Datron TW7000MS, Техническое руководство

Страница: 46 / 100

3: Theory of Operation
3-12 TW7000MS
3.5 Transmit Signal Flow
For a diagram of the transceiver transmit path, refer to the "Transceiver
Transmit Path" figure on page 3-13. Primary unbalanced audio comes from
the microphone connectors on the front panel (J1-25) and is routed to the
Audio board at J8-20. Balanced 600 ohm audio is obtained from accessory 1
or 2 (pins 15, 17), and enters the Audio board at J8-5,18. Modulated audio
(MODA) is sent to the 5 MHz IF board where it is mixed with the third LO
(BFO) to form the IF at 5 MHz. It is filtered and amplified, then mixed with
the second LO to form the IF at 75 MHz before routed to the 75-MHz IF
board. The signal is amplified and filtered, and mixed with the first LO to
form the low-level TX channel frequency RF. It is amplified again in the TX
exciter circuitry before sent to the RF power amplifier.
In the power amplifier, the signal is amplified to its final output power level
and routed to the RF Filter board. The RF Filter board provides the necessary
harmonic filtering before the signal reaches the antenna port. Just prior to this,
the signal passes through the VSWR circuit where it is sampled by a forward
and reflected power detector. These RF samples are fed back to the Reference/
Control board where they are processed to make up the transmitter automatic
level control (ALC) circuit and VSWR measurement. The raw ALC voltage is
sent back to the 75 MHz IF board where it controls the gain of the low-level
exciter amplifier.
3.6 Receive and Transmit Path Subassemblies
Standard subassemblies in the receive/transmit path include the following:
• VSWR circuit
• RF Filter board
• RF Power Amplifier board
• 75 MHz IF board
• 5 MHz IF board
• Audio board
• Squelch board