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, U302 and the DSP
microcontroller U301. These devices handle pre-emphasis, modulation limiting, and lowpass filtering on
transmit as well as de-emphasis and filtering on receive.
In transmit, the audio inputs are routed to the Codec IC. This IC has internal analog to digital converters
which convert the analog input to a digital data stream. This data stream is sent to the DSP processor where
the signal is adjusted in level, filtered, limited, and filtered again. The output digital signal is sent back to
the Codec IC where digital to analog converters convert the signal back to analog for routing to the RF
board for FM modulation.
In receive, the raw discriminator audio from the RF board is passed to the Codec and is converted to a
digital signal. The digital signal is processed by the DSP processor for level and frequency response and
sent back to the Codec. The Codec converts the signal back to analog where it 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 C372. If DC coupling is desired, an 0805 size
SMD zero ohm jumper resistor (RITRON P/N 47100000) must be soldered in the location of C372. (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 C352. 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, C352 must be removed and an 0805 size SMD zero ohm jumper resistor (RITRON P/N 47100000)
must be soldered in the C352 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.
9.2 RF BOARD OPTIONS
9.2.1 DISCRIMINATOR POLARITY
The polarity of the discriminator output at pin 14 of J102 is configured at the factory such that an increase
in RF frequency causes an increase in DC voltage. This is considered “normal” mode. An inverted mode
is available where an increase in frequency causes a decrease in voltage. This is effected by removing
R130 and placing it in the open pad pair denoted as R125