Transmitting; Astro Mode (Digital Mode) Of Operation; Transceiver Board Overview; Receiver Front End - Motorola ASTRO XTS 5000 Detailed Service Manual

Overall Characteristics: ASTRO Mode (Digital Mode) of Operation
In the VOCON board, the Patriot digital signal processor (DSP) processes the synchronous serial
interface (SSI) data from Abacus III. Voice data is sent to the coder/decoder (CODEC) for conversion
to an analog signal. The CODEC delivers the signal to the audio power amplifier (PA), which drives
the speaker. Subaudible signaling information is decoded by the DSP and passed to the MCU.
2.2.2

Transmitting

When the radio is transmitting voice, microphone audio is passed through gain stages to the
CODEC, where the signal is digitized. The CODEC then passes the digital data to the DSP, where
pre-emphasis and low-pass (splatter) filtering are done. The DSP then sends the signal to a digital-
to-analog (D/A) converter on the transceiver board.
In contrast to the way microphone audio— that is, voice—is processed for transmission, signaling
information is accepted by the DSP from the MCU, coded appropriately, and passed to a D/A
converter, which then handles it the same as a voice signal.
Modulation information is passed to the synthesizer along the modulation line. A modulated carrier is
provided to the RF power amplifier (PA), which transmits the signal under dynamic power control.
2.3

ASTRO Mode (Digital Mode) of Operation

In the ASTRO (digital) mode of operation, the transmitted or received signal is limited to a discrete
set of deviation levels, instead of continuously varying. The receiver handles an ASTRO mode signal
the same way it does an analog mode signal, up to the point where the DSP decodes the received
data.
In the ASTRO receive mode, the DSP uses a specific algorithm to recover information. In the
ASTRO transmit mode, microphone audio is processed the same as an analog mode signal, except
that an algorithm in the DSP encodes the information as deviation levels limited to discrete levels.
2.4

Transceiver Board Overview

The transceiver board is divided into the following sections:
• Receiver
• Transmitter
• Frequency Generation Unit (FGU)
2.4.1

Receiver Front End

The receiver front end
the first intermediate frequency (IF). Channel selection is by way of a tunable local oscillator, RXLO,
from the FGU.
The receiver front end consists of a preselector filter, an RF amplifier, a second preselector, mixer,
and an IF crystal filter. The preselectors are multi-layer ceramic filters with two surface-mount
varactor diodes placed on each filter. The RF amplifier is a discrete RF transistor with associated
circuitry. The mixer is a double-balanced, active mixer IC, coupled by transformers. The receiver
(RX) local oscillator (LO) is provided by the FGU.
68P81094C31-O
(Figure 2-5) tunes to the desired channel, and down converts the RF signal to
2-5
April 15, 2002