User Manual

SECTION 1: THEORY OF OPERATION
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Modem Converts parallel data into an analog audio waveform for
transmission and analog audio from the receiver to serial data.
Serial data appears on the base station’s DB25 serial port.
The radio supports a 115.2 KBPS data transmission rate on the
serial port, SLIP protocol, and a 19.2 KBPS OR 9.6 KBPS over-the-
air data transmission rate.
Within a single chip the modem provides forward error correction and
detection, bit interleaving for more robust data communications, and
third generation collision detection and correction capabilities.
Synthesizers Provides ultra stable signals for the three (3) receivers and the
transmitter. Two (2) independent synthesizers are used in the base
station to obtain full duplex (simultaneous receive and transmit)
operations. Both synthesizers incorporate phase lock loop (PLL)
technology and use separate reference oscillators.
RX Synthesizer The receive synthesizer provides a local oscillator signal 45 MHz
above or below the desired receive channel frequency. This is called
high side injection or low side injection.
TX Synthesizer The transmit synthesizer produces the desired frequency by
controlling a VCO. The VCO and the reference oscillator are both
frequency modulated by the modem output.
Diversity Reception Controller Circuitry selects one of three diversity receiver audio outputs for
processing by the modem by comparing the Received Signal
Strength Indication (RSSI) output from each receiver. Essentially,
audio from the receiver with the highest RSSI value is passed on to
the modem.
Transmitter Consists of an exciter and a power amplifier module covering various
frequency bands in segments. A different power amplifier module is
required for each segment. The transmitter power control is included
with the power supply circuitry on the same board.
Receiver 1/Receiver 2/ Uses three (3) discrete receivers tuned to the same frequency.
Receiver 3 The three (3) receivers are required to support IPMN’s base station
DRS.
NOTE
: Some installations use only two (2) receivers.
The receivers are double-conversion superhetrodynes with an
Intermediate Frequency (IF) of 45 MHz. Each receiver consist of
bandpass filters, RF amplifiers, a mixer, 45 MHz crystal filter, and a
one-chip IF system. The injection synthesizer provides the first local
oscillator signal and outputs from each receiver including RSSI and
analog audio for the diversity reception controller.
Power Supply Power supply circuitry derives the various operating voltages
required by the base station. Fixed voltage regulators are employed
through the radio for this purpose.