Vocon Basic Theory Of Operation - Motorola XTS 2500 Basic Service Manual

Basic Theory of Operation
VOCON Basic
Theory of
Operation
8
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.
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 4 kHz 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 2.4 MHz synthesis circuit to provide a clock
to the DSP-support IC on the VOCON circuitry. 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) circuitry contains the radio's microcontrol unit with
its memory and support circuits, the DSP, its memory devices, and the DSP-support IC,
voltage regulators, audio, and power control circuits. Connected to the VOCON circuitry
are the display board, RF circuitry, keypad board, and controls/universal flex.
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 16.8 MHz clock from the RF circuitry. 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 8 kHz interrupt signal generated by the DSP-support IC is also required
for functionality. It is programmed using parallel programming from the microcontrol unit.
The audio CODEC performs analog-to-digital (A/D), and digital-to-analog conversions on
audio signals. The DSP controls squelch, deviation, and executes receiver and transmitter
filtering. The DSP-support IC receives a 2.4 MHz clock, and also receives data, which it
formats for the DSP.