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Theory of Operation
Controller
els) provided by the external EEPROM, U703. There
are three basic types of codeplug information: informa-
tion on the trunked system on which the radio is autho-
rized to operate; information on the conventional
system, which is either of the repeater or talk-around
type on which the radio is authorized to operate, and
information on the conÞguration and tuning of the
radio itself. Tuning information is normally located in
the internal EEPROM of U709.
SPI Interface
The microprocessor communicates to several ICs and
modules through a dedicated on-chip serial-periph-
eral-interface (SPI) port which consists of transmit data
line MOSI (U709-P1), receive data line MISO (U709-
P80), and clock line SCK (U709-P2). In addition, each
IC that can be accessed by the microprocessor using the
SPI has a read/write select line associated with it. The
ICs or circuits and their associated select lines are:
¥ EEPROM (U703) with select line U709-P3
¥ ASFIC (U701) with select line U709-P34
¥ LCD Driver (U707) with select line U709-P23
¥ OTP/FLASH ROM (U705) with select line
U709-P29
¥ SRAM (U706) with select line U709-P28
¥ Transceiver board Synthesizer (U201) with
select line U709-P35
¥ Transceiver board DAC IC (U400) with select
line U709-P26
The LCD Driver uses the master out/slave in (MOSI)
line to send data to the display driver IC, and the mas-
ter in/slave out (MISO) line to send data back to the
microprocessor (U709). Note, however, that the key-
pad (or any other SPI device) can never initiate display
data; the microprocessor is at all times the SPI master
device. Thus the MOSI line and MISO line are always
in the master conÞguration.
LED Control
The bicolor LED on the top of the radio is indirectly
activated by SPI of U709 via the DAC IC (U400) on
Transceiver Board. When either input to the dual NPN
transistor (U410) is at logic high, the corresponding
output pin (pin 6 for the green LED, pin 3 for the red)
should be at approximately 4.3 Vdc. Note that it is pos-
sible to have both LED outputs on simultaneously, in
which case the LED emits a yellow/orange light.
Audio & Data Circuitry
The transmit and receive audio paths are disabled in
the standby mode and selectively enabled by the
2-6
GTX/GTX LTR/LTS 2000 Portable Radios Service Manual
microprocessor when the radio transmits or receives a
signal. Also, there are minor differences in the func-
tioning of both paths depending on whether an inter-
nal or external (accessory) microphone/speaker is
being used. The radio constantly monitors the received
data path for control-channel data in trunking opera-
tion or sub-audible data in conventional operation.
External PTT Sense Circuits
On connecting an external MIC through connector J3,
external PTT sense transistor Q408 switches ÒONÓ
when the external PTT switch is closed. Q408 collector
voltage is monitored by U709-P4. When collector volt-
age is logic ÒHIGHÓ, the microprocessor conÞgures the
radio for transmit mode. In PTT equipped accessories,
the PTT switch is series connected with the external
MIC element.
MIC Amplifier
MIC audio from internal MIC MK401 is coupled
through C429, L404, J3 and L403 to the MIC buffer cir-
cuit U405-1. External MIC plug insertion mechanically
disconnects the internal MIC. External MIC audio is
coupled through L403 to the MIC buffer input. The
buffer will route the MIC audio into MIC IN (U701-B8)
through ßex and connectors J200 and J700. Inside the
ASFIC, the MIC audio is Þltered to eliminate compo-
nents outside the 300-3000 Hz voice band, pre-empha-
sized, and then limited. The limited MIC audio is then
routed through a summer, which is used to add in PL
or DPL sub-audio band modulation, and then to a
splatter Þlter to eliminate high frequency spectral com-
ponents generated by the limiter. After the splatter Þl-
ter, the audio is routed to the 8 bit modulation
attenuators, which are tuned in the factory of the Þeld
to set the proper amount of FM deviation. The TX
audio emerges from the ASFIC at U701-J4 and is dc
coupled and applied through ßex and connectors J700
and J200 to the synthesizer (U201) pin 8.
TX Data Circuits
There are four major types of transmit data: sub-audi-
ble data (PL/DPL/Connect Tone) that gets summed
with voice, high speed data for trunking control chan-
nel communication, DTMF data for telephone commu-
nication in trunked and conventional systems, and
MDC data for use in Motorola proprietary MDC sys-
tems. The deviation levels of the latter three types are
tuned by a 5-bit digital attenuation inside the ASFIC.
For each data type and each band split, there is a dis-
tinct set of tuning values that are programmed into the
ASFIC before the data is generated and transmitted.
68P02945C75-A
June 1999
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Troubleshooting
