency divisionmultiplex)multiplex)

Channel codingChannel coding -- timetime

+frequency interleave+frequency interleave

Digital modulation ofDigital modulation ofsubcarrierssubcarriers

Inverse Fast FourierInverse Fast FourierTransform (IFFT)Transform (IFFT)

InterpolationInterpolation

RF frequency generationRF frequency generationand modulation in DDSand modulation in DDS

(direct digital synthesizer)(direct digital synthesizer) UpconversionUpconversion

Power amplificationPower amplification


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Audio ProcessingAudio Processing The minimum amount of control is automatic peakThe minimum amount of control is automatic peak

limiting, that is, reducing the amplification whenever thelimiting, that is, reducing the amplification whenever theinput level is so high that overload and clipping will beinput level is so high that overload and clipping will beproduced in later steps.produced in later steps.

Audio levels in program material vary with the sourceAudio levels in program material vary with the source

(live microphone, taped, satellite, CDs etc and with the(live microphone, taped, satellite, CDs etc and with theskill and attention level of the operator. It is desirable toskill and attention level of the operator. It is desirable tohave some automatic control of audio levels.have some automatic control of audio levels.

When most listeners are located in noisy environmentsWhen most listeners are located in noisy environmentssuch as automobiles, it is good to reduce the dynamicsuch as automobiles, it is good to reduce the dynamicrange (ratio of soft to loud sounds) so that soft music orrange (ratio of soft to loud sounds) so that soft music orspeech will not be lost in the ambient noise.speech will not be lost in the ambient noise.


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Fortunately its possible to reduce these bit rates drasticallyFortunately its possible to reduce these bit rates drastically

with little or no perceptible loss of audio quality.with little or no perceptible loss of audio quality.The coding technique for high quality audio signals usesThe coding technique for high quality audio signals usesthe properties of human sound perception by exploitingthe properties of human sound perception by exploitingthe spectral and temporal masking effects of the ear.the spectral and temporal masking effects of the ear.

Source CodingSource Coding The output bit rates of the A/D converter are much greaterThe output bit rates of the A/D converter are much greaterthan can be transmitted in reasonable broadcast bandwidths.than can be transmitted in reasonable broadcast bandwidths.(1536 kbits/second from the A/D converter for one channel.(1536 kbits/second from the A/D converter for one channel.

MPEG Advanced Audio Coding (AAC) perceptual codingMPEG Advanced Audio Coding (AAC) perceptual codingallows great bit rate reduction while maintaining excellentallows great bit rate reduction while maintaining excellentaudio quality. CD quality stereo requires only 96 to 128 kaudio quality. CD quality stereo requires only 96 to 128 kbits/s. and good monaural AM 16 to 24 kb/s. It is specifiedbits/s. and good monaural AM 16 to 24 kb/s. It is specified

as the source coding method for DRM.as the source coding method for DRM.


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Error Correction CodingError Correction Coding

Bit errors due to noise, interference from other transBit errors due to noise, interference from other trans--mitters, and multipath propagation produce audiblemitters, and multipath propagation produce audibleclicks, squawks, and other noises at the receiver.clicks, squawks, and other noises at the receiver.Forward Error Correction (FEC) coding adds calculatedForward Error Correction (FEC) coding adds calculatedbytes or bits to the signal to make it possible to detectbytes or bits to the signal to make it possible to detect

and in many cases correct erroneous bits at the receiver.and in many cases correct erroneous bits at the receiver.

Similar codes are used to make compact disks (CDs)Similar codes are used to make compact disks (CDs)and hard disk drives nearly error free in spite ofand hard disk drives nearly error free in spite ofimperfections in the recording medium.imperfections in the recording medium.

Good error correction is a major consideration in theGood error correction is a major consideration in thedesign of these systems. FEC is most effective whendesign of these systems. FEC is most effective whenerror bits are isolated or come in short bursts.error bits are isolated or come in short bursts.


