Lectrosonics SRb_SRb5P User Manual download pdf (Page 6)

SRb5P and SRb Dual Receivers

LECTROSONICS, INC.

RF Front-End and Mixer

Eachantennasignalisrstpassedthroughahighqual-

itySAWltertorejecthighpowerRFsignalsaboveand

belowtheoperatingfrequency.Ahighcurrentamplier

followstheSAWltersandpassesthesignaltoanin-

ternalsplittersothatbothantennasignalsareavailable

tobothreceiversforSmartDiversity™reception.

The mixer in each receiver takes the signal from the

PLLtogeneratetherstIFsignal,withbothPLLcircuits

fed from a common reference oscillator to maintain

signalphasethroughtheIFsections.

Microcontroller, PLL and VCO Circuits

The8-bitmicroprocessoristhe“heart”oftheSRb5P

andSRbreceivers.Itmonitorsusercommandinputs

fromthefrontpanelcontrolbuttonsandnumerousother

internalsignalssuchasRFlevel,audiolevels,pilottone

levelsandtransmitterbatterystatus.Outputsfromthe

microcontrollerdrivetheLCDdisplayandbacklight,and

controlthesquelch,theaudiooutputattenuator,the

PLL/VCOcircuitsandtheantennaphaseswitch.

IF Ampliﬁers and SAW Filters

TherstIFstageemploystwoSAW(surfaceacoustic

wave)lters.Theuseoftwolterssignicantlyincreas-

esthedepthoflteringwhilepreservingsharpskirts,

constantgroupdelay,andwidebandwidth.Thoughex-

pensive,thisspecialtypeoflterallowsprimaryltering

asearlyaspossible,atashighafrequencyaspossible,

beforehighgainisapplied,todelivermaximumimage

rejection.Sincetheseltersaremadeofquartz,they

areverytemperaturestable.

AftertheSAWlter,theIFsignalisconvertedto350

kHzinreceiver1and250kHzinreceiver2.Onlythen

isthemajorityofthegainapplied,justbeforethesignal

isconvertedtoaudiowithapulsecountingdetector.

AlthoughtheseIFfrequenciesareunconventionalina

widedeviation(±75kHz)system,itoffersoutstanding

AMrejectionoveraverywiderangeofsignalstrengths

andimprovednoiseperformance.

Digital Pulse Counting Detector

Thereceiverusesanelegantlysimple,yethighlyeffec-

tivedigitalpulsedetectortodemodulatetheFMsignal,

rather than a conventional quadrature detector. This

unusualdesigneliminatesthermaldrift,improvesAM

rejection,andprovidesverylowaudiodistortion.

DSP-Based Pilot Tone

The Digital Hybrid system design uses a DSP gener-

atedultrasonicpilottonetocontrolthereceiveraudio

muting(squelch).Thepilottonefrequencyisdifferent

foreachofthe256frequenciesinthetuningrangeofa

system(frequencyblock).Thiseliminatessquelchprob-

lemsinmultichannelsystemswhereapilottonesignal

canappearinthewrongreceiverviaintermodulation

products.

TheDSPgeneratedpilottonealsoeliminatesfragile

crystals, allowing the receiver to survive shocks and

mishandlingmuchbetterthanolderanalog-basedpilot

tone systems.

Note:ThisdescriptionappliesonlytotheDigital

Hybridmode.In200Series,IFBandMode6

compatibility,onlyonepilottonefrequencyisused

onallchannels,emulatingtheoriginalcrystal-

basedsystem.Inothercompatibilitymodes,no

pilottoneisused.

SmartSquelch

™

Anysquelchingsystemfacesinevitabletrade-offs:

squelch too aggressively and valuable audio information

may be lost. Squelch too little and excessive noise may

beheard.Respondtoorapidlyandtheaudiowillsound

“choppy.”Respondtoosluggishlyandsyllablesorentire

words can be cut off.

SmartSquelch

combines several techniques to achieve

anoptimalbalance,removingdistractingnoise,without

thesquelchingactionitselfbecomingadistraction.One

of these techniques involves waiting for a word or syl-

labletocompletebeforesquelching.Anothertechnique

incorporatesrecentsquelchinghistoryandrecentsignal

strength,adjustingsquelchingbehaviordynamicallyfor

the most serviceable result under variable conditions. Us-

ingtheseandothertechniques,theSRa5PandSRacan

deliveracceptableaudioqualityfromotherwiseunusable

signals.

Smart Noise Reduction (SmartNR

™

)

Note: The SmartNR setting is user selectable

onlyintheDigitalHybridcompatibilitymode.In

othermodes,noisereductionisappliedinsucha

way as to emulate the original analog system as

accuratelyaspossibleandisnotuseradjustable.

The wide dynamic range of digital hybrid technology,

combinedwithatresponseto20kHz,makesitpos-

sibletohearthe-120dBVnoiseoorinthemicpre-

amp,orthe(usually)greaternoisefromthemicrophone

itself.Toputthisinperspective,thenoisegenerated

by the recommended 4k bias resistor of many electret

lavalieremicsis–119dBVandthenoiselevelofthe

microphone’selectronicsisevenhigher.Inorderto

reducethisnoisethereceiverisequippedwithaSmart

Noise Reduction algorithm, which removes hiss without

sacricingaudiohighfrequencyresponse.

The Smart Noise Reduction algorithm works by at-

tenuatingonlythoseportionsoftheaudiosignalthatt

astatisticalproleforrandomnessor“electronichiss.”

Becauseitismuchmorethanasophisticatedvariable

lowpasslter,thetransparencyoftheaudiosignalis

preserved.Desiredhighfrequencysignalshavingsome

coherencearenotaffected,suchasspeechsibilance

and tones.

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Lectrosonics SRb_SRb5P User Manual Page 6

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