Technical Manual

28
Wh en th e radio is transm itting, m icroph on e audio is passed from th e audio
power am plifier (PA) to th e DSP-support IC, wh ere th e sign al is digitized. Th e
DSP-support IC passes digital data to th e DSP, wh ere pre-em ph asis an d low-
pass (splatter) filterin g are don e. Th e DSP return s th is sign al to th e DSP-
support IC, wh ere it is recon verted in to an an alog sign al an d scaled for
application to th e voltage-con trolled oscillator as a m odulation sign al.
Tran sm itted sign allin g in form ation is accepted by th e DSP from th e
m icrocon trol un it, coded appropriately, an d passed to th e DSP-support IC,
wh ich h an dles it th e sam e as a voice sign al. Modulation in form ation is passed
to th e syn th esizer alon g th e m odulation lin e. A m odulated carrier is provided
to th e RF PA, wh ich tran sm its th e sign al under dyn am ic power con trol.
ASTRO M ode
of Operat ion
In th e ASTRO m ode (digital m ode) of operation , th e tran sm itted or received
sign al is lim ited to a discrete set of deviation levels, in stead of con tin uously
varyin g. Th e receiver h an dles an ASTRO-m ode sign al iden tically to an an alog-
m ode sign al up to th e poin t wh ere th e DSP decodes th e received data. In th e
ASTRO receive m ode, th e DSP uses a specifically defin ed algorith m to recover
in form ation .
In th e ASTRO tran sm it m ode, m icroph on e audio is processed iden tically to an
an alog m ode with th e exception of th e algorith m th e DSP uses to en code th e
in form ation . Th is algorith m will result in deviation levels th at are lim ited to
discrete levels.
RF Board Basic
Theory of
Operat ion
Th e receiver fron t en d con sists of a preselector, an RF am plifier, a secon d
preselector, an d a m ixer. On th e 800MHz receiver fron t en d, both preselectors
are varactor-tun ed, two-pole, fixed-tun ed filters. Th e RF am plifier is a dual-
gate, gallium - arsen ide based IC. Th e m ixer is a double-balan ced, active m ixer
coupled by tran sform ers. In jection is provided by th e VCO th rough an
in jection filter. See Table 13 for local oscillator (LO) an d first IF in form ation .
Th e frequen cy gen eration fun ction is perform ed by th ree ICs an d associated
circuitry. Th e referen ce oscillator provides a frequen cy stan dard to th e
syn th esizer/prescaler IC, wh ich con trols th e VCO IC. Th e VCO IC actually
gen erates th e first LO an d tran sm it-in jection sign als an d buffers th em to th e
required power level. Th e syn th esizer/prescaler circuit m odule in corporates
frequen cy-division an d com parison circuitry to keep th e VCO sign als stable.
Th e syn th esizer/prescaler IC is con trolled by th e m icrocon trol un it th rough a
serial bus. Most of th e syn th esizer circuitry is en closed in rigid m etal can s on
th e RF board to reduce m icroph on ic effects.
Th e receiver back en d con sists of a two-pole crystal filter, an IF am plifier, a
secon d two-pole crystal filter, an d th e digital back-en d IC. Th e two-pole filters
are wide en ough to accom m odate 5kHz m odulation . Fin al IF filterin g is don e
digitally in th e DSP-support IC.
Table 13 Local Oscillator and First IF Frequencies
8 0 0 MH z
LO Frequen cy Ran ge 732.65-796.65MHz
First IF Frequen cy 73.35MHz