5-7
The audio signal then leaves the Mainboard Assembly (ASY-0245-01) as a 600
ohm balanced 0 dBm level and is received by the SEABUSS audio receiver in the
Front Panel/Controller Assembly consisting of balanced line amplifier U10A.
After passing through U10A, the signal is then sent to the VOLUME control and the
squelch limiter U8A and U8B. The signal from the VOLUME control wiper then
goes to AF Power Amplifier U14 where it is amplified to a 4-Watt level and is then
sent to the loudspeaker.
The limited audio signal from U8A is sent to an input of the Controller
microprocessor (U1) where it is used to generate the SQL signal. An algorithm in
U1 senses the presence (or absence) of a voice signal in the limited audio signal and,
when the Squelch function is activated, generates a SQL OUT signal. The SQL
OUT signal is used to turn on the squelch gate transistor Q6 to silence the
loudspeaker.
5.3
THE TRANSMITTER
5.3.1
BLOCK DIAGRAM
Figure 5.3.1 shows the block diagram of the SEA 245 in the transmit mode.
Microphone audio is amplified and converted to a balanced format for transmission
from the Front Panel/Controller Assembly to the Mainboard Assembly (ASY-0245-
01) via the SEAbuss audio lines. On the -01 board it is converted back to
unbalanced format and then digitally sampled by CODEC U1 on the CPU board
(ASY-0245-04). The CODEC transfers the data serially to the DSP, U7. The DSP
generates a SSB signal at the (nominal) subcarrier frequency of 16 kHz. The
CODEC converts the digital sample stream back to analog format. The resulting I
and Q SSB signals are fed back to the Mainboard and into the inputs of quadrature
modulator U6. The modulator mixes the I and Q signals with in-phase and
quadrature 45.016 MHz local oscillator signals. This results in a single sideband
signal at the 45 MHz intermediate frequency. This IF signal is passed through the
bilateral 45 MHz crystal filter into the IF port of mixer MIX1. This mixer
downconverts the IF signal into the MF/HF RF band. The RF signal is low-pass
filtered and amplified before being passed on to the PA/Filter Assembly (ASY-
0245-02) via a coaxial cable. Transmitter preamplifier Q1 boosts the signal level
sufficiently to drive the push-pull driver stage consisting of Q2 and Q3. The driver
output is then routed to the push-pull power amplifiers, Q6 and Q7. The output of
the amplifier is then routed through the T/R relay to a low pass filter that is relay
selected for the desired band of operation. The filtered output is fed to the antenna
jack on the SEA 245 rear panel.
5.3.2
THE MICROPHONE AUDIO CIRCUITRY
The 600 ohm dynamic microphone output is terminated by 620 ohm resistor R8 and
then passes through R94 and C13X to the input of the amplifier/phase splitter stage
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