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Error Correction Coding (2)Error Correction Coding (2)

Multipath propagation tends to make theMultipath propagation tends to make thedelayed signal interfere with later bits of thedelayed signal interfere with later bits of thedesired signal. It can also result in adesired signal. It can also result in adelayed signal being 180 deg. out of phasedelayed signal being 180 deg. out of phase

and canceling a desired signal. With aand canceling a desired signal. With amulticarrier signal this normally affects onemulticarrier signal this normally affects oneor a few adjacent carriers at a given time.or a few adjacent carriers at a given time.The bits of the data stream are scatteredThe bits of the data stream are scattered

among the carriers so that errors on severalamong the carriers so that errors on severaladjacent carriers become isolated errors inadjacent carriers become isolated errors inthe reconstituted bit stream.the reconstituted bit stream.


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Orthogonal Frequency DivisionOrthogonal Frequency DivisionMultiplexMultiplex

Uses a relatively large number of subcarriers that fill theUses a relatively large number of subcarriers that fill theallocated bandwidth with a low data rate per carrier.allocated bandwidth with a low data rate per carrier.

Used to minimize multipath problems.Used to minimize multipath problems.

DRM Mode ADRM Mode A -- 24 carriers/kHz = 204 in 9 kHz24 carriers/kHz = 204 in 9 kHz

Mode B (SW)Mode B (SW) -- 21 carriers/kHz = 182 in 9 kHz21 carriers/kHz = 182 in 9 kHz

Modulation of each carrier is QAM 4, 16, or 64 states.Modulation of each carrier is QAM 4, 16, or 64 states.

Synchronization signals are distributed thru each frameSynchronization signals are distributed thru each frameto give the receiver a phase and amplitude reference forto give the receiver a phase and amplitude reference foreach carrier.each carrier.


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History of OFDMHistory of OFDM

OFDM has been used in radio communicationOFDM has been used in radio communicationfor 50 years. The Collins Kineplex system infor 50 years. The Collins Kineplex system inthe 1950s put 24 300the 1950s put 24 300--baud teletype channelsbaud teletype channels

on a single SSB transmitter. The mathematicalon a single SSB transmitter. The mathematicaltheory of the system probably dates from thetheory of the system probably dates from the1930s.1930s.

Implementation of OFDM with analog circuitsImplementation of OFDM with analog circuitswas very complex. The 24 channel receiverwas very complex. The 24 channel receivertook up nearly all of a 6 foot rack.took up nearly all of a 6 foot rack.


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Phase and AmplitudePhase and AmplitudeModulation of SubcarriersModulation of Subcarriers

Various possibilitiesVarious possibilities

Simplest form isSimplest form isBPSK (binary phaseBPSK (binary phase

shift keying)shift keying)

Reference phase = 0Reference phase = 0degrees (0 Logical)degrees (0 Logical)

Other state = 180Other state = 180degrees (1 Logical)degrees (1 Logical)

1 bit / Hz1 bit / Hz


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Quadrature Amplitude ModulationQuadrature Amplitude Modulation


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Quadrature Amplitude Modulation (2)Quadrature Amplitude Modulation (2)


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Inverse Fast Fourier TransformInverse Fast Fourier Transform

Converts desired frequency spectrum to inConverts desired frequency spectrum to in--phase and quadrature waveformsphase and quadrature waveforms

Efficiently executed in DSP (digital signalEfficiently executed in DSP (digital signalprocessor)processor)

Interpolation (smoothing)Interpolation (smoothing)

Output of IFFT is a stepped waveformOutput of IFFT is a stepped waveform

Must be smoothed to avoid producingMust be smoothed to avoid producingspurious sidebandsspurious sidebands


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Power AmplificationPower Amplification

Typical peakTypical peak--toto--average power ratio for multiaverage power ratio for multi--carriercarrier

systems is 10:1 (10 dB) with infrequent higher peaks.systems is 10:1 (10 dB) with infrequent higher peaks.DRM requires high power for international broadcastingDRM requires high power for international broadcasting(25 to 100 kW average power)(25 to 100 kW average power)

A linear amplifier will have about 20 % efficiency for thisA linear amplifier will have about 20 % efficiency for this

type of signal. For example, 125 kW dc in for 25 kWtype of signal. For example, 125 kW dc in for 25 kWaverage power out.average power out.

