Larcan DTT250M Operating and maintenance instructions

GENERAL DESCRIPTION
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TSM 20-275D rev 0: Jul 1, 2010 1 DTT250M
DTT250M
–
250W Di
g
ital Television Transmitte
r
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INTRODUCTION
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ThismanualdescribestheLARCANmodelDTT250MVHFDigitalTelevisionTransmitter.
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LARCANall‐solid‐state250WVHFtransmittersaredesignedtooperateconservativelyat250Waverage
DTVpowerwithsuperbperformance,reliabilityandoperatingeconomy.
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Thetransmitterandexciterortranslatorchassisarepackagedinasingle19"cabinet.Thesimplicityof
design,thedeploymentofallmodularandothersubassemblies,andtheuseofstandardreadily
availablecomponents,enhancesserviceability.
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Importanttransmitterparametersaremonitored,andcanbedisplayedonthemeterbuiltintothe
amplifier.Additionally,allmeterreadingsaremadeavailableasDCsignalsfortelemetrybyremote
controlsystems.TheDTT250M,likeallotherLARCANtransmittingequipment,issuitablefor
automaticorremote‐controloperation.
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AMPLIFIERCHAIN
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TheRFoutputoftheexciterisfedtoaconservativelydesignedbroadbandsolid‐stateamplifier.This
amplifierrequiresnotuningoradjustmentwithinitsbandofoperation.Simplicityofoperation,
reducedmaintenancecostsandincreasedreliabilityareafewofthemajorbenefitsderivedfromthis
modularamplifier.Thismoduleisoperatedwellbelowitsmaximumratings.
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Theamplifierchainconsistsofthreestagesofamplification.
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Thepreamplifierstageisahighgain,broadband,thin‐filmintegratedcircuitamplifieroperatingclassA.
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TheIPAstageconsistsofapairofpush‐pullFETsinasinglecase,operatinginclassABasalinear
amplifier.ThisamplifierusestheidenticaldualFETdevicethatisusedbythePAmodule.
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Thefinalamplifierstageconsistsofsixpush‐pullFETamplifiersthatoperateinclassAB,inthreegroups
oftwoinquadrature,andarecombinedinquadratureandthenina3‐waycombiner.Theamplifier
moduleisratedfor250wattsaverageATSCoutput.Themoduleisprovidedwithsoft‐start,VSWR
protection,andamonitorport.
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Theamplifieroutputisfedtothebandpassfilterandthedirectionalcoupler,whichprovidesasmall
sampleofforwardandreflectedoutputpowerforAGCandVSWRsupervisoryfunctions.The
transmitteroutputthenpassestotheantennasystem.
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GENERAL DESCRIPTION
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TSM 20-275D rev 0: Jul 1, 2010 2 DTT250M
DTT250M
–
250W Di
g
ital Television Transmitte
r
TRANSMITTERCONTROL
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Thecontrolcircuitryinthissolidstatetransmitterissimple.Interlockingconsistsoftheenabling
circuitrynecessarytoensurethatanyexternalpatchpanellinkoperation,orRFswitching,canonlybe
donewithRFturnedoff.
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nableJ5‐5.
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Allcontrolwiringofthetransmitterpassesthroughacontrolcircuitboard(prefix5B),andfacilitiesare
providedonthisboardfortelemetry,status,andcontrolconnectionstoandfromaremotecontrol
system.Theseareavailableon15contactD‐shellconnectorJ5.
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Forlocaloperation,simplyplacetheLOC‐REMswitchintheLOCposition.Forremotecontrol
operationtheLOC‐REMswitchmustbeintheREMposition.Thisplaces+12VonRemoteE
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TheRemoteEnable+12VappearsasanarmingsignalatJ5‐5,andthemomentaryconnectionofthis
+12VtoJ5‐13turnsthetransmitterON,andmomentaryconnectionofthe+12VtoJ5‐8turnsthe
transmitterOFF.
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ThetransmittercontrolandinterlockwiringisalsobroughtoutonJ3,whichisprovidedwithaterminal
blockstyleofconnectorinterface.RemoteEnable,RemoteOn,RemoteOff,andExternalInterlocks1
and2areallbroughtoutonJ3forconnectionasrequired.
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AthermostatisprovidedinthePAheatsinktoopentheinterlockchainshouldanunlikelyoverheating
conditionoccur.

