SmarTrunk Systems ST-3118D Service Manual - page 8
ST-3118D SM R 1.1 SMARTRUNK SYSTEMS, INC.
10
and fed it to the audio power
amplifier (U8).
4.3. Transmitter System
4.3.1. ANALOG TX SIGNAL
The Mic signal coming from the
front connector, the external Mic
signal coming from the accessory
connector or the output of the PWM
filter and fed into an analog
selector/switch (U5), then fed into
the microphone pre-amplifier (U7).
The result of the conditioned and in
band-filtered signal is fed to the
Audio Processor (U12), which
provides
audio
compander,
emphasize, limitation, encryption,
etc.
The high audio band of the signal
from 300 to 3500 Hz are directly fed
to the transmitter (U2), which
performs a direct modulation of a
generated carrier.
Low frequency signaling audio are
fed directly to Y1 VC TXCO main
clock generator and U2 with a
corresponding balance controlled
by software.
4.3.2. DIGITAL TX AUDIO:
The Mic signal coming from the
front connector, the external Mic
signal coming from the accessory
connector or the output of the PWM
filter and fed into an analog
selector/switch (U5), then fed into
the microphone pre-amplifier (U7).
After
conditioning
and
band
limiting, the signal is fed directly to
the vocoder chip (U14) which
perform the data codification,
delivering the digitized and encoded
audio as digital frames, which are
processed by the Main Processor
(U13), which add a FEC (Frame
Error Correction), encrypt the
information and add the user
related information as user ID, etc.
The final data frame is fed to the
Audio Processor (U12), which has a
built in modem, which operates for
FDMA or TDMA signals, controlling
the modulation of the Transmitter
(U2) through the VC TXCO for DC
to low frequencies and the direct
modulation for higher frequencies.
AMPLIFIER
[U7]
AUDIO
PROCESSOR
[U12]
TRANSMITTER
[U2]
VC TXCO
[Y1]
Mic
Selector
[U5]
Mic
External
PW
Internal
External
AUDI
O
PROC
ESSOR
TRANSM
ITTER
[U2]
VC
TXCO
[Y1]
MAIN
PROC
ESSOR
[U13]
VOCO
DER
[u14]
AMP
LIFIE
R
Mic
Selec
tor
[U5]
Mic
PW
Figure 4: Digital Modulation path
Figure 5: Analog Modulation path