User's Manual

5.5 OPERATING MODES
5.5.1 RECEIVE
Carrier Detect and Squelch Operation
The DTX-165-RR-SIG 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
output, AUDIO OUT, is 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 Output
The AUDIO OUT is programmable as de-emphasized or a flat frequency response. Its gain is
also programmable. This output can drive 8-ohm speaker-type loads.
5.5.2 TRANSMIT
PTT Operation
The transmitter is activated by placing the PTT (Push-To-Talk) 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).
Transmitter Audio Input
The AUDIO 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 MIC IN signal can achieve 60% modulation with 4 mv rms in the low gain
position and 0.5 mv in the high gain position.
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 HI/LO input at a logic high state. Placing the HI/LO input at a
logic low state chooses low power. The HI/LO 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.
Antenna Placement
The DTX module is enclosed in a metal housing for RF shielding. However, RF emitting sources
located very close (less than 12 inches) to the unit can at times affect its operation. It is not
recommended that an antenna be connected directly to the module’s BNC connector unless the
RF output power is set for less than 5 watts or the module is placed within another RF-tight
enclosure.
5.6 RECEIVER SELF-QUIETING SPUR (BIRDIE) FREQUENCIES
Receiver self-quieting spurious signals (birdies) are internally generated signals that result from
the outputs and harmonics of the outputs of the various oscillators that form part of the DTX
radio. These usually sound like unmodulated carriers -- signals with "dead air." The DTX has
two oscillators that tend to cause birdies, the 14.4 MHz reference oscillator and the 12.288 MHz
oscillator on the control/loader board. Of these two, the 14.4 MHz is by far the most significant
due to its proximity to the sensitive receiver circuitry. Most of the spur frequencies in the following
table have sensitivity degraded by 3 dB or less. Frequencies where the sensitivity is degraded
more than 4 dB below specification are in bold. Frequencies that are integer multiples of the 14.4
MHz reference oscillator typically cause a reduction in sensitivity of 10 dB or more and are in
bold and in red. Note that a birdie is not the same thing as a receiver spurious response. A
spurious response is the receiver hearing a signal on a frequency other than that to which it has
been programmed.