CONTENTS
1BANDPASS FILTER ........................................................................................................................................................2
2RF DIRECTIONAL COUPLER......................................................................................................................................4
PUB96-26 Rev 1 September 13, 2005 26-1 RF Output: BP Filter & Directional Coupler

1 BANDPASS FILTER
Drawing References: Figure 1 and Figure 4.
The LARCAN bandpass filter implementation consists of a cascaded series of coupled helical resonators. A
helical resonator is essentially a self supporting high Q coil (the helix) mounted inside a metallic shield enclosure.
One end of the coil is solidly connected to the shield enclosure and the other end is open circuited except for a
small trimmer capacitance to ground. The dimensions of the coil are critical as to frequency of operation; the
assembly behaves as though it were a quarter wave coaxial transmission line resonator. Several sizes of coils
and enclosures are necessary to cover the desired frequency ranges. Figure 4 indicates the generic assembly of
a coupled helical resonator bandpass filter.
The referenced drawing in Figure 4 is a low band filter, but the high band unit is laid out identically and appears
almost the same, except the high band helixes have fewer turns of coarser winding pitch, and their shield
enclosure dimensions are somewhat smaller.
The desired response shape is presented as Figure 1, and the filter electrical equivalents are presented as Figure
2. When we examine the assembly, and take capacitances into account, the equivalent circuit of a helical
resonator becomes simply a parallel resonant LC tank circuit having low (trimmer) capacitance and relatively high
inductance. Adjustment of the trimmer produces a change of capacitance, and the trimmer's moveable slug is
shaped to appear as a shorted turn, which alters the inductance of the helix.
Matching from and to 50 ohm transmission lines is accomplished with taps on the input and output helixes.
Coupling between sections is electrically a bridged T network of capacitors, and is made up of the small
capacitance between the free ends of the coils, controllable by the amount of capacitance to ground that is
introduced by the coupling adjustment screws; the coupling is maximum when the screws are backed out fully
from the enclosure. Shielding partitions placed inside the enclosure between helixes, produce fixed area
apertures which affect the coupling capacitance between helixes. Helix #3 in Figure 4 has taller partitions on both
sides of it, giving lower capacitance and less coupling than the others.
For system use, the tuning and coupling is adjusted for a flat topped response with steep sides, and the desired
shape is such that fV- 4.5 MHz and fV+ 9.0 MHz are both 30 dB down, but the carriers must be fV< 0.6 dB and
fA< 0.7 dB departure from flatness. Input and output return loss must be 20 dB or better over the full 6 MHz
bandwidth. These sweep curves are shown below as Figure 1A.
There are nine screw adjustments and two I/O matching (with soldering iron) adjustments that need to be made
simultaneously. Factory adjustment is never attempted without the aid of a network analyzer, and for this reason
we say the unit is not user-adjustable.
PUB96-26 Rev 1 September 13, 2005 26-2 RF Output: BP Filter & Directional Coupler

Figure 1 5-Pole Bandpass Filter Curves
Figure 2 5-Pole Bandpass Filter Used in the TTS1000B
PUB96-26 Rev 1 September 13, 2005 26-3 RF Output: BP Filter & Directional Coupler

