Owner's manual

SRa5P and SRa Dual Receivers
LECTROSONICS, INC.
6
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SRa5P
andSRa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,
controlthesquelchandaudiooutputattenuator,the
PLL/VCOcircuitsandtheantennaphaseswitch.
IF Amplifiers and SAW Filters
TherstIFstageat244MHzemploystwostate-of-the-
artSAW(surfaceacousticwave)lters.Theuseoftwo
lterssignicantlyincreasesthedepthoflteringwhile
preservingsharpskirts,constantgroupdelay,andwide
bandwidth.Thoughexpensive,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244MHzIFsignalisconverted
to250kHzinreceiver1and350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detec-
tor.AlthoughtheseIFfrequenciesareunconventional
inawidedeviation(±75kHz)system,itoffersoutstand-
ingAMrejectiongureoveraverywiderangeofsignal
strengthsandproducesanexcellentnoiseimprovement
at low signal strengths.
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).Briefdelaysareappliedtoeliminate
thumps,popsorothertransientsthatcanoccurwhen
thepoweristurnedonoroff.Thepilottonefrequency
isdifferentforeachofthe256frequenciesinthetuning
rangeofasystem(frequencyblock).Thiseliminates
squelchproblems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in400Series
mode.In200Seriesmode,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
TM
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400Series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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.