A multicarrier signal can be amplified with 75 to 80%A multicarrier signal can be amplified with 75 to 80%efficiency by splitting it into two signals.efficiency by splitting it into two signals.1. A constant amplitude RF signal with the phase variations1. A constant amplitude RF signal with the phase variationsof the desired output signal (including phase reversals).of the desired output signal (including phase reversals).2. An amplitude (envelope) signal defining the2. An amplitude (envelope) signal defining theinstantaneous amplitude of the desired output.instantaneous amplitude of the desired output.


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Power Amplification (2)Power Amplification (2)

The phase modulated signal can be amplified by theThe phase modulated signal can be amplified by theclass C intermediate and driver amplifiers of a typicalclass C intermediate and driver amplifiers of a typicallocal AM or short wave transmitter.local AM or short wave transmitter.

A class C anode modulated amplifier effectivelyA class C anode modulated amplifier effectively

multiplies the phase modulated signal by its anodemultiplies the phase modulated signal by its anodevoltage (the envelope signal), producing the desiredvoltage (the envelope signal), producing the desiredoutput.output.

The time delay (phase shift) of the phase andThe time delay (phase shift) of the phase and

amplitude signals must be matched to a fraction of aamplitude signals must be matched to a fraction of amicrosecond . The OFDM signal contains phasemicrosecond . The OFDM signal contains phasereversals (points at which the amplitude goes rapidlyreversals (points at which the amplitude goes rapidlyto 0 and then increases with reversed phase).to 0 and then increases with reversed phase).


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MathematicalMathematicalanalysis shows thatanalysis shows thatthough the OFDM I/Qthough the OFDM I/Qsignal is limited to itssignal is limited to itsdesignated bandwidthdesignated bandwidthwith only a smallwith only a smallamount of outamount of out--ofof--

band energy, theband energy, thephase and amplitudephase and amplitudesignals both aresignals both arewideband. If they arewideband. If they areproperly matchedproperly matched

their outtheir out--ofof--bandbandsidebands cancel.sidebands cancel.

Spectra of signals forSpectra of signals foramplitudeamplitude--phasephase

amplificationamplification


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Transmitter RequirementsTransmitter Requirements

The fundamental requirement is that theThe fundamental requirement is that thetransmitter function as a linear amplifier and thistransmitter function as a linear amplifier and thissection shows how linear amplification can besection shows how linear amplification can beachieved with existing AM broadcast transmitterachieved with existing AM broadcast transmitterdesigns.designs.

As has been described in the previous section, theAs has been described in the previous section, theDRM signal from the OFDM modulator takes theDRM signal from the OFDM modulator takes theform of a group of equally spaced carriers, withform of a group of equally spaced carriers, with

the digital information being modulated onto thethe digital information being modulated onto thecarriers in terms of phase and amplitude.carriers in terms of phase and amplitude.

This signal is termed an I/Q signal, as it isThis signal is termed an I/Q signal, as it iscomplex, and contains Incomplex, and contains In--phase and Quadraturephase and Quadrature

components.components.