2 RF DIRECTIONAL COUPLER
A directional coupler is based on the principles of inductive (magnetic) coupling and capacitive coupling.
In the LARCAN quad directional coupler implementation as shown in Figure 3 (schematic equivalent) and Figure
5(assembly), the RF to be sampled passes through a microstrip transmission line that is connected between the
transmitter output filter at J3 and the antenna system at J4. The magnetic field surrounding the hot conductor of
this transmission line induces a small RF current flow in other conductors situated parallel to it. One end of each
sampling conductor is terminated by a resistor to ground. Sometimes small capacitors are connected across
these resistors to provide a termination that remains resistive over the band. The other end of each sampling
conductor connects to an external load, usually a 50 Ωinput of something such as an RF detector for AGC, the
station demodulator, or an RF detector for VSWR sensing.
If the sampling system as described in the forgoing paragraph were dependent only on magnetic coupling and
absolutely no capacitance were present, the external loads would be driven with RF samples regardless of the
direction they came from. Omnidirectionality is not wanted; our objective is that the system should be directional,
that is, a signal coming from the transmitter should be seen by the "forward" ports, and a signal reflected back
from the antenna should be seen by the "reflected" ports, but at the same time as little as possible of the forward
signal from the transmitter should be seen on these reflected ports.
The desired directivity is achieved by the capacitance between the main line and each sampling line. The
presence of this capacitance changes the relative phase of the RF signal seen in the sampling line such that the
capacitively coupled signal adds to the inductively coupled signal at the end of the line nearest the signal source,
and subtracts from it at the other end, thus the sample becomes directive.
This capacitance is trimmed by small "gimmick" capacitors designated L1 through L4. They are in reality short
pieces of Teflon sleeved magnet wire which, although they may possess a fraction of a nanohenry of inductance,
are mainly small capacitors which are factory adjusted by bending the wire to control the amount of coupling
capacitance between the transmission line and the sampling loop concerned. The position of the capacitor along
the loop does not seem to matter.
Terminations are provided at the subtractive ends of each of the four sampling lines.
In the enclosure shown in Figure 5, J3 and J4 are the filter and antenna ports respectively, and J1, J5 are
"forward" samples which are maximum amplitude for signals incident on J3; while J2, J6 are "reflected" samples
which are maximum amplitude for signals incident on J4.
Different coupling values are obtained from the spacing of conductors; the nearer the spacing, the greater the
coupling. Coupling is also greater according to frequency, and rises at a rate of about 6dB per octave. In the
boards shown in Figure 5, the J1 and J2 signals will be about 10dB greater amplitude (about 36dB below the
generator level at 70 MHz on low band or 200 MHz on high band) than the signals sampled from J5 and J6
(about -46dB). Generally for system purposes the reflected signal sample to the VSWR supervisory system
should be taken from the J2 connector because it has greater coupling and we need to measure a much smaller
signal in a detector having finite small-signal sensitivity. System forward signals can be taken from J1 for the
AGC detector, and J5 for the system monitoring demodulator.
A network analyzer and extremely accurate terminations are required for setting up the directional coupler. The
adjustments are made to the trimming capacitances L1 through L4, and the capacitors in parallel with resistors
R1 through R4. Our target is directivity of 30dB or better on each sampling port, and coupling (forward direction)
for J1 and J2 about 36dB down, J5 and J6 about 46dB down.
No user adjustments are possible or recommended. Very little can go wrong with the directional coupler other
than from the antenna being hit by lightning, and inspection is all that is recommended, nothing more.
PUB96-26 Rev 1 September 13, 2005 26-4 RF Output: BP Filter & Directional Coupler

Figure 3 Quad Directional Coupler Equivalent Schematic
PUB96-26 Rev 1 September 13, 2005 26-5 RF Output: BP Filter & Directional Coupler

