SmarTrunk Systems ST-9116C, Service Manual, Page: 11 / 67

ST-9116C SM R 1.1 SMARTRUNK SYSTEMS, INC. 11
4.3.2. ANALOG REPEATER LOOP
In repeater mode, the RF is detected
by the receiver front-end (U8) then
the discriminator audio is fed into
the receiver audio processor (U35)
to be conditioned and filtered.
The received audio is fed to
transmit audio processor (U20)
where the signal is emphasized and
limited to ensure the proper
transmission shape.
If sub audio signaling is required,
then it is also generated by transmit
audio processor (U20).
The result of the analog modulation
is divided in low band and high
band to be fed to the transmitter
(U2) and a fraction of the low band
information is modulated throw TX
VC TXCO (Y1).
4.3.3. DIGITAL TX AUDIO FROM
EXTERNAL MIC:
The Mic signal coming from the
front connector is fed into the
microphone pre-amplifier (U43) to
be conditioned.
The output of the conditioned mic
signal is fed into the RX audio
processor (U35) to be filtered, then
fed to one ADC of the main
processor (U37) where is converted
to AdPCM format.
If required, this signal is sent to the
IP port for wide area applications.
Once inside of the processor, the
PCM information is managed in
agree to the digital transmission
mode, being delivered to one of the
PWM filters related to any of the
vocoders, which may serve to the
digital transmission.
The output of the vocoder is
collected by the processor (U37)
where the data is encrypted in agree
to SDR protocol, adding a FEC
(Frame Error Correction), and
adding the user related information
as user ID, etc., arranging the
frames which are moved to 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.
TX AUDIO
PROCESSOR
[U20]
TRANSMITTER
[U2]
VC TXCO
[Y1]
RX Radio
U8
RX AUDIO
PROCESSOR
[U35]
Figure 5: Analog Repeater loop path