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In raising the level of this signal to the power requiredIn raising the level of this signal to the power required

for broadcast transmission it is imperative that thefor broadcast transmission it is imperative that thecorrect phase and amplitude relationship of the "I"correct phase and amplitude relationship of the "I"and "Q" components is maintained. In other words,and "Q" components is maintained. In other words,the DRM signal must not be distorted in the powerthe DRM signal must not be distorted in the power

amplification process.amplification process. If the signal is distorted, errors will be introduced andIf the signal is distorted, errors will be introduced and

the Bit Error Rate (BER) may fall to unacceptablethe Bit Error Rate (BER) may fall to unacceptablelevels and the DRM signal be unusable and/or outlevels and the DRM signal be unusable and/or out--ofof--

band radiation become excessive. To avoid distortion,band radiation become excessive. To avoid distortion,the power amplifier must therefore have a linearthe power amplifier must therefore have a lineartransfer function such that the output signal is antransfer function such that the output signal is anexact replica of the input but at a higher power level.exact replica of the input but at a higher power level.


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A factor that has to be borne in mind for a DRMA factor that has to be borne in mind for a DRMamplifier, is the peak to mean ratio of the DRMamplifier, is the peak to mean ratio of the DRM

signal. The power level of a DRM signal is generallysignal. The power level of a DRM signal is generallystated in terms of its mean or average value;stated in terms of its mean or average value;however the instantaneous peak amplitude of thehowever the instantaneous peak amplitude of thecombined carriers exceeds this value by a largecombined carriers exceeds this value by a largeamount. A typical DRM signal has a peak to meanamount. A typical DRM signal has a peak to meanratio of 10dB, thus an amplifier producing a signalratio of 10dB, thus an amplifier producing a signalhaving an average power of 10kW needs a peakhaving an average power of 10kW needs a peakpower capability of 100kW.power capability of 100kW.

In a practical amplifier, a point is reached whereIn a practical amplifier, a point is reached wheregain compression occurs and the transfergain compression occurs and the transfercharacteristic flattens out. The amplifier must becharacteristic flattens out. The amplifier must beoperated on the linear portion of its characteristic.operated on the linear portion of its characteristic.


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It is possible to construct a linear amplifier to provideIt is possible to construct a linear amplifier to providethe power level required for broadcast transmission, butthe power level required for broadcast transmission, but

its energy conversion efficiency is very poor, typicallyits energy conversion efficiency is very poor, typically2020 30%; significant cooling is required and operating30%; significant cooling is required and operatingcosts will be high.costs will be high.

Although some earlier low power transmitters usedAlthough some earlier low power transmitters used

linear amplifiers, high power AM transmitters invariablylinear amplifiers, high power AM transmitters invariablyuse nonuse non--linear class C operation to achieve highlinear class C operation to achieve highconversion efficiency. In a valve (electronic tube)conversion efficiency. In a valve (electronic tube)transmitter, the grid bias voltage is chosen such thattransmitter, the grid bias voltage is chosen such that

the valve conducts over a limited range of the RF cyclethe valve conducts over a limited range of the RF cycleand effectively delivers energy to the resonant anodeand effectively delivers energy to the resonant anodecircuit as a series of pulses. The resulting RF power iscircuit as a series of pulses. The resulting RF power iscoupled from the anode circuit to the antenna.coupled from the anode circuit to the antenna.


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With the use of modern high power valve technologyWith the use of modern high power valve technologyand efficient cooling systems, very high output powerand efficient cooling systems, very high output powercan be achieved for relatively low drive power with highcan be achieved for relatively low drive power with highconversion efficiency.conversion efficiency.

SolidSolid--state modular MF/LF transmitters use a switchingstate modular MF/LF transmitters use a switchingtechnique to achieve high conversion efficiency,technique to achieve high conversion efficiency,typically between 70 and 80%. The output stage oftypically between 70 and 80%. The output stage ofeach power amplifier module uses MOSFET transistorseach power amplifier module uses MOSFET transistorsas switches arranged in an "H Bridge" arrangement. RFas switches arranged in an "H Bridge" arrangement. RFpower is taken from a transformer connected betweenpower is taken from a transformer connected betweenthe midthe mid--points of each arm. In operation, diagonallypoints of each arm. In operation, diagonally

opposite transistors are sequentially switched at carrieropposite transistors are sequentially switched at carrierfrequency rate to produce alternate current reversals infrequency rate to produce alternate current reversals inthe output transformer primary. In this way high RFthe output transformer primary. In this way high RFpower levels are generated at high conversionpower levels are generated at high conversionefficiency.efficiency.