POWERAMPLIFIERLOWBAND
PUB96‐28Rev2Aug.2007 PAModule,

CONTENTS
FUNCTIONAL DESCRIPTION.....................................................................................................................................1
6-WAY SPLITTER/INPUT BOARD.............................................................................................................................1
FET RF AMPLIFIERS ...............................................................................................................................................1
6-WAY COMBINER/OUTPUT BOARD.......................................................................................................................2
VSWR CONTROL BOARD .......................................................................................................................................2
GREEN LED SENSITIVITY ADJUSTMENT.................................................................................................................5
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POWERAMPLIFIERLOWBAND
PUB96‐28Rev2Aug.2007 28-1PAModule,
FunctionalDescription
ThePowerAmplifiermoduleconsistsofasix‐waypowersplitter,sixFETamplifiers,asix‐waypowercombiner,a
VSWRprotectionboard,andpower&I/Oconnectors.Twofull‐sizeheatsinksprovidethecoolingfortheactive
devices.Itisdesignedfor1.5kWsyncpeakpoweroutputinLowBand54‐88MHzAnalogtelevisionsystems,
andprovidespowergainofapproximately20dB,with1.5kWpeaksyncvisualor900Wauraloutput.The
modulecanprovideupwardsof250Wofaveragedigitalpowerwhenusedwithappropriatepredistortion.Itis
fullyhot‐pluggable,incorporatingprotectivecircuitryforexcessVSWRpowercutback.
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6‐WaySplitter/InputBoard
Partnumber:40D1474G1/40D1474G2
References:Figure3andFigure4.
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The6‐WaypowersplitterreceivesitsRFinputsignalfromthedrivestageandprovidessixinputsignalsto
integralinputmatchingnetworksforthesixFETamplifiers.Theincomingsignalisfirstsplitinthreebya3‐way
Wilkinsonsplitter,andthethreeresultantsignalsaresplitagainbythree2‐wayWilkinsonsplitterstoprovide
thesixoutputsrequired.Terminationsforthe3‐waysplitterareprovidedbyR109,R110,andR111,with
reactivetrimmingbyL109,L110,andL111;andforthetwo‐waysplitters,terminationsareR101,R103andR105,
withreactivetrimmingbyC106,C116,andC126.ImpedancematchisprovidedbyC145,C142,C138,C144,
C145,C139,andC140whichmakethepathfromthe50Ωinputtothesixquarter‐wavematchingsections,into
alow‐passπnetwork.C148providesinputmatchingforthetransitionfromtheinputconnectortotheinput
transmissionline.
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Abuilt‐indetector(CR102andC147)isfedfromadirectionalcouplerontheinputtransmissionline,toprovidea
sampleoftheinputsignalformodulegainmonitoring.R117andR118terminatethedirectionalcoupler.
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FETRFAmplifiers
References:Figure3,Figure4,Figure5,Figure6.
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Eachofthesixamplifiersinthemoduleconsistsoftwo,sourcegroundedN‐channel,insulatedgateFieldEffect
Transistors(FETs)packagedinasinglecase,operatingclassABinapush‐pullconfiguration.BecausetheseFETs
are"enhancementmode"devices,theyrequirepositivegate‐to‐sourcebiasvoltageoneachgatetocause
source‐drainconduction.AquiescentClassABidlingbiascurrentissetindependentlyforeachhalf.Thegate
voltagerequiredtoproducethisidlingcurrentmayvarybetween2Vand5Vaccordingtothedevice
specificationsheet,andtheidlingcurrentused.FETgatethresholdvoltagesalsoaretemperaturesensitive,so
thermalcompensationisprovidedbyR9,RT1,andR10,RT2.Biascurrentissetto500mAperhalfofthedevice
foranalogoperationand750mAperhalffordigitaloperation.
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Gatebiasissuppliedfromanadjustablevoltagedividerfromthe+39Vregulatedbiasrail.ResistorsR1,R2,R3,
R4providegatebiasforonehalfoftheamplifier;R5,R6,R7,R8providebiasfortheotherhalf.
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TheRFinputsignalisappliedtobalunT1toprovidetwosignals180˚out‐of‐phase.Thesesignalsarestepped
downtomatchthelowinputimpedanceoftheFETdevicethroughaπ‐networkconsistingofC1,C2,C3,L1,L2,
C4,andthedeviceCG‐S.Thegateinputimpedanceattheoperatingfrequencyislowcomparedwiththevalues
ofR3andR6,whichhavelittleornoeffectatRF.


