Analog Mode Of Operation; Astro Mode (Digital Mode) Of Operation - Motorola ASTRO XTS 3000 Service Manual

Analog Mode
of Operation
ASTRO Mode
(Digital Mode)
of Operation
2-2
The display module is a four-line x 12-character, liquid-crystal display
(LCD) with associated circuitry. This module utilizes chip-on-board
technology and is not considered field repairable.
The keypad module is either a 6 x 3- or a 2 x 3-button module with
backlighting.
The encryption board (secure models only) connects directly to the
controller board and interfaces directly with the vocoder digital
circuitry. It contains an independent microcontroller and two custom
ICs to perform digital, numerical, encryption algorithms.
When the radio is receiving, the signal comes from the antenna/
antenna-switch connector to the transceiver board, passes through the
RX/TX switch and the receiver front end. The signal is then filtered,
amplified, and mixed with the first local-oscillator signal generated by
the voltage-controlled oscillator (VCO). The resulting intermediate
frequency (IF) signal is fed to the IF circuitry, where it is again filtered
and amplified. This amplified signal is passed to the digital back-end
IC, where it is mixed with the second local oscillator to create the
second IF at 450kHz. It is then converted to a digital bit stream and
mixed a third time to produce a baseband signal. This signal is passed
to the vocoder board through a current-driven differential output. On
the vocoder board, the ADSIC (ABACUS DSP Support IC) digitally
filters and discriminates the signal, and passes it to the digital-signal
processor (DSP). The DSP decodes the information in the signal and
identifies the appropriate destination for it. For a voice signal, the DSP
will route the digital voice data to the ADSIC for conversion to an
analog signal. The ADSIC will then present the signal to the controller
board's audio power amplifier, which drives the speaker. For signalling
information, the DSP will decode the message and pass it to the
microcontrol unit.
When the radio is transmitting, microphone audio is passed from the
audio power amplifier (PA) to the ADSIC, where the signal is digitized.
The ADSIC passes digital data to the DSP, where pre-emphasis and low-
pass (splatter) filtering are done. The DSP returns this signal to the
ADSIC, where it is reconverted into an analog signal and scaled for
application to the voltage-controlled oscillator as a modulation signal.
Transmitted signalling information is accepted by the DSP from the
microcontrol unit, coded appropriately, and passed to the ADSIC,
which handles it the same as a voice signal. Analog modulation
information is passed to the synthesizer along the modulation line. A
modulated carrier is provided to the RF PA, which transmits the signal
under dynamic power control.
In the ASTRO mode (digital mode) of operation, the transmitted or
received signal is limited to a discrete set of four deviation levels. The
receiver handles an ASTRO-mode signal identically to an analog-mode
signal up to the point where the DSP decodes the received data. In the
ASTRO receive mode, the DSP uses a specifically defined algorithm to
recover information. In the ASTRO transmit mode, microphone audio
is processed identically to an analog mode with the exception of the