5.5
OPERATING MODES
5.5.1
RECEIVE
Carrier Detect and Squelch Operation
The DTX 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 DTX 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.
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 Inputs
Two mutually exclusive audio inputs are available on the DTX module. The AUX IN is a general-purpose
input with an input impedance greater than 50 k-ohms and is capacitive coupled with a lower roll-off
frequency of about 25 Hz. The MIC IN input is a higher gain input designed for connection to a standard
electret or dynamic microphone. 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. At maximum
gain, the AUX IN signal input can achieve 60% rated modulation with about 25 mv rms input signal while
the MIC IN signal can achieve the same modulation with 4 mv rms in the low gain position and 0.5 mv in
the high gain position.
