3: Audio/Filter Board
PRC1099A-MS
3-3
Off-Board
Transmit Path
The output of U6 is applied to coax connector J10 (TX OUT) where it is
routed to the 1650 kHz IF board. On the 1650 kHz IF board, one of the
sidebands is stripped away. The 1650 kHz IF board is discussed in Chapter 4.
The single sideband transmit signal is then applied to the Mixer board where it
is up-converted from the BFO frequency of 1647 or 1650 kHz to an IF
frequency of 75 MHz and then down-converted to the selected channel
frequency. The Mixer board is discussed in Chapter 5.
From the Mixer board the transmit signal is routed to the Power Amplifier
board where it is amplified to the selected output power. Then it returns to the
Audio/Filter board through coax connector J11. The PA board is discussed in
Chapter 6.
Low-Pass Filters
From the J11 coax connector, the transmit signal is filtered through the
appropriate high performance, 5-pole, elliptic function low-pass filters
selected by the processor according to the selected channel frequency, as
discussed in “Relay Drivers and Decoder” below. These filters cover the
following frequency ranges:
These filters use high-Q toroidal inductors and precision capacitors to remove
high frequency noise and harmonics.
Relay Drivers
and Decoder
The microprocessor on the Processor board selects the correct low-pass filter
using latching relays K1 through K12. The latching relays use dual windings
and require separate current pulses to open or close the relay. Latching relays
are only energized to open or close the relay; otherwise they are not energized,
saving battery power.
Switching transistors Q8 through Q13 control the on-switching current pulse
to each pair of relays while reset transistor Q14 controls the off-switching
pulse to all the relays. Shift register U7 decodes the serial data from the
processor and drives the switching transistors. The firmware ensures that all
relays are reset before selecting a new filter. The program performs the same
function whenever the transceiver is switched on or there is a power
interruption. This ensures that the relays are always correctly reset.
From the low-pass filters the transmit signal passes through the detector for
the ALC circuit discussed in “ALC Circuit” on page 3-6. It is then applied to
coax connector J12 where it is routed to the Antenna Tuner board and then to
either the whip antenna connector or 50 Ohm antenna output connector.
• 2.0 to 2.99999 MHz
• 8.0 to 12.99999 MHz
• 3.0 to 4.99999 MHz
• 13.0 to 19.99999 MHz
• 5.0 to 7.99999 MHz
• 20.0 to 30.0 MHz