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These nonThese non--linear amplifiers cannot be used directlylinear amplifiers cannot be used directlyfor DRM signals. However a modulated nonfor DRM signals. However a modulated non--linearlinear

amplifier can be driven with suitable RF and baseamplifier can be driven with suitable RF and base--band signals derived from the original low levelband signals derived from the original low levelcomplex I/Q signal, such that the component signalscomplex I/Q signal, such that the component signalscombine in the modulated final amplifier to form acombine in the modulated final amplifier to form ahigh level replica of the original signal.high level replica of the original signal.

The overall effect is that the modulated amplifierThe overall effect is that the modulated amplifierfunctions as a linear amplifier even though thefunctions as a linear amplifier even though theamplifier itself continues to work in a nonamplifier itself continues to work in a non--linearlinear

manner.manner.

Although a modular solidAlthough a modular solid--state MF/LF transmitterstate MF/LF transmitterdoes not have a separate modulated amplifier asdoes not have a separate modulated amplifier assuch, the functionality is identical.such, the functionality is identical.


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The two signals derived from the basic DRMThe two signals derived from the basic DRMI/Q signal are termed the RF Phase (RFP) andI/Q signal are termed the RF Phase (RFP) and

Amplitude (A). The processing to derive theseAmplitude (A). The processing to derive thesesignals is contained within the DRM Modulatorsignals is contained within the DRM Modulatorand generally the modulator will provide bothand generally the modulator will provide bothI/Q and A/RFP outputs.I/Q and A/RFP outputs.

For this technique to work correctly there are aFor this technique to work correctly there are anumber of requirements that must be satisfiednumber of requirements that must be satisfiedby the transmitter. Firstly, there must be aby the transmitter. Firstly, there must be adirect (DC) connection between the modulatordirect (DC) connection between the modulatorand the final amplifier. Unfortunately thisand the final amplifier. Unfortunately thismeans that the A/RFP technique cannot bemeans that the A/RFP technique cannot beused with transmitters having Class Bused with transmitters having Class Btransformer coupled modulators.transformer coupled modulators.


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For this reason the widely used Continental 418For this reason the widely used Continental 418AA--F short wave transmitters cannot be used forF short wave transmitters cannot be used for

DRM without modification, but Continental canDRM without modification, but Continental cansupply a G upgrade using a pulse step modulatorsupply a G upgrade using a pulse step modulatorwhich makes the transmitter DRM capable.which makes the transmitter DRM capable.

Operation in a DRM simulcast mode does notOperation in a DRM simulcast mode does notrequire DC coupling, but it still requires a greaterrequire DC coupling, but it still requires a greateraudio bandwidth than is possible with typicalaudio bandwidth than is possible with typicalmodulation transformers.modulation transformers.

The Harris HF100 PDM modulator can be DCThe Harris HF100 PDM modulator can be DCcoupled but it is necessary to raise the PDMcoupled but it is necessary to raise the PDMswitching frequency to get the needed audioswitching frequency to get the needed audiobandwidthbandwidth..


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Secondly, the bandwidth of the audio path in theSecondly, the bandwidth of the audio path in thetransmitter needs to be significantly greater than thattransmitter needs to be significantly greater than that

required for normal AM working. Typically, it should berequired for normal AM working. Typically, it should bethree to four times the bandwidth of the wanted DRMthree to four times the bandwidth of the wanted DRMsignal.signal.

The sampling frequency of solidThe sampling frequency of solid--state Pulse Step orstate Pulse Step or

Pulse Duration Modulators (PDM/PSM) must be morePulse Duration Modulators (PDM/PSM) must be morethan twice this frequency limit to meet Nyquist criteria.than twice this frequency limit to meet Nyquist criteria.

