SmartTrunk Systems ST-1000C, Service Manual, Page: 10 / 35

ST-1000C SM R 1.0 SMARTRUNK SYSTEMS, INC. 10
high definition digital to analog
converter.
4.3. Transmitter System
4.3.1. ANALOG TX SIGNAL
The AF signal from the internal
microphone (MK1) or external
microphone (J4) are processed as
differential signal, then it is
conditioned and transformed into
common mode by the microphone
pre-amplifier (U11).
The result of the conditioned in-
band filtered signal is fed to the
Main
Processor (U8), which
provides audio gain control,
compander, emphasize, limitation,
encryption,
etc. by software
algorithms.
The high audio band of the signals
from 300 to 3500 Hz are directly
fed to the transmitter (U1), which
performs a direct modulation of a
generated carrier.
Low frequency signaling audio is
fed directly to Y1 VC TXCO main
clock generator and U1 with a
corresponding balance controlled
by software.
4.3.2. DIGITAL TX AUDIO:
The AF signal from the internal
microphone (MK1) or external
microphone (J4) are fed directly to
the main processor (U8), which
perform the signal filtering, the
data codification, into especial
firmware block. The result of the
digitized information is processed
by a FEC (Frame Error Correction)
module, adding spare bits to
ensure high robustness to the
data, then it is encrypted and the
user related information as user ID
is added.
The final data frame is fed to the
modem code, which generates the
FDMA signals, controlling the
modulation of the Transmitter (U1)
through the VC TXCO for DC to
low frequencies and the direct
modulation for higher frequencies.
4.3.3. Driver and Final Amplifier Stages
Final RF is amplified on Q1
(2SC3356) then filtered to be fed to
Q3
(2SK3475) then finally
amplified by Q2 (2SK3476) up to 2
watts output power.
The transmit signal then passes
through the antenna switch D1
(BAR88) and is low-pass filtered to
suppress
harmonic spurious
radiation before delivery to the
antenna.
AMPLIFIER
[U11]
MAIN
PROCESSOR
[U8]
TRANSMITTER
[U1]
VC TXCO
[Y1]
TRANSMITTER
[U1]
VC TXCO
[Y1]
MAIN
PROCESSOR
[U8]
AMPLIFIER
[U11]