Radio Power; General - Motorola ASTRO Digital Spectra Service Manual

2-8
The support-logic IC acts as an extension of the microcontrol unit by providing logic functions such
as lower address latch, reset, memory address decoding, and additional control lines for the radio.
The VOCON board controls a crystal-pull circuit to adjust the crystal oscillator frequency on the
microcontrol unit, so that the E-clock harmonics do not cause interference with the receive channel.
The vocoder circuitry on the VOCON board is powered by a switched +5-V regulator located on the
command board. This voltage is removed from the board when the radio is turned off by the control
head switch.
The DSP (digital-signal processing) IC performs signaling, voice encoding/decoding, audio filtering,
and volume control functions. This IC performs Private-Line/Digital Private Line (PL/DPL) encode
and alert-tone generation. The DSP IC transmits pre-emphasized analog signals and applies a low-
pass (splatter) filter to all transmitted signals. It requires a 33 MHz crystal to function. An 8 kHz
interrupt signal generated by the DSP-support IC is also required for functionality. This device is
programmed using parallel programming from the microcontrol unit and the DSP-support IC.
The DSP-support IC performs analog-to-digital and digital-to-analog conversions on audio signals. It
contains attenuators for volume, squelch, deviation, and compensation, and it executes receiver
filtering and discrimination. The IC requires a 2.4 MHz clock to function (generated by the digital
back-end IC) and is programmed by the microcontrol unit's Serial Peripheral Interface (SPI) bus.

2.11 Radio Power

This section provides information on DC power distribution in ASTRO radios.

2.11.1 General

In the ASTRO radio, power is distributed to seven boards: command, VOCON, control head,
synthesizer, receiver front end, RF, and RF power amplifier.
Power for the radio is supplied by the vehicle's 12-V battery. When using a desktop adapter unit, an
external DC power supply can be connected to replace the vehicle's battery source.
A+ (referred to as incoming unswitched battery voltage) enters the radio through the rear RF power
amplifier connector (P1) and is the main entry for DC power. The second path, through P2, pin 5,
provides ignition sense to inhibit the RF transmitter when the ignition switch is off.
July 1, 2002
General Overview: Radio Power
68P81076C25-C