User's Manual

8.3 CONTROL/LOADER BOARD
The control/loader board is responsible for controlling the operation of the RF board and for
processing the audio input and output signals to and from the RF board and to and from the
outside.
Audio Chain
The audio processing, both transmit and receive, is handled by the audio Codec IC, U101, PMR
processor U302, and the microcontroller U301. These devices handle pre-emphasis, modulation
limiting, analog to digital (A/D) & digital to analog (D/A) conversions, Digital stream
compression/de-compression and low-pass filtering on transmit as well as de-emphasis and
filtering on receive.
In transmit, the audio inputs are routed to the PMR processor. The analog signal is adjusted in
level, filtered, limited, and filtered again. This analog signal is sent to the PMR’s A/D converter
which changes the analog signal to a digital data stream. This uncompressed digital data stream
is sent to the Codec IC. The Codec compresses and sends this data stream back to the
microcontroller. The microcontroller then sends this digital stream to the PMR processor, where
the PMR interleaves this compress audio digital stream with the target address into a NXDN
format. This formatted stream is placed into the input of an D/A converter. The output of the D/A
forms a 4 level FSK analog signal that is routed to the RF board for FM modulation.
In receive, the raw discriminator audio from the RF board is fed to the PMR processor. Upon valid
NXDN frame sync reception, if the expected NXDN address is found, the voice packets are sent
to the microcontroller which passes them to the Codec. The Codec decompresses the digital
steam and pipes it to the PMR. This data stream is placed into the PMR’s D/A. The D/A’s output
is the analog which is sent to the I/O connector.
Other Analog Functions
To supply the proper voltages for the two boards, various voltage regulators are used. U310 is
used to produce +7.5 VDC for the RF board. U308 and U309 are used to supply +5 VDC and
+3.3 VDC for the control/loader board.
9 HARDWARE OPTIONS
The DTX is setup at the factory in a configuration that should be acceptable for most users. The
most common changes required are effected through the programmer without removing the cover
from the unit. There are, however, a number of component jumper changes which can be made
which may result in more satisfactory integration in a data system. These changes require the
soldering and unsoldering of SMD components and should be undertaken only by qualified
service personnel. Refer to the PCB component locator and schematic diagrams as needed.
9.1 CONTROL/LOADER BOARD OPTIONS
9.1.1 AUX OUT COUPLING
The AUX OUT output is normally AC coupled through C399. If DC coupling is desired, an 0805
size SMD zero ohm jumper resistor (RITRON P/N 47100000) must be soldered in the location of
MR384. (In lieu of a zero ohm jumper, a small piece of wire may be carefully soldered between
the pads.) The AUX OUT DC level then becomes nominally 1.67 volts.
9.1.2 AUX IN COUPLING
The AUX IN input is normally AC coupled through C343. DC coupling is possible, however, the
carrier frequency of the unit would then become directly affected by the DC voltage present. If
DC coupling is desired, an 0805 size SMD zero ohm jumper resistor (RITRON P/N 47100000)
must be soldered in the MR369 location. The DC voltage should be 2.5 volts nominal and very
well regulated. Note: The FCC Type Acceptance obtained by RITRON is invalid once this
modification is made. The user is responsible for obtaining type acceptance in a
configuration which includes the device which is connected to the AUX IN input.