6881076C20-E February 3, 2003
Basic Theory of Operation: Front-End Receiver Assembly 3-5
3.6.2 Power Control
A directional coupler and detector network controls power. It senses the forward power from the last
gain stage and feeds the detected voltage back to the command board control circuitry, where it is
compared to a reference voltage set during power-set procedures. The dc feed voltage is corrected
and supplied to the “controlled” stage of the power amplifier. Circuitry on the power amplifier board
controls the gain of the first stage and is proportional to the dc control voltage.
3.6.3 Circuit Protection
Current and temperature sensing circuitry on the power amplifier board feed sensed voltages to the
command board for comparison. If the command board suspects a fault condition, it overrides the
power control function and cuts the power back to a level that is safe for the conditions.
3.6.4 DC Interconnect
The ribbon cable connector carries sensed voltages for power and protection to the command board.
It also carries A+ feed to the command board for distribution throughout the internal transceiver
chassis and carries control voltage from the command board to the power amplifier board.
The rear battery connector carries A+ from the battery to the power amplifier board. The red lead
goes directly to the A+ terminal on the PA board. The black lead from the battery connector ties to
the chassis, and connection to the power amplifier board is made through the board mounting
screws.
A+ ground connection for the internal transceiver chassis is through the RF coax ground connectors
and through the mechanical connection of the power amplifier heatsink to the rest of the radio.
During test conditions in which the power amplifier assembly (board and heatsink) is physically
disconnected from the rest of the radio, it is acceptable to rely on the coax cable connections to carry
ground to the internal chassis.
3.7 Front-End Receiver Assembly
The receiver front-end consists of a preselector, a mixer circuit, and an injection filter. The receiver
injection (1st local oscillator) comes from the VCO assembly through a coax cable. The injection filter
is either fixed-tuned or tuned at the factory depending upon the bandsplit. The output of the filter is
connected to the mixer.
The preselector is a fixed-tuned filter. The receiver signal is fed to the preselector from the antenna
switch in the PA for the 800 MHz radios, or the preamp output for VHF and UHF. The signal is then
sent to the mixer integrated circuit where it is connected to the mixer transistor. The receiver injection
is also fed to this point. The mixer output is at the 1st IF center frequency of 109.65MHz. This signal
is sent to the 1st IF on the RF board through a coaxial cable.
3.8 Radio Frequency (RF) Board
The RF board contains the common synthesizer circuits and dual IF receiver and demodulation
circuits. A 4-pole crystal filter at 109.65MHz provides first IF selectivity. For HRN6014D, HRN6020C,
HRN6019C, HRN4009D, HRN4010C, and later RF board kits, two 2-pole crystal filters provide first
IF selectivity at 109.65MHz. The output of the filter circuit is fed directly to the custom digital back-
end circuit module. An amplifier (at 109.65MHz), the second mixer, the second IF amplifiers (at 450
kHz), the IF digital-to-analog converter, and the baseband down-converter are part of the digital
backend circuit module. The digital backend circuit module also converts a 2.1MHz reference to a
2.4MHz clock for the DSP support circuit module.
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