SmarTrunk Systems ST-9116C, Service Manual, Page: 12 / 67

ST-9116C SM R 1.1 SMARTRUNK SYSTEMS, INC. 12
4.3.4. REPEATER DIGITAL TX AUDIO:
Working on any digital modes, the
received carrier is detected by the
RX radio chip (U8), and then the
received audio in base band is fed
to the RX modem (U35).
RX modem recovers the received
data which is collected by the main
processor (U37), where is decrypted
and corrected from the received
FED.
The received data is routed as
required. In case of repeater mode,
a new frame is encoded, encrypted
and a new FEC is added, resulting
a new TX frame, which is fed into
the TX modem (U20).
The output of the modem is fed into
the TX radio chip and a portion of
that modulation (low frequencies) is
fed to the VC-TXCO. The balance of
that information is controlled by the
TX audio processor, depending on
the modulation type required.
4.3.5. DRIVE AND FINAL AMPLIFIER
STAGES
Transmit RF is amplified on Q1
(2SC5226)
adjusting the
impedance, then filtered to be fed to
Q2 (2SC3357) then finally amplified
by U3 (RA60H1317) up to 50 watts
output power.
The transmit signal then passes
through forward and reflected
power bridge (U1) then it is low-
pass filtered to suppress harmonic
spurious radiation before delivery
to the antenna
Figure 6: Digital Modulation path from external mic
TX AUDIO
PROCESSOR
[U20]
TRANSMITTER
[U2]
VC TXCO
[Y1]
Main Processor
[U37]
Audio Processor
RX Modem
[U35]
Vocoder
[U21]
AMPLIFIER
[U43]
Mic
Connector
TX AUDIO
PROCESSOR
[U20]
TRANSMITTER
[U2]
VC TXCO
[Y1]
Main Processor
[U37]
Audio Processor
RX Modem
[U35]
RX Radio
U8
Figure 7: Repeater digital path