Chapter 3 Basic Theory of Operation
3.1 Introduction
The following theory will help isolate the problem to a particular board. Using circuit board
replacement as the basic service approach maximizes the working time of the radio.
3.2 General Overview
The ASTRO Digital Spectra and Digital Spectra Plus radios are wideband, synthesized, fixed-tuned
radios. The ASTRO Digital Spectra is available with VHF, UHF, and 800 MHz bands. The ASTRO
Digital Spectra Plus is available with VHF and 800 MHz bands only. All ASTRO Digital Spectra radios
are capable of both analog operation and ASTRO mode (digital) in 12.5 kHz, 20 kHz, 25 kHz, or 30
kHz bandwidths.
The ASTRO Digital Spectra and Digital Spectra Plus radios consist of seven major assemblies, six of
which are in the main radio chassis. They are:
• Control Head Assembly (Dash- or Remote-Mount) — is connected, directly or remotely, to the
front of the transceiver by the interconnect board or remote interconnect board and control
cable. This assembly contains a vacuum fluorescent (VF) display, VF driver, microprocessor
and serial bus interface.
• Power Amplifier (PA) — contains antenna switch, directional coupler/detector, and amplifier(s).
• Front-End Receiver Assembly — contains pre-amplifier, preselector, mixer, and injection filter.
• Radio-Frequency (RF) Board — contains receiver IF amplifier, demodulator, synthesizer logic
and filtering circuitry, and digital receiver back-end integrated circuit (IC).
• VCO/Buffer/Divider Board — contains voltage-controlled oscillator (VCO), divider, receive and
transmit buffers.
• Command Board — contains power control/regulator, digital-to-analog (D/A) IC, serial bus inter-
face, and audio power amplifier (PA).
• VOCON (Vocoder/Controller) Board (Digital Spectra radio) — contains the microcomputer unit
(MCU), its associated memory and memory management integrated circuit, and the digital-sig-
nal processor (DSP) and its associated memories and support IC.
For the ASTRO Digital Spectra Plus radio, the VOCON board architecture is based on a Dual-Core
processor, which contains a DSP Core, an MCORE 210 Microcontroller Core, and custom
peripherals. The board also contains memory ICs and DSP-support ICs.
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