Astro Mode Of Operation - Motorola ASTRO XTS 3500 Basic Service Manual

ASTRO Mode
of Operation
RF Board Basic
Theory of
Operation
30
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 through gain
stages to the CODEC, where the signal is digitized. The CODEC passes digital
data to the DSP, where pre-emphasis and low-pass (splatter) filtering are done.
The DSP passes this signal to a digital/analog (D/A) converter, 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 D/A converter,
which 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 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 deviation levels, instead of continuously
varying. 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 algorithm the DSP uses to encode the
information. This algorithm will result in deviation levels that are limited to
discrete levels.
The receiver front end consists of a preselector, an RF amplifier, a second
preselector, and a mixer. Both preselectors are varactor-tuned, two-pole filters,
controlled by the microcontroller unit through the digital/analog (D/A) IC.
The RF amplifier is a dual-gate, gallium-arsenide-based IC. The mixer is a
double-balanced, active mixer, coupled by transformers. Injection is provided
by the VCO through an injection filter. See Table 14 for local oscillator (LO)
and first IF information.
Table 14 Local Oscillator and First IF Frequencies
LO Frequency Range
First IF Frequency
The frequency generation function is performed by three ICs and associated
circuitry. The reference oscillator provides a frequency standard to the
synthesizer/prescaler IC, which controls the VCOB IC. The VCOB IC actually
generates the first LO and transmit-injection signals and buffers them to the
required power level. The synthesizer/prescaler circuit module incorporates
frequency-division and comparison circuitry to keep the VCO signals stable.
The synthesizer/prescaler IC is controlled by the microcontrol unit through a
serial bus. Most of the synthesizer circuitry is enclosed in rigid metal cans on
the RF board to reduce microphonic effects.
VHF
180.85-218.85MHz
44.85MHz
UHF
376.65-446.65MHz
73.35MHz