Service Manual

MM-E 2072-09333-00
2-4
(3) The RF power amplifier in the HI POWER module generates an RF signal at the power
level selected by the operator (LOW, MED, HIGH, or MAX). The transmit signal is
connected by the TX/RX switch to the harmonic filter, which attenuates harmonics and
unwanted spurious signals. The filtered signal then passes to the ANT connector through
a power sensor, which generates DC voltages, VFWD and VREV, proportional to the
forward and reflected power.
The ANT connector can be connected either directly to a broadband antenna, or to an
external ATU unit. DC power for the ATU unit is provided from the internal protected
DC power line, through a switch controlled by the LORD module.
An automatic level control (ALC) loop maintains the transmit output power close to the
selected transmit power. The automatic level control loop compares the VFWD sample
with the transmit power control signal provided by the microcomputer subsystem in the
LORD module, and adjusts the input RF signal power reaching the amplifier.
The VREV sample is used for mismatch protection: if the VSWR (Voltage Standing
Wave Ratio) exceeds 2:1, the transmit power is proportionally reduced, to prevent
damage.
The RF power amplifier also includes thermal protection: when the internal temperature
exceeds the maximum permitted, the transmit power is automatically reduced until the
temperature returns to normal. If nevertheless the temperature continues to increase even
when operating at the LOW level, the transmission is stopped until the temperature
returns to normal.
2-1.1.3.2 Receive Path
a. Processing of Received RF Signal.
(1) The received RF signal is filtered by the harmonic filter, and is then connected by the
TX/RX switch to the preselector in the LORD module.
(2) The preselector provides further filtering of the RF signal, to improve the rejection of
out-of-band signals. The resulting signal is supplied to the first mixer, which converts the
signal to the first IF frequency, 45.1 MHz, by mixing it with the local oscillator 1 signal
provided by synthesizer 1 (para. 2-1.1.3.3).
b. IF Signal Path.
(1) The first IF processor filters and amplifies the 45.1 MHz provided by the first IF mixer.
The filtered signal is then converted to the second IF frequency, 450 kHz, by the second
mixer, which mixes the 45.1 MHz signal with the local oscillator signal provided by the
synthesizer 2.
(2) The second IF signal is filtered and amplified by the second IF processor, and is then
converted to the third IF frequency, 20 kHz, by the third mixer. The local oscillator signal
for the third mixer, 430.77 kHz, is obtained by dividing the 16.8 MHz reference
frequency.
(3) The third IF signal is converted to a digital data stream, which is supplied to the DSP
subsystem. The DSP demodulates the IF signal, and extracts the original modulation
signal.
c. Audio Signal Paths.
(1) The data stream demodulated signal, provided by the DSP, is converted by the D/A
converter section of the codec to an audio signal. The audio signal is amplified and
supplied to the speaker located on the front panel, and to the other audio and data outputs
in the ACCESSORY connector.
(2) When the ISB option is installed, the DSP provides an additional data stream which
represents the slave sideband. This data stream is converted by a codec to an audio signal,
RX_AUDIO_ISB, which is available in the ACCESSORY connector.