User Manual
SECTION 1: THEORY OF OPERATION
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Baseband Routing
This circuitry selects audio from the appropriate receiver to route to the modem for demodulation.
Power and Ground
Power from the vehicle’s battery appears at the VBATT pad. The supply line powers a series of voltage
regulators and the transmitter control circuitry, as follows:
Voltage regulator VR3 provides a continuous 5-volts for the microcontroller, RS232 interface, and
modem circuitry. This power supply is always live allowing user-defined parameters to be stored in
the radio.
Voltage regulator VR5 provides switched 8-volt power for most other sections in the radio. VR1 is
switched on by the microcontroller.
Voltage regulator VR6 powers the analog circuitry in the radio.
In the transmit control circuitry, to transmit, the microcontroller makes TXKEYOUT* low. This causes Q5
to conduct, grounding the gate terminal of Q2. This forces the P-channel device to conduct, applying 5-
volts via 5VKEY to the transmitter exciter. At the same time Q4 conducts, grounding the gate terminal of
Q7. This forces the P-channel device to conduct, applying 12-volts via 12VKEY to the transmitter power
amplifier.
Transmitter over-temperature protection is provided by a temperature sensor (U18). When the heatsink
becomes too hot, U18 shunts the bias from transistors Q5 and Q4 to ground, turning off Q2 and Q7. This
shuts down the transmitter. After the heatsink cools off, the shunt is removed and transmission can
resume.
Receiver 2 Front-End
The circuitry for Receiver 2 is identical to Receiver 1 with the exception of the T/R switch and an
additional amplifier (U33).
Incoming signals pass through a filter (FT4B). The filter is a double-tuned device that provides a high
degree of selectivity. Selected signals are amplified by a low noise amplifier (U6B). A triple-tuned filter
(FT3B) provides additional selectivity and the signal is further amplified by an RF amplifier chip (Q6B).
The output from Q6B passes through another triple-tuned filter (FT2B) and it goes to a double-balanced
mixer (U29B), which heterodynes the amplified signal from U33. The result is a 45 MHz IF signal.
Spurious signals are filtered out by a 4-pole bandpass filter (FL5B). The IF signal goes to Receiver 2 IF
for further processing.
There are five (5) filter sets (FT2B, FT3B, and FT4B) available and each one covers a 20-30 MHz
portion of the UHF band. Should replacement of the filters be required, exact replacement parts
must be used.
Receiver 2 IF