Motorola VRS750, Service Manual

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Hardware Detailed
Theory of Operation
6
The VRS750 contains two main modules: a transceiver and an
interface board.
Transceiver
This module is an Analog synthesized FM device that contains the
receiver, transmitter, and controller section. A flex cable connects the
transceiver’s two accessory connectors to a single connector, J30, on
the interface board. The RF jack is connected to W101 on the interface
board via the 50 ohm coaxial cable.
The VRS750 software is embedded in the digital portion of the
transceiver’s controller which consists of a microcontroller and
associated EEPROM, RAM, and ROM memories.
NOTE: This transceiver cannot be used as a stand-alone
portable.
Interface Board
This board provides power regulation, bus translation circuitry, audio
routing circuitry, Single Tone Encoder and Decoder circuitry,
programming circuitry, and mode configuration.
Power Regulation
The VRS750 is powered by the mobile’s Ra (13.8 VDC) at the
connector P102-22. L101 and C137 are used to filter alternator whine
from this signal before providing it as SWB+ to the interface board. The
VRS750 utilizes the mobile’s analog (P102-2, 10, 11, 21) and digital
grounds (P102-18).
U111 regulates the SWB+ down to +5 VDC. This voltage provides
power for several switching transistors and voltage dividers on the
VRS750 board. U118 i a micropower low dropout regulator used to
p9.6 V for all of the analog audio circuitry. R162 and R164
form a divider network from this 9.6 V to produce the 4.8 V (VAG)
used to bias the audio circuits. Finally, U129 is a three-terminal
regulator used to provide the regulated 7.5V to the transceiver via
Q113 in the reset circuit. SW7.5 V is provided to the transceiver at pad
P31.
Bus Translation
Circuitry
The devices U100-2, U103, Q100, Q101, Q102, Q103, and Q104 are
used to separate messages from the bi-directional SB9600 serial bus
Bus+, Bus-, and Busy into and from the single direction RXD, TXD,
BUSY OUT, and BUSY IN signals at the transceiver.
The transceiver’s microcontroller can be reset by an external device by
generating an active high reset pulse at pin P102-19, RADIO_RESET.
This will cause Q109 to drive J30-5 low.