User Manual

SGD-SB2025NT-TUM, Part 1
Jan 12 Page 35 TECHNICAL DESCRIPTION
between Rx audio and Tx looped back audio for output to line via amplifiers IC40A and IC40B.
Secondary inputs to these amplifiers are the Tx audio from the line modem and microphone audio
to line. The output on SKH-2 forms an unbalanced 600 Ω VF output to line and, alternatively,
outputs SKH-2 and SKH-11 form a differential output.
Discriminator audio is also fed via a low pass filter IC31 to the CTCSS decoder as well as to a level
detector D9 the output of which is connected to ADC1 on the microprocessor.
Discriminator audio is also fed to the mute detection circuit. This comprises a high pass filter IC41
followed by RV6/6B, which sets the mute threshold. Following amplification by IC42A and
rectification by IC42B and IC43A, a comparator, IC43B, determines when the detected signal
passes a fixed threshold. The output of the comparator is fed to the microprocessor and via a
selectable inverter to SKH-4.
Discriminator audio is also fed to Rx port of the RF modem IC54.
5.4.4 RF Power Control
Forward power is controlled by the microprocessor through two mechanisms. Based on pre-
programmed per channel adjustments, the microprocessor sets the digipot IC10 to a reference
setting. IC23A serves as a comparator and, with the non-inverting input connected to the wiper of
the digipot, is set up with a reference voltage. The detected actual PA forward power is fed to the
inverting input of IC23A. The error voltage at the output of IC23A is fed to the exciter output power
control circuit via SKC-4 and the action of the control loop is to set the RF power such that the
actual detected volts equals the reference volts. The digipot setting is static for each channel
unless the required forward power is changed.
The voltage to the top of the digipot is set up by the microprocessor through the Pulse Width
Modulator (PWM) output PWM1. On PTT, the ON duty cycle of the PWM1 output is progressively
increased and the filtered result of this forms a ramp to the top of the power control digipot. Once
100% duty cycle is reached, full power is produced. This results in a fast but controlled RF power
rise characteristic.
5.4.5 User Interface
All user interfaces to the SB2025, except the RF connections, are made by way of the front panel
USB interface on the Aux PCB and then via an internal serial connection to the NI or ITX for the
ET. The MxTools application for radio configuration also uses this route to the NI but is passed on
over another serial connection to the Micro Controller Board.
Output latch IC18 drives the six LED indicators on the front panel. The function is described in
Section 4.1.1.
Input latch IC14 serves to accept the programmed status of the eight bit binary channel select DIP
switch SW1. Refer to Appendix B for programming instructions.
Input latch IC13 serves to accept the programmed status of the six bit binary mode select DIP
switch SW2. Refer to Section 3.2.2.2 for mode selection instructions.
IC22 provides CMOS to RS232 conversion for the serial port.
5.5 S OLAR T RAFFIC M ANAGER
The TM is a software application running on the ITX board and manages all aspects of a channel
including NIs other than its physically associated one if necessary and a DFSI conforming to
TIA.102-BAHA.