User Manual

SECTION 1: THEORY OF OPERATION
~\Technical Documentation\System Manuals\FCC-Reports\DR4B-FCCRpt.doc Page 8
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Baseband
This circuitry amplifies the audio from each receiver, routes it through solid-state switches, and selects the
audio from the receiver with the highest quality value. The comparator circuit on the previous sheet
controls it.
There are three (3) channels of audio, with separate gain and DC offset adjustments to compensate for
performance differences in the receivers. For example, incoming audio from receiver 1 appears at
AUDIO 1. An op amp (U12D) is then amplifies the audio. A pot (R72) adjusts the gain. The amplifier
output drives a switch (U4).
The remaining audio circuits work in the same manner.
Two (2) solid-state switches route audio from the selected receiver. U4 selects audio from either
Receiver 1 or 2. When the RX1/RX2_SELECT input from the comparator circuitry is high, Receiver 1
audio is passed through the device. Another switch (U7) selects audio from either a selected Receiver (1
or 2) or receiver 3. When the RX3_SELECT input from the comparator circuitry is high, Receiver 3’s
audio is passed through the device. The output from U7 appears on DISC_AUD, which goes to a header
(J2). From there the audio is demodulated by the modems on the System Controller Board.
The radio also offers a voice grade audio output via the rear panel DB25 connector. DISC_AUD also
drives low pass filter (U14), and buffer (U9D). The AUDIO OUT line from the buffer goes to pin 12 of a
connector (J3).
This section displays manual receiver switching and DC power distribution. Manual receiver switching is
primarily intended as a diagnostic tool. By closing various switches on S1, the RX1/RX2_SELECT or
RX3_SELECT LINES may be forced high or low, as appropriate. This overrides the operation of the
comparator circuitry, forcing one receiver to operate at all times. Resistors (R98 and R97) provide power
supply short circuit protection in the event the wrong switches are closed.
Pads (TP4 and TP5) represent the power distribution bus (TP4 and TP5). Incoming power from the rear
panel fuse holder appears on pad TP4. The B+ output goes to the power supply circuitry. TP5 connects
to the transmitter power amplifier.
Receiver Board
Please be aware that the radio uses three (3) identical receiver boards. As a result, the circuitry will be
described only once.
Front end. Incoming signals pass through a filter (FT1). The filter (FT1) is a double-tuned device, which
provides a high degree of selectivity. The desired signals are amplified by U4, a low noise amplifier.
Additional selectivity is provided by triple-tuned filter (FT2). An RF amplifier (Q1) amplifies the signal.
The output from Q1 passes through triple-tuned filter (FT3), and then to mixer (U5). U5 is a double-
balanced mixer, which heterodynes a local oscillator signal from the Injection Synthesizer Board. The IF
output leaves the mixer as the 45 MHz line. It goes to an IF filter on the following sheet.
There are five (5) filter sets (FT1, FT2, and FT3) available, and each covers a 20-30 MHz
portion of the UHF band. Should replacement of the filters be required, exact replacement
parts must be used.