Any bandwidth limiting filters in the audio path mustAny bandwidth limiting filters in the audio path must

be removed and the modulator output filter will needbe removed and the modulator output filter will needto be modified to achieve the required bandwidth. Into be modified to achieve the required bandwidth. Inmodifying the filter response it is important to ensuremodifying the filter response it is important to ensurethat a substantially flat group delay characteristic isthat a substantially flat group delay characteristic ismaintained over the passmaintained over the pass--band.band.


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In modifying filters there are two points toIn modifying filters there are two points tobear in mind. Firstly, in some solidbear in mind. Firstly, in some solid--statestate

modulator designs the modulator output ismodulator designs the modulator output isslightly inductive and this will need to beslightly inductive and this will need to betaken into account when designing the filter.taken into account when designing the filter.Secondly, the final amplifier valve providesSecondly, the final amplifier valve provides

the load for the filter. It is important tothe load for the filter. It is important toremember that although this is predominantlyremember that although this is predominantlyresistive, the transmitter RF output circuit willresistive, the transmitter RF output circuit willprovide some shunt capacitance. Dependingprovide some shunt capacitance. Depending

on output circuit configuration, the shunton output circuit configuration, the shuntcapacitance existing at some tuning settingscapacitance existing at some tuning settingsmay be significant and need to be taken intomay be significant and need to be taken intoaccount in the filter design.account in the filter design.


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Current HF transmitter designs use a tetrode valve asCurrent HF transmitter designs use a tetrode valve asthe modulated amplifier. In order to achieve linearthe modulated amplifier. In order to achieve linear

modulation to 100% with a tetrode, it is necessary tomodulation to 100% with a tetrode, it is necessary toapply modulation to the screen grid electrode. This isapply modulation to the screen grid electrode. This isusually done by applying a fraction of the modulatingusually done by applying a fraction of the modulatingsignal to the screen grid, or by allowing the screen gridsignal to the screen grid, or by allowing the screen grid

to "self modulate", by including an inductance in theto "self modulate", by including an inductance in thescreen grid circuit. The audio component in the screenscreen grid circuit. The audio component in the screencurrent develops an AC voltage across the inductor andcurrent develops an AC voltage across the inductor andthis AC voltage serves to modulate the screen.this AC voltage serves to modulate the screen.

It is also necessary to decouple the screen grid, so thatIt is also necessary to decouple the screen grid, so thatit is at RF ground potential, for proper operation of theit is at RF ground potential, for proper operation of theamplifier.amplifier.


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In some transmitter designs the value of decouplingIn some transmitter designs the value of decouplingcapacitance used may be sufficiently high to becomecapacitance used may be sufficiently high to become

significant at the higher frequencies now required tosignificant at the higher frequencies now required tobe handled in the amplitude path. This can degradebe handled in the amplitude path. This can degradethe response at the upper end of the pass band. Itthe response at the upper end of the pass band. Itmay therefore be necessary to reduce the value ofmay therefore be necessary to reduce the value ofthe screen decoupling capacitor, but this must bethe screen decoupling capacitor, but this must be

done with great care in order not to disturb the RFdone with great care in order not to disturb the RFperformance of the amplifier.performance of the amplifier.

The decoupling capacitor is often an integral part ofThe decoupling capacitor is often an integral part ofthe final amplifier valve socket made in the form of anthe final amplifier valve socket made in the form of an

annular ring of dielectric film, with electrodesannular ring of dielectric film, with electrodesdeposited on each side of the ring. One possibledeposited on each side of the ring. One possibletechnique is to add a second ringtechnique is to add a second ring this effectivelythis effectivelyhalves the decoupling capacitance.halves the decoupling capacitance.


