User Manual

SECTION 1: THEORY OF OPERATION
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The modem supports a 115.2 Kbps data transmission rate on the serial
port, SLIP protocol, and up to 19.2 Kbps over-the-air rate. Within a single
chip it provides forward error correction and detection, bit interleaving for
more robust data communications, and third generation collision detection
and correction capabilities.
Injection Synthesizer Provides programmable, ultra stable signals for the radio. One synthesizer
incorporates phase lock loop technology used for both receiving and
transmitting.
High Side Injection In the receive mode, the synthesizer provides a local oscillator signal of 45
MHz above the selected receive channel frequency.
Low Side Injection In the receive mode, the synthesizer provides a local oscillator signal of 45
MHz below the selected receive channel frequency.
In the DT450, due to limitations in synthesizer frequency coverage, high
side injection is used for receive frequencies below 456 MHz.
Low side injection is used for all other receive frequencies. In the
transmit mode, the desired frequency is produced, and it is directly
modulated by the modem. A 10 MHz reference oscillator controls the
synthesizer.
Baseband Routing Allows the microcontroller to select one of the two diversity receiver audio
outputs for demodulation by the modem. Switching is done by the
microcontroller comparing the Received Signal Strength Indication (RSSI)
outputs from each receiver. Provision is also made for switching an
external modulation source from the DB25 data connector to the
transmitter input.
Transmitter Consists of an exciter and power amplifier module. The transmitter covers
the various frequency bands in segments. A different power amplifier
module is required for each segment. The transmitter circuitry includes a
T/R switch, switching the antenna between transmitter and receiver 1
(TX/RX1).
Receiver 1/Receiver 2 Required to support the mobile DRS; two (2) discrete receivers are tuned
to the same channel and use two (2) antennas.
The receivers are double-conversion superheterodyne with a first
Intermediate Frequency (IF) of 45 MHz and a second IF frequency of 455
KHz. Each receiver consists of bandpass filters, RF amplifiers, a crystal
filter, a double-balanced mixer, and a one-chip IF system. The injection
synthesizer provides the first local oscillator signal. Outputs from each
receiver include RSSI and analog audio for the baseband routing circuitry
and modem.
Power Supply Consists of circuitry that derives the various operating voltages for the
radio. A group of fixed and adjustable voltage regulators are used for this
purpose. The transmitter power control circuitry is also found in this
section.