Vocon Board Basic Theory Of Operation - Motorola ASTRO XTS 3500 Basic Service Manual

VOCON Board
Basic Theory of
Operation
The receiver back end consists of a two-pole crystal filter, an IF amplifier, a
second two-pole crystal filter, and the digital back-end IC. The two-pole filters
are wide enough to accommodate 4kHz modulation. Final IF filtering is done
digitally in the DSP.
The digital back-end IC consists of an amplifier, the second mixer, an IF
analog-to-digital converter, a baseband down-converter, and a 2.4MHz
synthesis circuit to provide a clock to the DSP-support IC on the VOCON
board. The second LO is generated by discrete components external to the IC.
The output of the digital back-end IC is a digital bit stream that is current
driven on a differential pair for a reduction in noise generation.
The transmitter consists of an RF driver IC that gets an injection signal from
the VCO and a final-stage power amplifier. Transmit power is controlled by a
power-control IC that monitors the output of a directional coupler and adjusts
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. Antenna selection is done mechanically in the
control top.
The vocoder and controller (VOCON) board contains the radio's microcontrol
unit with its memory and support circuits, the digital-signal processor (DSP),
its memory devices, and the DSP-support IC, voltage regulators, audio, and
power control circuits. Connected to the VOCON board are the display board,
RF board, keypad board, controls/universal flex, and (optional) encryption
module.
The microcontrol unit controls receive/transmit frequencies, power levels,
display, and other radio functions, using either direct logic control or serial
communications paths to the devices.The microcontrol unit executes a stored
program located in the FLASH ROM. Data is transferred to and from memory
by the microcontrol unit data bus. The memory location from which data is
read, or to which data is written, is selected by the address lines.
The DSP-support IC is supplied with a 16.8MHz clock from the RF board. Both
the DSP and the microprocessor have their clocks generated by the DSP-
support IC. They can both be adjusted so that the harmonics do not cause
interference with the radio's receive channel.
The regulator and power-control circuits include 3.3-volt analog, 3.3-volt
digital, and 5-volt regulators. The audio PA is sourced from 7.5V. The
regulator's power-down mode is controlled by the microcontrol unit, which
senses the position of the on/off switch. The 5-volt regulator has an error pin
for low-voltage resets.
The DSP performs signalling and voice encoding and decoding as well as audio
filtering and volume control. This IC performs Private-Line®/Digital Private
Line™ (PL/DPL) encode and alert-tone generation. The IC transmits pre-
emphasis on analog signals and applies a low-pass (splatter) filter to all
transmitted signals. It requires a clock on the EXTAL pin. An 8kHz interrupt
signal generated by the DSP-support IC is also required for functionality. It is
programmed using parallel programming from the microcontrol unit.
31