POWERAMPLIFIERLOWBAND
PUB96‐28Rev2Aug.2007 28-2PAModule,
R3andR6provideaDCpathforbias,andprovideloadingatlowerfrequencieswheregateimpedanceishigh,in
ordertoassistinmaintainingamplifierstability.ThechoiceofC6andC7values,andtheseriesinductanceof
boardtraces,alsoensureseffectivebypassingatcriticalfrequenciesofinterest.

Theoutputmatchingπ‐network,consistingofinductorsL3thruL8,andcapacitancesC13thruC16,transforms
theverylowoutputimpedanceoftheFET,to12.5Ω.Thetwoantiphaseoutputsignalsarefinallycombinedin
balunT2,L9.JumpersplacedacrosspartsofL7andL8,plusthechangedvaluesofC13,C14,C15andC16,
configuresthesystemforchannels5&6operation.

DCisappliedtotheFETdrainsthroughL3,L4fortheQ1Ahalf,andL5,L6fortheQ1Bhalf.L3andL6areshort
sectionsofmicrostriplinewhichtransformtheimpedancesofL4andL5tohighervaluesasseenbytheFET.RF
andlowerfrequenciesarebypassedwithparalleledC5,C9,C10foronehalfoftheamplifier,andC8,C11,C12
fortheotherhalf.Thesegroupsofcapacitorsareselectedinvalueandfortheirinternalequivalentseries
inductancessothattheywillbeaneffectivebypassatcriticalfrequenciesofinterest,includingvideo,toassistin
maintainingstability.

NotethatfusesareprovidedforthevoltagesuppliedtotheFETdrainconnections.Theintentofthesefusesis
toprotectthesurroundingcircuitryintheeventofadevicefailure.Thenormalfailuremodeofactivedevices
suchastheseisshort‐circuit,andthefusewillblowinthiscase,isolatingthedefectivedevicefromtherestof
themoduleandtransmitterpowersupply,allowingtheremainingdevicestokeepoperatingnormally.Ablown
fusecanserveasavaluabletroubleshootingaid,whentryingtoidentifyfaileddevices.


6‐WayCombiner/OutputBoard
Partnumber:40D1472G1/40D1472G2
References:Figure6andFigure3.

Thesixamplifieroutputsareappliedtothree2‐wayWilkinsoncombinersandphasedelayedtocorrectthe
quadratureconditionimposedbytheinputsplitterboard.ThethreeoutputsoftheseWilkinsoncombinersare
againcombinedbya3‐wayWilkinsoncombinerintoasingle50ohmoutput.TerminationsfortheWilkinson
networksaresimilartothoseprovidedontheInputboarddescribedabove,andconsistofR100,C105,R102,
C115,R104,C125forthe2‐ways;andR106,L106,R107,L107,andR108,L108forthe3‐waycombiner.An
outputmatchingπnetworkisformedbyC131thruC134,C136,C137,andC141,alongwiththeseries
inductanceoftheboardtrace.

AdirectionalcouplerfeedsaBNCconnectoronthemodulefrontpanel,andcanbeusedforoutputmonitoring.
Thebi‐directionalcouplerprovidesDCsamplescorrespondingtobothforwardandreflectedpowertotheVSWR
protectionboardformonitoringmodulegainandVSWRprotection.Terminationsforthesecouplerlinesections
areprovidedbyR113,R114,andR115;theRFsamplesforVSWRmonitoringaredetectedbyCR100,C143,and
R112for"forward"andbyCR101,C146,andR116for"reflected".

VSWRControlBoard
Partnumber:20B1549G1
References:Figure7andFigure8.

TheVSWRcontrolboardperformsanumberoffunctions:itprovidesregulatedbiasvoltagestotheFETpower
amplifierstages,itprovideshot‐plug‐incapabilitytoprotecttheamplifiermodulewhenpluggedintoan