SmarTrunk Systems ST-9116C, Руководство по обслуживанию, Страница: 8 / 67

4. - ST-9116C RF DESCRIPTION
4.1. CIRCUIT CONFIGURATION
The receiver is a single receiver with
built DSP fully integrated.
Incoming signals from the antenna,
after passing through LPF filter, are
band limited by an active bandpass
filter then fed direct to the DSP
down converter to get the baseband
voice from 0Hz to 3500Hz.
Demodulated signals are filtered
and conditioned onto a second DSP
based filter, which also includes a
high efficiency 4FSK modem.
For digital demodulation, the
recovered data is fed into a vocoder,
which converts the data to voice.
Analog voice from the analog path
or the analog voice recovered from
the vocoder, are fed into an audio
power amplifier.
The transmit signal frequency is
generated by the integrated VCO
and PLL. RF frequency generated by
the integrated RF chip is amplified
into a 3-step amplifier then filtered
by a low pass filter to be applied to
the antenna.
4.2. RECEIVER SYSTEM
4.2.1. FRONT-END RF RECEIVER
Incoming RF signals from the RX
antenna are delivered to the
Receiver Unit and passing through
an active band pass filter.
D12 & D13 varicaps works with D9
& D11 as a tuned filter controlled
by U10.
In the middle of the filter chain. An
amplifier compensates the loss
introduced by the filter. The gain of
Q10, the amplifier, can be adjusted
to increase the overall RX gain up
to 10dB.
ST-9116 firmware adjusts the gain
of this amplifier to ensure the best
signal to noise relationship.
4.2.2. ANALOG AUDIO PROCESSING
The RF signal is tuned by U8, which
includes a base band DSP audio
processing, recovering flat audio
from DC up to 3500 hz.
Figure 1: Receiver block Diagram
Figure 2: Receiver Analog Audio Path Bloc Diagram
RX
Radio
U8
LPF
AMPLIFIER
Q10
FILTER
D12/D13
FILTER
D9/D11
Filter
CTRL
[U10]
Band pass control
Gain control
Main Processor
[U37]
RX Radio
U8
AUDIO
PA
[U36]
PWM
FILTER
Analog
Loop
Audio
Processor
RX Modem
[U35]