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Even when all of the above points have beenEven when all of the above points have been

taken into account, it may not always be possibletaken into account, it may not always be possibleto directly achieve the required bandwidth andto directly achieve the required bandwidth andgroup delay response from the transmitter. Ingroup delay response from the transmitter. Inthis situation some degree of prethis situation some degree of pre--correction maycorrection may

be needed. DRM Modulators now include somebe needed. DRM Modulators now include someform of preform of pre--correction. In some implementations,correction. In some implementations,the prethe pre--correction is usercorrection is user--adjustable and is set onadjustable and is set ontest to give the best response, in others, the pretest to give the best response, in others, the pre--

correction is set at the factory and detailedcorrection is set at the factory and detailedresponse measurements of the transmitter areresponse measurements of the transmitter aretaken and provided to the DRM Modulatortaken and provided to the DRM Modulatorsupplier, who sets the presupplier, who sets the pre--correction.correction.


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For frequency agile HF transmitters, it willFor frequency agile HF transmitters, it willgenerally be necessary to use some form ofgenerally be necessary to use some form of

dynamic phase equalisation in order to retaindynamic phase equalisation in order to retainthe correct phase relationship between the Athe correct phase relationship between the Aand RFP signals as the transmitter switchesand RFP signals as the transmitter switches

between the various broadcast bands.between the various broadcast bands.Dynamic phase equalisation is generallyDynamic phase equalisation is generallyprovided as an option with a DRM Modulator.provided as an option with a DRM Modulator.


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Transmitter RequirementsTransmitter Requirements The HC100 and other modern transmitters using pulseThe HC100 and other modern transmitters using pulse--

step modulation can be modified to transmit OFDM signal instep modulation can be modified to transmit OFDM signal inconformance with the DRM specification. They typicallyconformance with the DRM specification. They typicallyhave 10 to 150 microseconds of delay in their amplitudehave 10 to 150 microseconds of delay in their amplitudecircuits but only a few microseconds in the RF amplifiers.circuits but only a few microseconds in the RF amplifiers.

(1) An audio server includes A/D conversion, audio(1) An audio server includes A/D conversion, audioprocessing and MPEG source encoding to deliver a signal atprocessing and MPEG source encoding to deliver a signal atthe required bitrate (An industrial PC).the required bitrate (An industrial PC).

(2) An OFDM exciter implements Forward Error Correction,(2) An OFDM exciter implements Forward Error Correction,

Channel Coding, the IFFT and interpolation, inChannel Coding, the IFFT and interpolation, in--phase (I)phase (I)and quadrature (Q) signal generation, derives Amplitudeand quadrature (Q) signal generation, derives Amplitudeand Phase signals from I and Q, compensates for timeand Phase signals from I and Q, compensates for timedelay of the amplitude signal, and sends phase anddelay of the amplitude signal, and sends phase andoperating frequency data to the Direct Digital Synthesizeroperating frequency data to the Direct Digital Synthesizer(one PCB or DSP and many lines of embedded software).(one PCB or DSP and many lines of embedded software).


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HC100 ChangesHC100 Changes

(1) An Audio Controller with a higher switching rate to(1) An Audio Controller with a higher switching rate to

produce the fast amplitude transitions of the OFDMproduce the fast amplitude transitions of the OFDMsignal and control the screen supply. (One PCB)signal and control the screen supply. (One PCB)

(2) A high voltage low pass filter with wider bandwidth(2) A high voltage low pass filter with wider bandwidthand less phase shiftand less phase shift

(3) A modulated Power Amplifier Screen supply.(3) A modulated Power Amplifier Screen supply.Basically a simplified low power version of the pulseBasically a simplified low power version of the pulse--step modulated anode (plate) supply.step modulated anode (plate) supply.

(4) Redesign of the fiber optic links and switching(4) Redesign of the fiber optic links and switchingmodules in the modulator for more uniform turnmodules in the modulator for more uniform turn--on andon andturnturn--off times.off times.

This redesign is in process at the Engineering Center inThis redesign is in process at the Engineering Center in

ElkhartElkhart


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