SmartTrunk Systems ST-1000C, Service Manual, Page: 8 / 35

ST-1000C SM R 1.0 SMARTRUNK SYSTEMS, INC. 8
4. - ST-1000 RF DESCRIPTION
4.1. Circuit Configuration
The receiver is a single receiver
with built-in DSP fully integrated.
Incoming signals from the
antenna, after passing through
LPF filter, are fed direct to the DSP
down
converter to get the
baseband voice from 0Hz to
3500Hz.
Demodulated signals are fed to the
DSP processor where are filtered
and conditioned. This processor
also includes a high efficiency FSK
modem.
For digital demodulation, once the
data is decoded, and free of errors,
it is fed into the into a vocoder
firmware module, which converts
the data to voice.
Analog voice from the analog path
or the analog voice recovered from
the vocoder firmware, are fed into
an audio power amplifier direct
from a digital to analog converter.
Transmit signal frequency is
generated by 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
antenna are delivered to the
Receiver Unit and pass through a
Low-pass filter, antenna switching
diode, and then fed to the receiver
(U1) passing through a limiter
BPF.
4.2.2. ANALOG Audio Processing
The RF signal is tuned by U1,
which includes a base band DSP
audio processing, recovering flat
audio from DC up to 3500 hz.
The detected audio is amplified,
filtered and conditioned inside of
U8 which also includes a de-
emphasize filtering shape for
received audio signals by firmware.
The output of the filtered and
conditioned audio is delivered to
an audio power amplifier (U9) then
to the speaker passing through the
external audio connector switch.
4.2.3. Virtual Squelch Circuit
S/N ratio is measured by the
receiver (U1) as the result of the
analysis of the signal and the noise
of the carrier. The output is sent to
the main processor as a digital
frame, which is analyzed by radio
LPF
SWITCH
PROTECTION
FILTER
RECEIVER
[U1]
RECEIVER
[U1]
MAIN
PROCESSOR
[U8]
AMPLIFIER
[U9]