GENERALDESCRIPTION
The 900 MHz MDXMobile Radio is a synthesized,
wideband radio that uses integrated circuits and microcom-
puter technology to provide high performance trunked op-
eration. This radio operates in the Enhanced Digital Access
Communications System (EDACS) environment, and in
conventional communication systems. The radio provides 25
Watts of RF power output in the 396-902 MHz and 935-941
MHz bands. The receiver operates in the 935-941 MHz
band.
All radio functions are stored in a programmable Electri-
cally Erasable PROM (EEPROM). The radio is field pro-
grammable using an IBM compatible personal computer
with the following equipment:
•Serial Programming Interface Module TQ3370
•Programming Cable (19B801417P10) TQ3372
•MDX Series Programming
Software (EDACS) TQ3365
With the interface equipment and software, the computer
can be used to program (or re-program) customer systemfre-
quencies, Channel Guard tones and options. Selection of op-
tions is done during radio initialization using the PC
programmer.
The 900 MHz MDXMobile Radio assembly contains
the following circuit boards and assemblies:
•Power Amplifier Board 19D902944Gxx
•RF Board 19D902123Gxx
•System Board 19D901891Gxx
•Audio/Logic Board 19D903963Gxx
•Audio Amplifier Board 19D904025Gxx
•Front Cap Assembly 19D904151Gxx
The circuit boards are all mounted on a main casting to
provide easy access for servicing. Interconnect plugs are
used to connect the boards to eliminate pinched wires or
other wiring problems.
RF BOARD
The RF Board includes the programmable frequency
synthesizer, transmitter exciter, receiver front end and IF cir-
cuitry.
Synthesizer
The synthesizer circuit generates all transmit and receive
RF frequencies. The synthesizer frequency is controlled by
the microprocessor located on the Audio/Logic Board. Fre-
quency stability is maintained by a temperature compensated
reference oscillator module. Transmit audio is processed on
the Audio/Logic Board and applied to the synthesizer to
modulate the VCO and TCXO. The buffered VCO output
drives both the transmitter exciter and the receiver mixer.
Transmitter
The transmitter consists of a fixed-tuned exciter module,
a PA module and a power control circuit. The PA module
provides RF output to drive the antenna. The power control
circuit controls the PA module to maintain a constant output
power across the band. The RF output level is internally ad-
justable for rated power. Thermistors in the control circuit
protect the PA from overheating by reducing the power out-
put level.
Receiver
The dual conversion receiver circuit consists of a front
end section, 39.5 MHz first IF, a 455 kHz second IF, and FM
detector. All audio processing and squelch functions are ac-
complished on the Audio Board.
POWER AMPLIFIER BOARD
The PA board amplifies the RF board output then con-
nects it back to the RF board where it is coupled through a
PIN diode antenna switch, the low-pass filter and the direc-
tional coupler to provide 25 watts of power output at the an-
tenna connector.
AUDIO/LOGIC BOARD
The Audio/Logic Board provides all audio and digital
processing of the receive and transmit audio for digital proc-
essing by the Logic Board. The board also contains audio fil-
tering, conventional analog tone processing, and the receiver
squelch. The Audio/Logic Board controls the operation of
the radio and digitally processes the receive and transmit
audio. The board contains a microprocessor and associated
memory circuits including an EPROM for controlling the
processor and a programmable "personality" memory (an
Electrically Erasable PROM - EEPROM) to store cus-
tomer frequencies, tones and options. The microprocessor
provides control data to the Audio Signal Processor (ASP),
conventional tone generation and detection, frequency data
for the synthesizer, and sends and receives data to another
microprocessor on the Display Board for the LCD.
SYSTEM BOARD
The system board controls the main input power to the ra-
dio. An IGNITION SENSE input lead provides the necessary
signals to the MOSFET switching circuit. The board also inter-
faces all option connections from the internal boards in the ra-
dio with the optional items outside of the radio. All external
options for the radio interconnect to the System Board through
the back of the radio using an optional cable.
FRONT CAPASSEMBLY
The Front Cap Assembly contains the Audio Amplifier
Board. This board provides compression of the microphone
audio. It also provides audio compression for the received
audio in the discriminator and internal/external speaker audio
paths. A 10-watt power amplifier is provided on the board to
drive a 4-ohm internal/external speaker.
ACCESSORIES AND OPTIONS
PC PROGRAMMER OPTIONS
The radio is programmed using an IBM compatible per-
sonal computer equipped with a RS-232 port. Option TQ3370
provides the RS-232 serial interface unit and the cable between
the PC and the unit. An auxiliary power supply for the unit is
also included but is not needed to program the radio.
Option TQ3372 provides the radio programming cable be-
tween the PC interface unit and the radio microphone jack.
MDX PC programming software Option TQ3373 (EDACS) is
provided on 3.5 inch diskettes.
PC PROGRAMMED OPTIONS
Carrier Control Timer (CCT)
The Carrier Control Timer turns off the transmitter after the
microphone push-to-talk (PTT) switch has been keyed for a
pre-programmed time period. A pulsing alert tone warns the
operator to unkey and then rekey the PTT to continue the trans-
mission. The timer can be programmed, using the PC program-
mer. Any time periods between 30 seconds and 7.5 minutes can
be programmed in 30 second increments. The timer can be en-
abled or disabled for each channel.
Channel Guard
Channel Guard provides a means of restricting calls to spe-
cific radios through the use of a Continuous Tone Coded
Squelch System (CTCSS), or a Continuous Digital Coded
Squelch System (CDCSS). Tone frequencies range from 67.0
Hz to 210.7 Hz in 0.1 steps. There are eighty three standard PC
programmable digital codes. The Channel Guard tone frequen-
cies and codes are listed in Table 1 - Channel Guard Tone
Frequencies and Table 2 - Digital Channel Guard Codes
(see below).
67.0 71.9 74.4 77.0 79.7 82.5 85.4 88.5 91.5 94.8 97.4
100.0 103.5 107.2 110.9 114.8 118.8 123.0 127.3 131.8 136.5 141.3
146.2 151.4 156.7 162.2 167.9 173.8 179.9 186.2 192.8 203.5 210.7
1. Do not use 179.9 Hz or 118.8 Hz in areas served by 60 Hz power distribution systems (or 100.0 Hz or 151.4 Hz in areas supplied with 50Hz power).
Hum modulation of co-channel stations may "false" Channel Guard decoders.
2. Do not use adjacent Channel Guard tone frequencies in systems employing multiple Channel Guard tones. Avoid same-areas co-channel use of adjacent
Channel Guard tones whenever possible. As stated in EIA Standard RS-220, there is a possibility of decoder falsing.
3. To minimize receiver turn-on time delay, especially in system using Channel Guard repeaters or receiver voting, choose the highest usable Channel
Guardtone frequency. Do not use tones below 100 Hz when it is necessary to meet the receiver response time requirements of EIA Standard RS-220.
Table 1 - Standard Tone Frequencies (Hz)
To reverse the polarity of the digital Channel Guard
codes, in the PC programmer, type I (inverted) be-
fore the code number, i.e. I023.)
NOTE
LBI-38915B
2
