5.5
OPERATING MODES
5.5.1
RECEIVE
Carrier Detect and Squelch Operation
The DTXM is a transceiver; i.e. it can receive and transmit, although not at the same time. A
carrier detect system exists within the unit to detect the presence of a carrier which controls the
logic state of the DCD (data carrier detect) output. The RF levels at which this output changes
state are programmable. In addition, the unit may be programmed such that the audio outputs,
AUDIO OUT and AUX OUT, are muted (squelched) in conjunction with DCD operation. In units
where squelch operation has been enabled, the RX MON input can be used to override a
squelched condition. The DCD output is not affected by the RX MON operation.
Receiver Audio Outputs
Two receiver audio outputs are present on the DTXM module. The AUX OUT is a general
purpose output which can have pre-emphasis enabled or bypassed. Its gain can be controlled
and its output is designed to drive 1000 ohm or higher loads. The coupling from the discriminator
on the RF board may be set to AC or DC by the programmer. DC coupling allows for internal
bandwidth to extend to DC, but if a high gain value is chosen for the AUX OUT, clipping may
occur on the waveform due to discriminator voltage offsets. AC coupling removes this issue. The
output stage is AC coupled, but can be modified for DC coupling. This requires replacing a
coupling capacitor with a zero-ohm resistor. Contact Ritron for details on this modification. The
AUDIO OUT is always de-emphasized, but its gain can be programmed. This output can drive 8-
ohm speaker-type loads. In the programmer menu, there is provision to set both the AUX OUT
levels and the AUDIO OUT levels. However, the AUDIO OUT level is dependent upon the AUX
OUT level. Therefore, the AUX OUT level should be set before setting the AUDIO OUT level.
5.5.2
TRANSMIT
PTT Operation
The transmitter is activated by placing the PTT/RTS (Push-To-Talk/Request-To-Send) input in its
asserted state. This state is programmable. If the unit is to operate in simplex (transmitter and
receiver on the same frequency), one should check for activity on the channel before transmitting.
This can be done by checking the state of the DCD output. In addition, the unit can be
programmed so that transmit operation is inhibited if the DCD threshold has been exceeded
(busy channel lockout option).
CTS Output
The CTS (Clear To Send) output goes to its active state when the unit has powered up the
transmitter, it is locked on the correct transmit frequency, and it is ready to accept modulation.
This output may be used to signal a modem to start transmitting data. If this output is not used, to
avoid losing data, a delay of at least 12 ms is required between PTT/RTS activation and the
application of data.
Transmitter Audio Input
The choice of the input source is made via the programmer. The selected signal can be set for a
pre-emphasized or flat audio response and the gain can also be set. The Flat signal input can
achieve 60% rated modulation with about 25 mv rms input signal, or the Pre-emphasized signal
can achieve the same modulation with 0.5 - 4.0 mv rms in the depending on the TX Gain setting.
High/Low Power
High and low power levels can be programmed into the unit on a channel by channel basis. High
power is selected by placing the CSN input at a logic high state. Placing the CSN input at a logic
low state chooses low power. The CSN input has an internal pull-up resistor; it will assume the
high state and the module will be set for high power when left unconnected.
