User Manual

27
Basic Theory of
Operat ion 7
General
Overview
Th e ASTRO Digital XTS 3000 radio is a wideban d, syn th esized, fixed-tun ed
radio available in th e 800MHz ban d. All ASTRO Digital XTS 3000 radios are
capable of both an alog operation an d ASTRO m ode (digital) operation in
12.5kHz or 25kHz ban dwidth s.
Th e ASTRO Digital XTS 3000 radio in cludes th e followin g m ajor assem blies:
• Con troller Board - con tain s th e m icrocon trol un it (MCU) an d its
associated m em ory an d m em ory m an agem en t in tegrated circuit (IC), th e
audio power am plifier, an d a switch in g regulator.
• RF Board - con tain s all tran sm it, receive, an d frequen cy gen eration
circuitry in cludin g th e digital receiver back-en d IC an d th e referen ce
oscillator.
• Vocoder Board - con tain s th e digital sign al processor (DSP) an d its support
IC an d associated m em ories.
• Con trols/Un iversal Flex - con tain s volum e/on /off switch , frequen cy
selector switch , push -to-talk (PTT) switch , m on itor button , several
fun ction -selectable switch es, un iversal con n ector, speaker, an d
m icroph on e.
• Display (Full-Featured Model On ly) - a four-lin e, 12-ch aracter liquid
crystal display (LCD).
• Keypad (Full-Featured Model On ly) - a 3 x 6 keypad.
Analog M ode
of Operat ion
Wh en th e radio is receiving, th e sign al com es from th e an ten n a con n ector to
th e RF board, passes th rough th e RX/TX switch an d th e receiver fron t en d. Th e
sign al is th en filtered, am plified, an d m ixed with th e first local-oscillator sign al
gen erated by th e voltage-con trolled oscillator (VCO).
Th e resultin g in term ediate frequen cy (IF) sign al is fed to th e IF circuitry, wh ere
it is again filtered an d am plified. Th is am plified sign al is passed to th e digital
back-en d IC, wh ere it is m ixed with th e secon d local oscillator to create th e
secon d IF at 450kHz. It is th en con verted to a digital bit stream an d m ixed a
th ird tim e to produce a baseban d sign al. Th is sign al is passed to th e vocoder
board th rough a curren t-driven differen tial output.
On th e vocoder board, th e digital- sign al-processor-support IC digitally filters
an d discrim in ates th e sign al, an d passes it to th e digital-sign al processor (DSP).
Th e DSP decodes th e in form ation in th e sign al an d iden tifies th e appropriate
destin ation for it. For a voice sign al, th e DSP will route th e digital voice data
to th e DSP-support IC for con version to an an alog sign al. Th e DSP-support IC
will th en presen t th e sign al to th e audio power am plifier, wh ich drives th e
speaker. For sign allin g in form ation , th e DSP will decode th e m essage an d pass
it to th e m icrocon trol unit.