Vocoder Circuitry Overview - Motorola XTS 2500 Service Manual

UHF2 Overall Characteristics: Vocoder Circuitry Overview
The digital back-end IC (ABACUS-III) consists of the following:
• an amplifier
• the second mixer
• an IF analog-to-digital converter
• a baseband down-converter, and
• a 18 MHz synthesis circuit to provide a sampling clock to the circuitry. The second LO is
generated by discrete components external to the IC. The output of the ABACUS-III IC is a
current-driven digital bit stream.
The transmitter consists of an RF driver IC and a final stage power amplifier. The RF driver IC gets
an injection signal from the VCO. Transmit power is controlled by a power-control IC that monitors
the output of a directional coupler and adjusts the PA control voltages correspondingly. The signal
passes through a RX/TX switch that uses PIN diodes to automatically provide an appropriate
interface to transmit or receive signals.

2.3.6 Vocoder Circuitry Overview

In the VOCON circuitry, the digital-signal processor (DSP) supports IC digital filters, discriminates the
signal, and passes it to the DSP. The DSP decodes the information in the signal and identifies the
following destination for the signal:
• For voice signal, the DSP routes the digital voice data to the Coder-Decoder (CODEC) for
conversion to an analog signal. The CODEC then presents the signal to the audio pre-amplifier
and audio power amplifier (integrated into the MAKO IC), which drives the speaker.
• For signaling information, the DSP decodes the message and passes it to the microcontroller
unit.
6816985H01-F
2.3-3
February 14, 2012