User Manual

SGD-SB2025NT-TUM, Part 1
Jan 12 Page 29 GENERAL DESCRIPTION
The Press (Push) To Talk (PTT) signal enables the amplifier circuit by providing bias to the
transistors. A thermistor TS1, physically located on the PA heatsink monitors the heatsink’s
temperature and is monitored by the Micro Controller.
The PA is very compact and efficient for high reliability and low cost. The heatsink has minimal
temperature rise even under continuous operation, ensuring the best Mean Time Between Failure
(MTBF) obtainable for a practical design.
4.3.4 Micro Controller Board
The Micro Controller Board is physically located behind the rear panel connectors and all signal
connections (apart from the RF connections) external to the transceiver are made via the controller
card. User settable jumpers and DIP switches are located on the card as are level adjustment
potentiometers.
The Micro Controller controls the operation of the RF modules and acts as the interface between
the Solar NI, indicators and the RF modules. It also processes transmit and received audio to and
from the Exciter and Rx modules.
The circuit board has an onboard Electrically Erasable Read-Only Memory (EEROM), which stores
all of the user channel related data such as frequencies. A serial port at the front of the SB2025
provides access to the NI and Controller card and, in conjunction with the Simoco ‘Engineering
Terminal’ and ‘MxTools’ programming utilities, allows the user fully configure the SB2025 base
stations.
Digipots under the control of the processor ensure that user set up levels for Tx deviation and
power levels are correctly set for each channel.
4.3.5 Network Interface
The Network Interface is physically located on a sub-chassis mounted above the Micro Controller
Board. Signal connections are made to the Micro Controller Board for audio, Tx/Rx control and a
serial data link for configuration of the Micro Controller. Other connections are made via Molex
and RJ45 connectors for serial data, 1PPS timing/NMEA data and Ethernet. On receive, the
Network Interface samples the Rx demodulator output at 24 kHz and processes the data before
sending it to the TM. On an analogue call, the Digital Signals Processor (DSP) carries out
CTCSS/Continuous Digital Coded Squelch System (CDCSS) decoding and g.726 encoding of the
received audio. The micro controller adds additional information such as RSSI and any detected
sub-audio tone/code before packaging and sending to the TM. On a P25 call the DSP provides
Continuous 4 Level Frequency Modulation (C4FM) demodulation.
On transmit, the NI receives Ethernet packets from the TM and generates appropriate analogue or
P25 signals for passing to the wideband modulation input of the Exciter via the Micro Controller
Board. CTCSS/CDCSS is generated in the correct phase and mixed with the analogue audio.
A PLL is maintained locked to the 1PPS timing reference and along with time stamps in the
Ethernet packets ensures transmitted audio/P25 data is correctly in phase for simulcast
transmission.
4.3.6 ITX
The ITX (also known as Single Board Computer (SBC)) uses the Linux operating system and runs
the TM application.
4.3.7 Compact Flash
Contains the operating system, the TM application and its configuration files.